In computing, entropy is the randomness
collected by an operating system or application for use in cryptography
or other uses that require random data. This randomness is often
collected from hardware sources (variance in fan noise or HDD), either
pre-existing ones such as mouse movements or specially provided
randomness generators. A lack of entropy can have a negative impact on
performance and security.
Linux kernel
The Linux kernel generates entropy from keyboard timings, mouse movements, and IDE timings and makes the random character data available to other operating system processes through the special files/dev/random and /dev/urandom. This capability was introduced in Linux version 1.3.30.
There are some Linux kernel patches allowing one to use more entropy sources. The audio_entropyd project, which is included in some operating systems such as Fedora, allows audio data to be used as an entropy source. Also available are video_entropyd which calculates random data from a video-source and entropybroker which includes these three and can be used to distribute the entropy data to systems not capable of running any of these (e.g. virtual machines). Furthermore, one can use the HAVEGE algorithm through haveged to pool entropy. In some systems, network interrupts can be used as an entropy source as well.
On systems using the Linux kernel, programs needing significant amounts of random data from /dev/urandom cannot co-exist with programs reading little data from /dev/random, as /dev/urandom depletes /dev/random whenever it is being read.
OpenBSD kernel
OpenBSD
has integrated cryptography as one of its main goals and has always
worked on increasing its entropy for encryption but also for randomising
many parts of the OS, including various internal operations of its
kernel. Around 2011, two of the random devices were dropped and linked
into a single source as it could produce hundreds of megabytes per
second of high quality random data on an average system. This made
depletion of random data by userland programs impossible on OpenBSD once
enough entropy has initially been gathered. This is due to OpenBSD
utilising an arc4random function to maximise the efficiency or minimise the wastage of entropy that the system has gathered.
Hurd kernel
A driver ported from the Linux kernel has been made available for the Hurd kernel.
Solaris
/dev/random and /dev/urandom have been available as Sun packages or patches for Solaris since Solaris 2.6, and have been a standard feature since Solaris 9.
As of Solaris 10, administrators can remove existing entropy sources or
define new ones via the kernel-level cryptographic framework.
A 3rd-party kernel module implementing /dev/random is also available for releases dating back to Solaris 2.4.
OS/2
There is a software package for OS/2 that allows software processes to retrieve random data.
Windows
Microsoft Windows releases newer than Windows 95 use CryptoAPI to gather entropy in a similar fashion to Linux kernel's /dev/random.
Windows's CryptoAPI uses the binary registry key HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Cryptography\RNG\Seed to store a seeded value from all of its entropy sources.
Because CryptoAPI is closed-source, some free and open source software
applications running on the Windows platform use other measures to get
randomness. For example, GnuPG, as of version 1.06, uses a variety of
sources such as the number of free bytes in memory that combined with a
random seed generates desired randomness it needs.
Programmers using CAPI can get entropy by calling CAPI's CryptGenRandom(), after properly initializing it.
CryptoAPI was deprecated from NT 6.0 and higher. New API is called Cryptography API: Next Generation (CNG).
Newer version of Windows are able to use a variety of entropy sources:
Embedded Systems
have difficulty gathering enough entropy as they are often very simple
devices with short boot times, and key generation operations that
require sufficient entropy are often one of the first things a system
may do. Common entropy sources may not exist on these devices, or will
not have been active long enough during boot to ensure sufficient
entropy exists. Embedded devices often lack rotating disk drives, human
interface devices, and even fans, and the network interface, if any,
will not have been active for long enough to provide much entropy.
Lacking easy access to entropy, some devices may use hard-coded keys to
seed random generators, or seed random generators from easily-guessed
unique identifiers such as the device's MAC address. A simple study
demonstrated the widespread use of weak keys by finding many embedded
systems such as routers using the same keys. It was thought that the
number of weak keys found would have been far higher if simple and often
attacker determinable one-time unique identifiers had not been
incorporated into the entropy of some of these systems.
Other systems
There
are some software packages that allow one to use a userspace process to
gather random characters, exactly what /dev/random does, such as EGD,
the Entropy Gathering Daemon.
Hardware-originated entropy
Modern CPUs
and hardware often feature integrated generators that can provide
high-quality and high-speed entropy to operating systems. On systems
based on the Linux kernel, one can read the entropy generated from such a device through /dev/hw_random. However, sometimes /dev/hw_random may be slow; usually around 80 KiB/s.
There are some companies manufacturing entropy generation devices, and some of them are shipped with drivers for Linux.
On Debian, one can install the rng-tools package (apt-get install rng-tools) that supports the true random number generators (TRNGs) found in CPUs supporting the RdRand instruction, Trusted Platform Modules and in some Intel, AMD, or VIAchipsets,
effectively increasing the entropy collected into /dev/random and
potentially improving the cryptographic potential. This is especially
useful on headless systems that have no other sources of entropy.
Practical implications
System administrators,
especially those supervising Internet servers, have to ensure that the
server processes will not halt because of entropy depletion. Entropy on
servers
utilising the Linux kernel, or any other kernel or userspace process
that generates entropy from the console and the storage subsystem, is
often less than ideal because of the lack of a mouse and keyboard, thus
servers have to generate their entropy from a limited set of resources
such as IDE timings.
The entropy pool size in Linux is viewable through the file /proc/sys/kernel/random/entropy_avail and should generally be at least 2000 bits (out of a maximum of 4096). Entropy changes frequently.
Administrators responsible for systems that have low or zero entropy should not attempt to use /dev/urandom as a substitute for /dev/random as this may cause SSL/TLS connections to have lower-grade encryption.
Some software systems change their Diffie-Hellman keys often, and this may in some cases help a server to continue functioning normally even with an entropy bottleneck.
On servers with low entropy, a process can appear hung when it is
waiting for random characters to appear in /dev/random (on Linux-based
systems). For example, there was a known problem in Debian that caused exim4 to hang in some cases because of this.
Security
Entropy sources can be used for keyboard timing attacks.
Entropy can affect the cryptography (TLS/SSL) of a server: If a server fails to use a proper source of randomness, the keys generated by the server will be insecure.
In some cases a cracker (malicious attacker) can guess some bits of entropy from the output of a pseudorandom number generator (PRNG), and this happens when not enough entropy is introduced into the PRNG.
Potential sources
Commonly
used entropy sources include the mouse, keyboard, and IDE timings, but
there are other potential sources. For example, one could collect
entropy from the computer's microphone, or by building a sensor to measure the air turbulence inside a disk drive.
For Unix/BSD derivatives there exists a USB based solution that
utilizes an ARM Cortex CPU for filtering / securing the bit stream
generated by two entropy generator sources in the system.
Biotic stress is stress that occurs as a result of damage done to an organism by other living organisms, such as bacteria, viruses, fungi, parasites, beneficial and harmful insects, weeds, and cultivated or native plants. It is different from abiotic stress,
which is the negative impact of non-living factors on the organisms
such as temperature, sunlight, wind, salinity, flooding and drought.
The types of biotic stresses imposed on an organism depend the climate
where it lives as well as the species' ability to resist particular
stresses. Biotic stress remains a broadly defined term and those who
study it face many challenges, such as the greater difficulty in
controlling biotic stresses in an experimental context compared to
abiotic stress.
The damage caused by these various living and nonliving agents can appear very similar. Even with close observation, accurate diagnosis can be difficult. For example, browning of leaves on an oak tree caused by drought stress may appear similar to leaf browning caused by oak wilt, a serious vascular disease caused by a fungus, or the browning caused by anthracnose, a fairly minor leaf disease.
Agriculture
Biotic
stressors are a major focus of agricultural research, due to the vast
economic losses caused to cash crops. The relationship between biotic
stress and plant yield affects economic decisions as well as practical
development. The impact of biotic injury on crop yield impacts population dynamics, plant-stressor coevolution, and ecosystem nutrient cycling.
Biotic stress also impacts horticulturalplant health and natural habitats ecology. It also has dramatic changes in the host recipient. Plants are exposed to many stress factors, such as drought, high salinity or pathogens, which reduce the yield
of the cultivated plants or affect the quality of the harvested
products. Although there are many kinds of biotic stress, the majority
of plant diseases are caused by fungi. Arabidopsis thaliana is often used as a model plant to study the responses of plants to different sources of stress.
In history
Biotic stresses have had huge repercussions for humanity; an example of this is the potato blight, an oomycete which caused widespread famine in England, Ireland and Belgium in the 1840s. Another example is grape phylloxera coming from North America in the 19th century, which led to the Great French Wine Blight.
Today
Losses to pests and disease in crop plants continue to pose a significant threat to agriculture and food security.
During the latter half of the 20th century, agriculture became
increasingly reliant on synthetic chemical pesticides to provide control
of pests and diseases, especially within the intensive farming
systems common in the developed world. However, in the 21st century,
this reliance on chemical control is becoming unsustainable. Pesticides
tend to have a limited lifespan due to the emergence of resistance in
the target pests, and are increasingly recognised in many cases to have negative impacts on biodiversity, and on the health of agricultural workers and even consumers.
Tomorrow
Due to the implications of climate change, it is suspected that plants will have increased susceptibility to pathogens. Additionally, elevated threat of abiotic stresses (i.e. drought and heat) are likely to contribute to plant pathogen susceptibility.
Effect on plant growth
Photosynthesis
Many
biotic stresses affect photosynthesis, as chewing insects reduce leaf
area and virus infections reduce the rate of photosynthesis per leaf
area. Vascular-wilt fungi compromise the water transport and
photosynthesis by inducing stomatal closure.
Response to stress
Plants
have co-evolved with their parasites for several hundred million years.
This co-evolutionary process has resulted in the selection of a wide
range of plant defences against microbial pathogens and herbivorous
pests which act to minimise frequency and impact of attack. These
defences include both physical and chemical adaptations, which may
either be expressed constitutively, or in many cases, are activated only
in response to attack. For example, utilization of high metal ion
concentrations derived from the soil allow plants to reduce the harmful
effects of biotic stressors (pathogens, herbivores etc.); meanwhile
preventing the infliction of severe metal toxicity by way of
safeguarding metal ion distribution throughout the plant with protective
physiological pathways.
Such induced resistance provides a mechanism whereby the costs of
defence are avoided until defense is beneficial to the plant. At the
same time, successful pests and pathogens have evolved mechanisms to
overcome both constitutive and induced resistance in their particular
host species. In order to fully understand and manipulate plant biotic
stress resistance, we require a detailed knowledge of these interactions
at a wide range of scales, from the molecular to the community level.
Inducible defense responses to insect herbivores.
In
order for a plant to defend itself against biotic stress, it must be
able to differentiate between an abiotic and biotic stress. A plants
response to herbivores starts with the recognition of certain chemicals
that are abundant in the saliva of the herbivores. These compounds that
trigger a response in plants are known as elicitors or
herbivore-associated molecular patterns (HAMPs).
These HAMPs trigger signalling pathways throughout the plant,
initiating its defence mechanism and allowing the plant to minimise
damage to other regions.These HAMPs trigger signalling pathways
throughout the plant, initiating its defence mechanism and allowing the
plant to minimise damage to other regions. Phloem feeders, like aphids,
do not cause a great deal of mechanical damage to plants, but they are
still regarded as pests and can seriously harm crop yields. Plants have
developed a defence mechanism using salicylic acid pathway, which is
also used in infection stress, when defending itself against phloem
feeders. Plants perform a more direct attack on an insects digestive
system. The plants do this using proteinase inhibitors. These proteinase
inhibitors prevent protein digestion and once in the digestive system
of an insect, they bind tightly and specifically to the active site of
protein hydrolysing enzymes such as trypsin and chymotrypsin. This mechanism is most likely to have evolved in plants when dealing with insect attack.
Plants detect elicitors in the insects saliva. Once detected, a
signal transduction network is activated. The presence of an elicitor
causes an influx of Ca2+ ions to be released in to the
cytosol. This increase in cytosolic concentration activates target
proteins such as Calmodulin and other binding proteins. Downstream
targets, such as phosphorylation and transcriptional activation of
stimulus specific responses, are turned on by Ca2+ dependent protein kinases.
In Arabidopsis, over expression of the IQD1 calmodulin-binding
transcriptional regulator leads to inhibitor of herbivore activity. The
role of calcium ions in this signal transduction network is therefore
important.
Calcium Ions also play a large role in activating a plants
defensive response. When fatty acid amides are present in insect saliva,
the mitogen-activated protein kinases (MAPKs) are activated. These
genes when activated, play a role in the jasmonic acid pathway.
The jasmonic acid pathway is also referred to as the Octadecanoid
pathway. This pathway is vital for the activation of defence genes in
plants. The production of jasmonic acid, a phytohormone, is a result of
the pathway. In an experiment using virus-induced gene silencing of two
calcium-dependent protein kinases (CDPKs) in a wild tobacco ( Nicotiana attenuata),
it was discovered that the longer herbivory continued the higher the
accumulation of jasmonic acid in wild-type plants and in silenced
plants, the production of more defence metabolites was seen as well as
the decrease in the growth rate of the herbivore used, the tobacco
hornworm (Manduca sexta). This example demonstrates the importance of MAP kinases in plant defence regulation.
Inducible defense responses to pathogens
Plants are capable of detecting invaders through the recognition of non-self signals despite the lack of a circulatory or immune system
like those found in animals. Often a plant's first line of defense
against microbes occurs at the plant cell surface and involves the
detection of microorganism-associated molecular patterns (MAMPs).
MAMPs include nucleic acids common to viruses and endotoxins on
bacterial cell membranes which can be detected by specialized
pattern-recognition receptors.
Another method of detection involves the use of plant immune receptors
to detect effector molecules released into plant cells by pathogens.
Detection of these signals in infected cells leads to an activation of effector-triggered immunity (ETI), a type of innate immune response.
Both the pattern recognition immunity (PTI) and
effector-triggered immunity (ETI) result from the upregulation of
multiple defense mechanisms including defensive chemical signaling
compounds. An increase in the production of salicylic acid (SA) has been shown to be induced by pathogenic infection. The increase in SA results in the production of pathogenesis related (PR) genes which ultimately increase plant resistance to biotrophic and hemibiotrophic pathogens. Increases in jasmonic acid (JA) synthesis near the sites of pathogen infection have also been described. This physiological response to increase JA production has been implicated in the ubiquitination
of jasmonate ZIM domains (JAZ) proteins, which inhibit JA signaling,
leading to their degradation and a subsequent increase in JA activated
defense genes.
Studies regarding the upregulation of defensive chemicals have
confirmed the role of SA and JA in pathogen defense. In studies
utilizing Arabidopsis mutants with the bacterial NahG
gene, which inhibits the production and accumulation of SA, were shown
to be more susceptible to pathogens than the wild-type plants. This was
thought to result from the inability to produce critical defensive
mechanisms including increased PR gene expression. Other studies conducted by injecting tobacco plants and Arabidopsis
with salicylic acid resulted in higher resistance of infection by the
alfalfa and tobacco mosaic viruses, indicating a role for SA
biosynthesis in reducing viral replication. Additionally, studies performed using Arabidopsis
with mutated jasmonic acid biosynthesis pathways have shown JA mutants
to be at an increased risk of infection by soil pathogens.
Along with SA and JA, other defensive chemicals have been implicated in plant viral pathogen defenses including abscisic acid (ABA), gibberellic acid (GA), auxin, and peptide hormones.
The use of hormones and innate immunity presents parallels between
animal and plant defenses, though pattern-triggered immunity is thought
to have arisen independently in each.
Cross tolerance with abiotic stress
Evidence
shows that a plant undergoing multiple stresses, both abiotic and
biotic (usually pathogen or herbivore attack), can produce a positive
effect on plant performance, by reducing their susceptibility to biotic
stress compared to how they respond to individual stresses. The
interaction leads to a crosstalk between their respective hormone
signalling pathways which will either induce or antagonize another
restructuring genes machinery to increase tolerance of defense
reactions.
Reactive oxygen species
(ROS) are key signalling molecules produced in response to biotic and
abiotic stress cross tolerance. ROS are produced in response to biotic
stresses during the oxidative burst.
Dual stress imposed by ozone (O3) and pathogen affects tolerance of
crop and leads to altered host pathogen interaction (Fuhrer, 2003).
Alteration in pathogenesis potential of pest due to O3 exposure is of
ecological and economical importance.
Tolerance to both biotic and abiotic stresses has been achieved. In
maize, breeding programmes have led to plants which are tolerant to
drought and have additional resistance to the parasitic weed Striga hermonthica.
Remote sensing
The Agricultural Research Service
(ARS) and various government agencies and private institutions have
provided a great deal of fundamental information relating spectral
reflectance and thermal emittance properties of soils and crops to their
agronomic and biophysical characteristics. This knowledge has
facilitated the development and use of various remote sensing methods
for non-destructive monitoring of plant growth and development and for
the detection of many environmental stresses that limit plant
productivity. Coupled with rapid advances in computing and position
locating technologies, remote sensing from ground-, air-, and
space-based platforms is now capable of providing detailed spatial and
temporal information on plant response to their local environment that
is needed for site specific agricultural management approaches.
This is very important in today's society because with increasing
pressure on global food productivity due to population increase, result
in a demand for stress-tolerant crop varieties that has never been
greater.
In anthropology, high-context culture and low-context culture is a measure of how explicit the messages exchanged in a culture are, and how important the context
is in communication. High and low context cultures fall on a continuum
that describes how a person communicates with others through their range
of communication abilities: utilizing gestures, relations, body language, verbal messages, or non-verbal messages. These concepts were first introduced by the anthropologistEdward T. Hall in his 1976 book Beyond Culture.
According to Hall, in a low-context culture, the message will be
interpreted through just the words (whether written or spoken) and their
explicit meaning. In a high-context culture, messages are also
interpreted using tone of voice, gesture, silence or implied meaning, as
well as context or situation. There, the receiver is expected to use the situation, messages and cultural norms to understand the message.
High-context cultures often stem from less direct verbal and
nonverbal communication, utilizing small communication gestures and
reading into these less direct messages with more meaning.
Low-context cultures are the opposite; direct verbal communication is
needed to properly understand a message being said and doing so relies
heavily on explicit verbal skills.
"High" and "low" context cultures typically refer to language groups, nationalities, or regional communities. However, they have also been applied to corporations, professions and other cultural groups, as well as settings such as online and offline communication.
Examples of higher and lower context cultures
Cultural
contexts are not absolutely "high" or "low". Instead, a comparison
between cultures may find communication differences to a greater or
lesser degree. Typically a high-context culture will be relational, collectivist,
intuitive, and contemplative. They place a high value on interpersonal
relationships and group members are a very close-knit community.
Typically a low-context culture will be less close-knit, and so
individuals communicating will have fewer relational cues when
interpreting messages. Therefore, it is necessary for more explicit
information to be included in the message so it is not misinterpreted. Not all individuals in a culture can be defined by cultural stereotypes,
and there will be variations within a national culture in different
settings. For example, Hall describes how Japanese culture has both low-
and high-context situations.
However, understanding the broad tendencies of predominant cultures can
help inform and educate individuals on how to better facilitate
communication between individuals of differing cultural backgrounds.
Although the concept of high- and low-context cultures is usually
applied in the field of analyzing national cultures, it can also be
used to describe scientific or corporate cultures, or specific settings
such as airports or law courts. A simplified example mentioned by Hall
is that scientists working in "hard science" fields (like chemistry and physics) tend to have lower-context cultures: because their knowledge and models have fewer variables, they will typically include less context for each event they describe. In contrast, scientists working with living systems need to include more context because there can be significant variables which impact the research outcomes.
Croucher’s study examines the assertion that culture influences
communication style (high/low context) preference. Data was gathered in
India, Ireland, Thailand, and the United States where the results
confirm that "high-context nations (India and Thailand) prefer the
avoiding and obliging conflict styles more than low-context nations
(Ireland and the United States), whereas low-context nations prefer the
uncompromising and dominating communication style more than high-context
nations."
In addition, Hall identified countries such as Japan, Arabic
countries and some Latin American Countries to practice high-context
culture; “High context communication carries most of its information
within physical acts and features such as avoiding eye contact or even
the shrug of a shoulder.”
On the other hand, he identified countries such as Germany, the United
States and Scandinavia as low context cultures. These countries are
quite explicit and elaborate without having prior knowledge to each
member’s history or background.
Cultures and languages are defined as higher or lower context on a spectrum. For example, it could be argued[by whom?] that the Canadian French language is higher context than Canadian English, but lower context than Spanish or French French. An individual from Texas (a higher-context culture) may communicate with a few words or use of a prolonged silence characteristic of Texan English, where a New Yorker would be very explicit (as typical of New York City English), although both speak the same language (American English) and are part of a nation (the United States of America) which is lower-context relative to other nations. Hall notes a similar difference between Navajo-speakers and English-speakers in a United States school.
Hall and Hall proposed a "spectrum" of national cultures from "High-Context cultures" to "Low-Context Cultures. This has been expanded to further countries by Copeland & Griggs (1985).
Cultural context can also shift and evolve. For instance, a study has argued that both Japan and Finland (high-context cultures) are becoming lower-context with the increased influence of Western European and United States culture.
The overlap and contrast between context cultures
The
categories of context cultures are not totally separate. Both often
take many aspects of the other's cultural communication abilities and
strengths into account.
The terms high- and low-context cultures are not classified with strict
individual characteristics or boundaries. Instead, many cultures tend
to have a mixture or at least some concepts that are shared between
them, overlapping the two context cultures.
Ramos suggests that "in low context culture, communication
members’ communication must be more explicit. As such, what is said is
what is meant, and further analysis of the message is usually
unnecessary." This
implies that communication is quite direct and detailed because members
of the culture are not expected to have knowledge of each other's
histories, past experience or background. Because low-context
communication concerns more direct messages, the meaning of these
messages is more dependent on the words being spoken rather than on the
interpretation of more subtle or unspoken cues.
The Encyclopedia of Diversity and Social Justice states that,
"high context defines cultures that are relational and collectivist, and
which most highlight interpersonal relationships. Cultures and
communication in which context is of great importance to structuring
actions is referred to as high context."In
such cultures, people are highly perceptive of actions. Furthermore,
cultural aspects such as tradition, ceremony, and history are also
highly valued. Because of this, many features of cultural behavior in
high-context cultures, such as individual roles and expectations, do not
need much detailed or thought-out explanation.
According to Watson, "the influence of cultural variables
interplays with other key factors – for example, social identities,
those of age, gender, social class and ethnicity; this may include a
stronger or weaker influence."
A similarity that the two communication styles share is its influence
on social characteristics such as age, gender, social class and
ethnicity. For example, for someone who is older and more experienced
within a society, the need for social cues may be higher or lower
depending on the communication style. The same applies for the other
characteristics in varied countries.
On the other hand, certain intercultural communication
skills are unique for each culture and it is significant to note that
these overlaps in communication techniques are represented subgroups
within social interactions or family settings. Many singular cultures that are large have subcultures inside of them, making communication and defining them more complicated than the low context and high context culture scale. The diversity
within a main culture shows how the high and low scale differs
depending on social settings such as school, work, home, and in other
countries; variation is what allows the scale to fluctuate even if a
large culture is categorized as primarily one or the other.
Miscommunication within culture contexts
Between each type of culture context, there will be forms of miscommunication because of the difference in gestures, social cues,
and intercultural adjustments; however, it is important to recognize
these differences and learn how to avoid miscommunication to benefit
certain situations.
Since all sets of cultures differ, especially from a global standpoint
where language also creates a barrier for communication, social
interactions specific to a culture normally require a range of
appropriate communication abilities that an opposing culture may not
understand or know about.
This significance follows into many situations such as the workplace,
which can be prone to diversified cultures and opportunities for
collaboration and working together.
Awareness of miscommunication between high and low context cultures
within the workplace or intercultural communication settings advocates
for collected unification within a group through the flexibility and
ability to understand one another.
How higher context relates to other cultural metrics
Diversity
Families, subcultures and in-groups typically favour higher-context communication.
Groups that are able to rely on a common background may not need to use
words as explicitly to understand each other. Settings and cultures
where people come together from a wider diversity of backgrounds such as
international airports, large cities, or multi-national firms, tend to use lower-context communication forms.
Language
Hall links language to culture through the work of Sapir-Whorf on linguistic relativity. A trade language will typically need to explicitly explain more of the context than a dialect which can assume a high level of shared context. Because a low-context setting cannot rely on shared understanding of potentially ambiguous
messages, low-context cultures tend to give more information, or to be
precise in their language. In contrast, a high-context language like
Japanese or Chinese can use a high number of homophones but still be understood by a listener who knows the context.
Elaborated and restricted codes
The concept of elaborated and restricted codes is introduced by sociologist Basil Bernstein in his book Class, Codes and Control.
An elaborated code indicates that the speaker is expressing his/her
idea by phrasing from an abundant selection of alternatives without
assuming the listener shares significant amounts of common knowledge,
which allows the speaker to explain their idea explicitly. In contrast,
restricted codes are phrased from more limited alternatives, usually
with collapsed and shortened sentences. Therefore, restricted codes
require listeners to share a great deal of common perspective to
understand the implicit meanings and nuances of a conversation.
Restricted codes are commonly used in high-context culture
groups, where group members share the same cultural background and can
easily understand the implicit meanings "between the lines" without further elaboration.
Conversely, in cultural groups with low context, where people share
less common knowledge or ‘value individuality above group
identification’, detailed elaboration becomes more essential to avoid
misunderstanding.
Collectivism and individualism
The concepts of collectivism and individualism have been applied to high- and low-context cultures by Dutch psychologist Geert Hofstede in his Cultural Dimensions Theory.
Collectivist societies prioritize the group over the individual, and
vice versa for individualist ones. In high-context cultures, language
may be used to assist and maintain relationship-building and to focus on
process. India and Japan are typically high-context, highly
collectivistic cultures, where business is done by building
relationships and maintaining respectful communication.
Individualistic cultures promote the development of individual
values and independent social groups. Individualism may lead to
communicating to all people in a group in the same way, rather than
offering hierarchical respect to certain members. Because individualistic cultures may value cultural diversity,
a more explicit way of communicating is often required to avoid
misunderstanding. Language may be used to achieve goals or exchange
information. The USA and Australia are typically low-context, highly individualistic cultures, where transparency and competition in business are prized.
Stability and durability of tradition
High-context
cultures tend to be more stable, as their communication is more
economical, fast, efficient and satisfying; but these are gained at a
price of devoting time into preprogramming cultural background, and their high stability might come with a price of a high barrier for development. By contrast, low-context cultures tend to change more rapidly and drastically, allowing extension to happen at faster rates. This also means that low-context communication may fail due to the overload of information, which makes culture lose its screening function.
Therefore, higher-context cultures tend to correlate with cultures that also have a strong sense of tradition and history, and change little over time. For example, Native Americans in the United States have higher-context cultures with a strong sense of tradition and history, compared to general American culture.
Focusing on tradition creates opportunities for higher context
messages between individuals of each new generation, and the
high-context culture feeds back to the stability hence allows the
tradition to be maintained. This is in contrast to lower-context
cultures in which the shared experiences upon which communication is
built can change drastically from one generation to the next, creating
communication gaps between parents and children, as in the United States.
Facial expression and gesture
Culture also affects how individuals interpret other people's facial expressions. An experiment performed by the University of Glasgow
shows that different cultures have different understanding of the
facial expression signals of the six basic emotions, which are the
so-called "universal language of emotion"—happiness, surprise, fear, disgust, anger and sadness.
In high-context cultures, facial expressions and gestures take on
greater importance in conveying and understanding a message, and the
receiver may require more cultural context to understand "basic"
displays of emotions.
Marketing and advertising perspective
Cultural differences in advertising and marketing may also be explained through high- and low-context cultures. One study on McDonald's online advertising compared Japan, China, Korea, Hong Kong,
Pakistan, Germany, Denmark, Sweden, Norway, Finland, and the United
States, and found that in high-context countries, the advertising used
more colors, movements, and sounds to give context, while in low-context
cultures the advertising focused more on verbal information and linear
processes.
Three representations of hexabenzocoronene,
a polycylic aromatic hydrocarbon. Top: standard line-angle schematic,
where carbon atoms are represented by the vertices of the hexagons and
hydrogen atoms are inferred. Middle: ball-and-stick model showing all carbon and hydrogen atoms. Bottom: atomic force microscopy image.
Polycyclic aromatic hydrocarbons (PAHs, also polyaromatic hydrocarbons or polynuclear aromatic hydrocarbons) are hydrocarbons—organic compounds containing only carbon and hydrogen—that are composed of multiple aromatic rings (organic rings in which the electrons are delocalized). The simplest such chemicals are naphthalene, having two aromatic rings, and the three-ring compounds anthracene and phenanthrene.
PAHs are uncharged, non-polar molecules found in coal and in tar deposits. They are also produced by the thermal decomposition of organic matter (for example, in engines and incinerators or when biomass burns in forest fires).
PAHs are abundant in the universe, and have recently been found to have formed possibly as early as the first couple of billion years after the Big Bang, in association with formation of new stars and exoplanets. Some studies suggest that PAHs account for a significant percentage of all carbon in the universe.
By definition, polycyclic aromatic hydrocarbons have multiple cycles, precluding benzene from being considered a PAH. Naphthalene is considered the simplest polycyclic aromatic hydrocarbon by the US EPA and US CDC for policy contexts. Other authors consider PAHs to start with the tricyclic species phenanthrene and anthracene.
PAHs with five or six-membered rings are most common. Those composed only of six-membered rings are called alternant PAHs, which include benzenoid PAHs. The following are examples of PAHs that vary in the number and arrangement of their rings:
PAHs are nonpolar and lipophilic. Larger PAHs are generally insoluble in water, although some smaller PAHs are soluble and known contaminants in drinking water. The larger members are also poorly soluble in organic solvents and in lipids. They are usually colorless.
The aromaticity varies for PAHs. According to Clar's rule, the resonance structure of a PAH that has the largest number of disjoint aromatic pi sextets—i.e. benzene-like moieties—is the most important for the characterization of the properties of that PAH.
Benzene-substructure resonance analysis for Clar's rule
Phenanthrene
Anthracene
Chrysene
For example, in phenanthrene
one Clar structure has two sextets—the first and third rings—while the
other resonance structure has just one central sextet; therefore in this
molecule the outer rings have greater aromatic character whereas the
central ring is less aromatic and therefore more reactive. In contrast, in anthracene
the resonance structures have one sextet each, which can be at any of
the three rings, and the aromaticity spreads out more evenly across the
whole molecule. This difference in number of sextets is reflected in the differing ultraviolet–visible spectra of these two isomers, as higher Clar pi-sextets are associated with larger HOMO-LUMO gaps; the highest-wavelength absorbance of phenanthrene is at 293 nm, while anthracene is at 374 nm. Three Clar structures with two sextets each are present in the four-ring chrysene
structure: one having sextets in the first and third rings, one in the
second and fourth rings, and one in the first and fourth rings.[citation needed]
Superposition of these structures reveals that the aromaticity in the
outer rings is greater (each has a sextet in two of the three Clar
structures) compared to the inner rings (each has a sextet in only one
of the three).
Polycyclic aromatic compounds characteristically reduce to the radicalanions. The redox potential correlates with the size of the PAH.
Half-cell potential of aromatic compounds against the SCE
Polycyclic aromatic hydrocarbons are primarily found in natural sources such as creosote. They can result from the incomplete combustion of organic matter. PAHs can also be produced geologically when organic sediments are chemically transformed into fossil fuels such as oil and coal. PAHs are considered ubiquitous in the environment and can be formed from either natural or manmade combustion sources. The dominant sources of PAHs in the environment are thus from human activity: wood-burning and combustion of other biofuels
such as dung or crop residues contribute more than half of annual
global PAH emissions, particularly due to biofuel use in India and
China.
As of 2004, industrial processes and the extraction and use of fossil
fuels made up slightly more than one quarter of global PAH emissions,
dominating outputs in industrial countries such as the United States. Wildfires are another notable source.
Substantially higher outdoor air, soil, and water concentrations of
PAHs have been measured in Asia, Africa, and Latin America than in
Europe, Australia, the U.S., and Canada.
PAHs are typically found as complex mixtures. Lower-temperature combustion, such as tobacco smoking or wood-burning,
tends to generate low molecular weight PAHs, whereas high-temperature
industrial processes typically generate PAHs with higher molecular
weights.
In the aqueous environment
Most PAHs are insoluble in water, which limits their mobility in the environment, although PAHs sorb to fine-grained organic-rich sediments. Aqueous solubility of PAHs decreases approximately logarithmically as molecular mass increases.
Two-ringed PAHs, and to a lesser extent three-ringed PAHs, dissolve in
water, making them more available for biological uptake and degradation. Further, two- to four-ringed PAHs volatilize
sufficiently to appear in the atmosphere predominantly in gaseous form,
although the physical state of four-ring PAHs can depend on
temperature.
In contrast, compounds with five or more rings have low solubility in
water and low volatility; they are therefore predominantly in solid state, bound to particulateair pollution, soils, or sediments.
In solid state, these compounds are less accessible for biological
uptake or degradation, increasing their persistence in the environment.
In galaxies
Spiral galaxy NGC 5529 has been found to have amounts of PAHs.
Human exposure
Human
exposure varies across the globe and depends on factors such as smoking
rates, fuel types in cooking, and pollution controls on power plants,
industrial processes, and vehicles.
Developed countries with stricter air and water pollution controls,
cleaner sources of cooking (i.e., gas and electricity vs. coal or
biofuels), and prohibitions of public smoking tend to have lower levels
of PAH exposure, while developing and undeveloped countries tend to have
higher levels.
Surgical smoke plume have been proven to contain PAHs in several
independent research studies. ref. Int J Occup Med Environ Health. 2014
Apr;27(2):314-25. doi: 10.2478/s13382-014-0250-3. Epub 2014 Apr 9. https://www.ncbi.nlm.nih.gov/pubmed/24715421
For indoor contaminants, surgical plume needs to be noticed as a
serious potential health risk for the 59 million health care workers
around the world.
A wood-burning open-air cooking stove. Smoke from solid fuels like wood is a large source of PAHs globally.
Burning solid fuels such as coal and biofuels
in the home for cooking and heating is a dominant global source of PAH
emissions that in developing countries leads to high levels of exposure
to indoor particulate air pollution containing PAHs, particularly for women and children who spend more time in the home or cooking.
In industrial countries, people who smoke tobacco products, or who are exposed to second-hand smoke, are among the most highly exposed groups; tobacco smoke contributes to 90% of indoor PAH levels in the homes of smokers.
For the general population in developed countries, the diet is
otherwise the dominant source of PAH exposure, particularly from smoking
or grilling meat or consuming PAHs deposited on plant foods, especially
broad-leafed vegetables, during growth. PAHs are typically at low concentrations in drinking water.
Smog in Cairo. Particulate air pollution, including smog, is a substantial cause of human exposure to PAHs.
Emissions from vehicles such as cars and trucks can be a substantial outdoor source of PAHs in particulate air pollution. Geographically, major roadways are thus sources of PAHs, which may distribute in the atmosphere or deposit nearby. Catalytic converters are estimated to reduce PAH emissions from gasoline-fired vehicles by 25-fold.
PAHs typically disperse from urban and suburbannon-point sources through road run-off, sewage, and atmospheric circulation and subsequent deposition of particulate air pollution. Soil and river sediment near industrial sites such as creosote manufacturing facilities can be highly contaminated with PAHs. Oil spills, creosote, coal mining dust, and other fossil fuel sources can also distribute PAHs in the environment.
Two- and three-ringed PAHs can disperse widely while dissolved in
water or as gases in the atmosphere, while PAHs with higher molecular
weights can disperse locally or regionally adhered to particulate matter
that is suspended in air or water until the particles land or settle
out of the water column. PAHs have a strong affinity for organic carbon, and thus highly organic sediments in rivers, lakes, and the ocean can be a substantial sink for PAHs.
Algae and some invertebrates such as protozoans, mollusks, and many polychaetes have limited ability to metabolize PAHs and bioaccumulate
disproportionate concentrations of PAHs in their tissues; however, PAH
metabolism can vary substantially across invertebrate species. Most vertebrates metabolize and excrete PAHs relatively rapidly. Tissue concentrations of PAHs do not increase (biomagnify) from the lowest to highest levels of food chains.
PAHs transform slowly to a wide range of degradation products. Biological degradation by microbes is a dominant form of PAH transformation in the environment. Soil-consuming invertebrates such as earthworms speed PAH degradation, either through direct metabolism or by improving the conditions for microbial transformations.
Abiotic degradation in the atmosphere and the top layers of surface
waters can produce nitrogenated, halogenated, hydroxylated, and
oxygenated PAHs; some of these compounds can be more toxic,
water-soluble, and mobile than their parent PAHs.
PAHs in urban soils
The British Geological Survey reported the amount and distribution of PAH compounds including parent and alkylated forms in urban soils at 76 locations in Greater London.
The study showed that parent (16 PAH) content ranged from 4 to 67 mg/kg
(dry soil weight) and an average PAH concentration of 18 mg/kg (dry
soil weight) whereas the total PAH content (33 PAH) ranged from 6 to
88 mg/kg and fluoranthene and pyrene were generally the most abundant PAHs. Benzo[a]pyrene (BaP), the most toxic of the parent PAHs, is widely considered a key marker PAH for environmental assessments; the normal background concentration of BaP in the London urban sites was 6.9 mg/kg (dry soil weight). London
soils contained more stable four- to six-ringed PAHs which were
indicative of combustion and pyrolytic sources, such as coal and oil
burning and traffic-sourced particulates. However, the overall
distribution also suggested that the PAHs in London soils had undergone
weathering and been modified by a variety of pre-and post-depositional
processes such as volatilization and microbial biodegradation.
PAHs in peatlands
Managed burning of moorland vegetation in the UK has been shown to generate PAHs which become incorporated into the peat surface. Burning of moorland vegetation such as heather
initially generates high amounts of two- and three-ringed PAHs relative
to four- to six-ringed PAHs in surface sediments, however, this pattern
is reversed as the lower molecular weight PAHs are attenuated by biotic decay and photodegradation.
Evaluation of the PAH distributions using statistical methods such as
principal component analyses (PCA) enabled the study to link the source
(burnt moorland) to pathway (suspended stream sediment) to the
depositional sink (reservoir bed).
PAHs in rivers, estuarine and coastal sediments
Concentrations of PAHs in river and estuarine sediments
vary according to a variety of factors including proximity to municipal
and industrial discharge points, wind direction and distance from major
urban roadways, as well as tidal regime which controls the diluting
effect of generally cleaner marine sediments relative to freshwater
discharge. Consequently, the concentrations of pollutants in estuaries tends to decrease at the river mouth. Understanding of sediment hosted PAHs in estuaries is important for the protection of commercial fisheries (such as mussels) and general environmental habitat conservation because PAHs can impact the health of suspension and sediment feeding organism.
River-estuary surface sediments in the UK tend to have a lower PAH
content than sediments buried 10–60 cm from the surface reflecting lower
present day industrial activity combined with improvement in
environmental legislation of PAH. Typical PAH concentrations in UK estuaries range from about 19 to 16,163 µg/kg (dry sediment weight) in the River Clyde and 626 to 3,766 µg/kg in the River Mersey. In general estuarine sediments with a higher natural total organic carbon content (TOC) tend to accumulate PAHs due to high sorption capacity of organic matter. A similar correspondence between PAHs and TOC has also been observed in the sediments of tropical mangroves located on the coast of southern China.
Minor sources
Volcanic eruptions may emit PAHs. Certain PAHs such as perylene can also be generated in anaerobic
sediments from existing organic material, although it remains
undetermined whether abiotic or microbial processes drive their
production.
Human health
Cancer is a primary human health risk of exposure to PAHs. Exposure to PAHs has also been linked with cardiovascular disease and poor fetal development.
Cancer
PAHs have been linked to skin, lung, bladder, liver, and stomach cancers in well-established animal model studies. Specific compounds classified by various agencies as possible or probable human carcinogens are identified in the section "Regulation and Oversight" below.
Historically, PAHs contributed substantially to our understanding of adverse health effects from exposures to environmental contaminants, including chemical carcinogenesis. In 1775, Percivall Pott, a surgeon at St. Bartholomew's Hospital in London, observed that scrotal cancer was unusually common in chimney sweepers and proposed the cause as occupational exposure to soot. A century later, Richard von Volkmann reported increased skin cancers in workers of the coal tar
industry of Germany, and by the early 1900s increased rates of cancer
from exposure to soot and coal tar was widely accepted. In 1915,
Yamigawa and Ichicawa were the first to experimentally produce cancers,
specifically of the skin, by topically applying coal tar to rabbit ears.
In 1922, Ernest Kennaway
determined that the carcinogenic component of coal tar mixtures was an
organic compound consisting of only carbon and hydrogen. This component
was later linked to a characteristic fluorescent pattern that was similar but not identical to benz[a]anthracene, a PAH that was subsequently demonstrated to cause tumors. Cook, Hewett and Hieger then linked the specific spectroscopic fluorescent profile of benzo[a]pyrene to that of the carcinogenic component of coal tar, the first time that a specific compound from an environmental mixture (coal tar) was demonstrated to be carcinogenic.
In the 1930s and later, epidemiologists from Japan, the UK, and the US, including Richard Doll and various others, reported greater rates of death from lung cancer following occupational exposure to PAH-rich environments among workers in coke ovens and coal carbonization and gasification processes.
The structure of a PAH influences whether and how the individual compound is carcinogenic. Some carcinogenic PAHs are genotoxic and induce mutations that initiate cancer; others are not genotoxic and instead affect cancer promotion or progression.
PAHs that affect cancer initiation are typically first chemically modified by enzymes into metabolites that react with DNA, leading to mutations. When the DNA sequence is altered in genes that regulate cell replication, cancer can result. Mutagenic PAHs, such as benzo[a]pyrene,
usually have four or more aromatic rings as well as a "bay region", a
structural pocket that increases reactivity of the molecule to the
metabolizing enzymes. Mutagenic metabolites of PAHs include diol epoxides, quinones, and radical PAH cations. These metabolites can bind to DNA at specific sites, forming bulky complexes called DNA adducts that can be stable or unstable. Stable adducts may lead to DNA replication errors, while unstable adducts react with the DNA strand, removing a purine base (either adenine or guanine). Such mutations, if they are not repaired, can transform genes encoding for normal cell signaling proteins into cancer-causing oncogenes. Quinones can also repeatedly generate reactive oxygen species that may independently damage DNA.
Enzymes in the cytochrome family (CYP1A1, CYP1A2, CYP1B1) metabolize PAHs to diol epoxides.
PAH exposure can increase production of the cytochrome enzymes,
allowing the enzymes to convert PAHs into mutagenic diol epoxides at
greater rates. In this pathway, PAH molecules bind to the aryl hydrocarbon receptor (AhR) and activate it as a transcription factor
that increases production of the cytochrome enzymes. The activity of
these enzymes may at times conversely protect against PAH toxicity,
which is not yet well understood.
Low molecular weight PAHs, with two to four aromatic hydrocarbon rings, are more potent as co-carcinogens
during the promotional stage of cancer. In this stage, an initiated
cell (a cell that has retained a carcinogenic mutation in a key gene
related to cell replication) is removed from growth-suppressing signals
from its neighboring cells and begins to clonally replicate. Low-molecular-weight PAHs that have bay or bay-like regions can dysregulate gap junction channels, interfering with intercellular communication, and also affect mitogen-activated protein kinases that activate transcription factors involved in cell proliferation.
Closure of gap junction protein channels is a normal precursor to cell
division. Excessive closure of these channels after exposure to PAHs
results in removing a cell from the normal growth-regulating signals
imposed by its local community of cells, thus allowing initiated
cancerous cells to replicate. These PAHs do not need to be enzymatically
metabolized first. Low molecular weight PAHs are prevalent in the
environment, thus posing a significant risk to human health at the
promotional phases of cancer.
In laboratory experiments, animals exposed to certain PAHs have shown increased development of plaques (atherogenesis) within arteries. Potential mechanisms for the pathogenesis
and development of atherosclerotic plaques may be similar to the
mechanisms involved in the carcinogenic and mutagenic properties of
PAHs. A leading hypothesis is that PAHs may activate the cytochrome enzyme CYP1B1 in vascular smooth muscle
cells. This enzyme then metabolically processes the PAHs to quinone
metabolites that bind to DNA in reactive adducts that remove purine
bases. The resulting mutations may contribute to unregulated growth of
vascular smooth muscle cells or to their migration to the inside of the
artery, which are steps in plaque formation. These quinone metabolites also generate reactive oxygen species that may alter the activity of genes that affect plaque formation.
Oxidative stress following PAH exposure could also result in cardiovascular disease by causing inflammation, which has been recognized as an important factor in the development of atherosclerosis and cardiovascular disease. Biomarkers
of exposure to PAHs in humans have been associated with inflammatory
biomarkers that are recognized as important predictors of cardiovascular
disease, suggesting that oxidative stress resulting from exposure to
PAHs may be a mechanism of cardiovascular disease in humans.
Developmental impacts
Multiple epidemiological studies of people living in Europe, the United States, and China have linked in utero
exposure to PAHs, through air pollution or parental occupational
exposure, with poor fetal growth, reduced immune function, and poorer neurological development, including lower IQ.
Regulation and oversight
Some governmental bodies, including the European Union as well as NIOSH and the Environmental Protection Agency (EPA) in the US, regulate concentrations of PAHs in air, water, and soil. The European Commission has restricted concentrations of 8 carcinogenic PAHs in consumer products that contact the skin or mouth.
PAHs possess very characteristic UV absorbance spectra. These often possess many absorbance bands and are unique for each ring structure. Thus, for a set of isomers,
each isomer has a different UV absorbance spectrum than the others.
This is particularly useful in the identification of PAHs. Most PAHs are
also fluorescent,
emitting characteristic wavelengths of light when they are excited
(when the molecules absorb light). The extended pi-electron electronic
structures of PAHs lead to these spectra, as well as to certain large
PAHs also exhibiting semi-conducting and other behaviors.
PAHs may be abundant in the universe. They seem to have been formed as early as a couple of billion years after the Big Bang, and are associated with new stars and exoplanets. More than 20% of the carbon in the universe may be associated with PAHs. PAHs are considered possible starting material for the earliest forms of life.
Light emitted by the Red Rectangle nebula and found spectral signatures that suggest the presence of anthracene and pyrene.
This report was considered a controversial hypothesis that as nebulae
of the same type as the Red Rectangle approach the ends of their lives,
convection currents cause carbon and hydrogen in the nebulae's cores to
get caught in stellar winds, and radiate outward. As they cool, the
atoms supposedly bond to each other in various ways and eventually form
particles of a million or more atoms. Adolf Witt and his team inferred that PAHs—which may have been vital in the formation of early life on Earth—can only originate in nebulae.
Two extremely bright stars illuminate a mist of PAHs in this Spitzer image.
More recently, fullerenes (or "buckyballs"), have been detected in other nebulae. Fullerenes are also implicated in the origin of life;
according to astronomer Letizia Stanghellini, "It's possible that
buckyballs from outer space provided seeds for life on Earth." In September 2012, NASA scientists reported results of analog studies in vitro that PAHs, subjected to interstellar medium (ISM) conditions, are transformed, through hydrogenation, oxygenation, and hydroxylation, to more complex organics—"a step along the path toward amino acids and nucleotides, the raw materials of proteins and DNA, respectively". Further, as a result of these transformations, the PAHs lose their spectroscopic signature which could be one of the reasons "for the lack of PAH detection in interstellar icegrains, particularly the outer regions of cold, dense clouds or the upper molecular layers of protoplanetary disks."
NASA's Spitzer Space Telescope includes instruments for obtaining both images and spectra of light emitted by PAHs associated with star formation. These images can trace the surface of star-forming clouds in our own galaxy or identify star forming galaxies in the distant universe.
In October 2018, researchers reported low-temperature chemical pathways from simple organic compounds to complex PAHs. Such chemical pathways may help explain the presence of PAHs in the low-temperature atmosphere of Saturn's moon Titan, and may be significant pathways, in terms of the PAH world hypothesis, in producing precursors to biochemicals related to life as we know it.
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