High-context and low-context cultures

From Wikipedia, the free encyclopedia

 

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 anthropologist Edward 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).

Higher-context culture: Afghans, African, Arabic, Brazilians, the Chinese, Filipinos, French Canadians, the French, Greeks, Hawaiian, Hungarians, Indians, Indonesian, Italians, Irish, Japanese, Koreans, Latin Americans, Nepali, Pakistani, Persian, Portuguese, Russians, Southern United States, the Spanish, Thai, Turks, Vietnamese, South Slavic, West Slavic.

Lower-context culture: Australian, Dutch, English Canadians, the English, Finnish, Germans, Israelis, New Zealand, Scandinavia, Switzerland, United States.

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.

Polycyclic aromatic hydrocarbon

From Wikipedia, the free encyclopedia

 

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.

Polycyclic aromatic hydrocarbons are discussed as possible starting materials for abiotic syntheses of materials required by the earliest forms of life.

Nomenclature, structure, properties

Nomenclature and structure

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 are not generally considered to contain heteroatoms or carry substituents.

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:

Principal PAH Compounds

• 

  Anthracene
  
• 

  Phenanthrene
  
• 

  Phenalene
  
• 

  Tetracene
  
• 

  Chrysene
  
• 

  Triphenylene
  
• 

  Pyrene
  
• 

  Pentacene
  
• 

  Benzo[a]pyrene
  
• 

  Corannulene
  
• 

  Benzo[ghi]perylene
  
• 

  Coronene
  
• 

  Ovalene
  
• 

  Benzo[c]fluorene
  

Physicochemical properties and bonding

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 radical anions. The redox potential correlates with the size of the PAH.

Half-cell potential of aromatic compounds against the SCE

Compound	Potential (V)
benzene	−3.42
biphenyl[15]	−2.60
naphthalene	−2.51
anthracene	−1.96
phenanthrene	−1.96
perylene	−1.67
pentacene	−1.35

Sources and distribution

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 particulate air 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.

People can also be occupationally exposed during work that involves fossil fuels or their derivatives, wood-burning, carbon electrodes, or exposure to diesel exhaust. Industrial activity that can produce and distribute PAHs includes aluminum, iron, and steel manufacturing; coal gasification, tar distillation, shale oil extraction; production of coke, creosote, carbon black, and calcium carbide; road paving and asphalt manufacturing; rubber tire production; manufacturing or use of metal working fluids; and activity of coal or natural gas power stations.

Environmental distribution and degradation

Crude oil on a beach after a 2007 oil spill in Korea.

 

PAHs typically disperse from urban and suburban non-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.

Historical significance

An 18th-century drawing of chimney sweeps.

 

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.

Mechanisms of carcinogenesis

An adduct formed between a DNA strand and an epoxide derived from a benzo[a]pyrene molecule (center); such adducts may interfere with normal DNA replication.

 

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.

Cardiovascular disease

Adult exposure to PAHs has been linked to cardiovascular disease. PAHs are among the complex suite of contaminants in tobacco smoke and particulate air pollution and may contribute to cardiovascular disease resulting from such exposures.

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.

Priority polycyclic aromatic hydrocarbons identified by the US EPA, the US Agency for Toxic Substances and Disease Registry (ATSDR), and the European Food Safety Authority (EFSA) due to their carcinogenicity or genotoxicity and/or ability to be monitored are the following:

Compound Agency EPA MCL in water [mg L-3] acenaphthene EPA, ATSDR acenaphthylene EPA, ATSDR anthracene EPA, ATSDR benz[a]anthracene EPA, ATSDR, EFSA 0.0001 benzo[b]fluoranthene EPA, ATSDR, EFSA 0.0002 benzo[j]fluoranthene ATSDR, EFSA benzo[k]fluoranthene EPA, ATSDR, EFSA 0.0002 benzo[c]fluorene EFSA benzo[g,h,i]perylene EPA, ATSDR, EFSA benzo[a]pyrene EPA, ATSDR, EFSA 0.0002 benzo[e]pyrene ATSDR chrysene EPA, ATSDR, EFSA 0.0002 coronene ATSDR

Compound Agency EPA MCL in water [mg L-3] cyclopenta[c,d]pyrene EFSA dibenz[a,h]anthracene EPA, ATSDR, EFSA 0.0003 dibenzo[a,e]pyrene EFSA dibenzo[a,h]pyrene EFSA dibenzo[a,i]pyrene EFSA dibenzo[a,l]pyrene EFSA fluoranthene EPA, ATSDR fluorene EPA, ATSDR indeno[1,2,3-c,d]pyrene EPA, ATSDR, EFSA 0.0004 5-methylchrysene EFSA naphthalene EPA phenanthrene EPA, ATSDR pyrene EPA, ATSDR

Detection and optical properties

A spectral database exists for tracking polycyclic aromatic hydrocarbons (PAHs) in the universe. Detection of PAHs in materials is often done using gas chromatography-mass spectrometry or liquid chromatography with ultraviolet-visible or fluorescence spectroscopic methods or by using rapid test PAH indicator strips. 

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.

Origins of life

The Cat's Paw Nebula lies inside the Milky Way Galaxy and is located in the constellation Scorpius. Green areas show regions where radiation from hot stars collided with large molecules and small dust grains called "polycyclic aromatic hydrocarbons" (PAHs), causing them to fluoresce. (Spitzer space telescope, 2018)

 

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 ice grains, 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 June 2013, PAHs were detected in the upper atmosphere of Titan, the largest moon of the planet Saturn.

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.