Cytokine

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Cytokine

3D medical animation still showing secretion of cytokines

Cytokines are a broad and loose category of small proteins (~5–25 kDa) important in cell signaling. Due to their size, cytokines cannot cross the lipid bilayer of cells to enter the cytoplasm and therefore typically exert their functions by interacting with specific cytokine receptors on the target cell surface. Cytokines have been shown to be involved in autocrine, paracrine and endocrine signaling as immunomodulating agents.

Cytokines include chemokines, interferons, interleukins, lymphokines, and tumour necrosis factors, but generally not hormones or growth factors (despite some overlap in the terminology). Cytokines are produced by a broad range of cells, including immune cells like macrophages, B lymphocytes, T lymphocytes and mast cells, as well as endothelial cells, fibroblasts, and various stromal cells; a given cytokine may be produced by more than one type of cell. They act through cell surface receptors and are especially important in the immune system; cytokines modulate the balance between humoral and cell-based immune responses, and they regulate the maturation, growth, and responsiveness of particular cell populations. Some cytokines enhance or inhibit the action of other cytokines in complex ways. They are different from hormones, which are also important cell signaling molecules. Hormones circulate in higher concentrations, and tend to be made by specific kinds of cells. Cytokines are important in health and disease, specifically in host immune responses to infection, inflammation, trauma, sepsis, cancer, and reproduction.

The word comes from the ancient Greek language: cyto, from Greek κύτος, kytos, 'cavity, cell' + kines, from Greek κίνησις, kinēsis, 'movement'.

Discovery

Interferon-alpha, an interferon type I, was identified in 1957 as a protein that interfered with viral replication. The activity of interferon-gamma (the sole member of the interferon type II class) was described in 1965; this was the first identified lymphocyte-derived mediator. Macrophage migration inhibitory factor (MIF) was identified simultaneously in 1966 by John David and Barry Bloom.

In 1969, Dudley Dumonde proposed the term "lymphokine" to describe proteins secreted from lymphocytes and later, proteins derived from macrophages and monocytes in culture were called "monokines". In 1974, pathologist Stanley Cohen, M.D. (not to be confused with the Nobel laureate named Stanley Cohen, who was a PhD biochemist; nor with the MD geneticist Stanley Norman Cohen) published an article describing the production of MIF in virus-infected allantoic membrane and kidney cells, showing its production is not limited to immune cells. This led to his proposal of the term cytokine. In 1993, Ogawa described the early acting growth factors, intermediate acting growth factors and late acting growth factors.

Difference from hormones

Classic hormones circulate in aqueous solution in nanomolar (10^-9 M) concentrations that usually vary by less than one order of magnitude. In contrast, some cytokines (such as IL-6) circulate in picomolar (10^-12 M) concentrations that can increase up to 1,000 times during trauma or infection. The widespread distribution of cellular sources for cytokines may be a feature that differentiates them from hormones. Virtually all nucleated cells, but especially endo/epithelial cells and resident macrophages (many near the interface with the external environment) are potent producers of IL-1, IL-6, and TNF-α. In contrast, classic hormones, such as insulin, are secreted from discrete glands such as the pancreas. The current terminology refers to cytokines as immunomodulating agents.

A contributing factor to the difficulty of distinguishing cytokines from hormones is that some immunomodulating effects of cytokines are systemic (i.e., affecting the whole organism) rather than local. For instance, to accurately utilize hormone terminology, cytokines may be autocrine or paracrine in nature, and chemotaxis, chemokinesis and endocrine as a pyrogen. Essentially, cytokines are not limited to their immunomodulatory status as molecules.

Cytokines typically activate second messenger systems, like JAK-STAT pathways, as illustrated on the left side of the diagram. Conversely, hormones typically activate different signaling pathways, like G protein-coupled receptors, seen at the top of the figure.

Nomenclature

Cytokines have been classed as lymphokines, interleukins, and chemokines, based on their presumed cell of secretion, function, or target of action. Because cytokines are characterised by considerable redundancy and pleiotropism, such distinctions, allowing for exceptions, are obsolete.

• The term interleukin was initially used by researchers for those cytokines whose presumed targets are principally white blood cells (leukocytes). It is now used largely for designation of newer cytokine molecules and bears little relation to their presumed function. The vast majority of these are produced by T-helper cells.
• Lymphokines: produced by lymphocytes
• Monokines: produced exclusively by monocytes
• Interferons: involved in antiviral responses
• Colony stimulating factors: support the growth of cells in semisolid media
• Chemokines: mediate chemoattraction (chemotaxis) between cells.

Classification

Structural

Structural homogeneity has been able to partially distinguish between cytokines that do not demonstrate a considerable degree of redundancy so that they can be classified into four types:

• The four-α-helix bundle family (InterPro: IPR009079): member cytokines have three-dimensional structures with a bundle of four α-helices. This family, in turn, is divided into three sub-families:

1. the IL-2 subfamily. This is the largest family. It contains several non-immunological cytokines including erythropoietin (EPO) and thrombopoietin (TPO). They can be grouped into long-chain and short-chain cytokines by topology. Some members share the common gamma chain as part of their receptor.
2. the interferon (IFN) subfamily.
3. the IL-10 subfamily.

• The IL-1 family, which primarily includes IL-1 and IL-18.
• The cysteine knot cytokines (IPR029034) include members of the transforming growth factor beta superfamily, including TGF-β1, TGF-β2 and TGF-β3.
• The IL-17 family, which has yet to be completely characterized, though member cytokines have a specific effect in promoting proliferation of T-cells that cause cytotoxic effects.

Functional

A classification that proves more useful in clinical and experimental practice outside of structural biology divides immunological cytokines into those that enhance cellular immune responses, type 1 (TNFα, IFN-γ, etc.), and those that enhance antibody responses, type 2 (TGF-β, IL-4, IL-10, IL-13, etc.). A key focus of interest has been that cytokines in one of these two sub-sets tend to inhibit the effects of those in the other. Dysregulation of this tendency is under intensive study for its possible role in the pathogenesis of autoimmune disorders. Several inflammatory cytokines are induced by oxidative stress. The fact that cytokines themselves trigger the release of other cytokines and also lead to increased oxidative stress makes them important in chronic inflammation, as well as other immunoresponses, such as fever and acute phase proteins of the liver (IL-1,6,12, IFN-a). Cytokines also play a role in anti-inflammatory pathways and are a possible therapeutic treatment for pathological pain from inflammation or peripheral nerve injury. There are both pro-inflammatory and anti-inflammatory cytokines that regulate this pathway.

Receptors

Main article: Cytokine receptor

In recent years, the cytokine receptors have come to demand the attention of more investigators than cytokines themselves, partly because of their remarkable characteristics and partly because a deficiency of cytokine receptors has now been directly linked to certain debilitating immunodeficiency states. In this regard, and also because the redundancy and pleomorphism of cytokines are, in fact, a consequence of their homologous receptors, many authorities think that a classification of cytokine receptors would be more clinically and experimentally useful.

A classification of cytokine receptors based on their three-dimensional structure has, therefore, been attempted. Such a classification, though seemingly cumbersome, provides several unique perspectives for attractive pharmacotherapeutic targets.

• Immunoglobulin (Ig) superfamily, which are ubiquitously present throughout several cells and tissues of the vertebrate body, and share structural homology with immunoglobulins (antibodies), cell adhesion molecules, and even some cytokines. Examples: IL-1 receptor types.
• Hemopoietic Growth Factor (type 1) family, whose members have certain conserved motifs in their extracellular amino-acid domain. The IL-2 receptor belongs to this chain, whose γ-chain (common to several other cytokines) deficiency is directly responsible for the x-linked form of Severe Combined Immunodeficiency (X-SCID).
• Interferon (type 2) family, whose members are receptors for IFN β and γ.
• Tumor necrosis factors (TNF) (type 3) family, whose members share a cysteine-rich common extracellular binding domain, and includes several other non-cytokine ligands like CD40, CD27 and CD30, besides the ligands on which the family is named.
• Seven transmembrane helix family, the ubiquitous receptor type of the animal kingdom. All G protein-coupled receptors (for hormones and neurotransmitters) belong to this family. Chemokine receptors, two of which act as binding proteins for HIV (CD4 and CCR5), also belong to this family.
• Interleukin-17 receptor (IL-17R) family, which shows little homology with any other cytokine receptor family. Structural motifs conserved between members of this family include: an extracellular fibronectin III-like domain, a transmembrane domain and a cytoplasmic SERIF domain. The known members of this family are as follows: IL-17RA, IL-17RB, IL-17RC, IL17RD and IL-17RE.

Cellular effects

Each cytokine has a matching cell-surface receptor. Subsequent cascades of intracellular signaling then alter cell functions. This may include the upregulation and/or downregulation of several genes and their transcription factors, resulting in the production of other cytokines, an increase in the number of surface receptors for other molecules, or the suppression of their own effect by feedback inhibition. The effect of a particular cytokine on a given cell depends on the cytokine, its extracellular abundance, the presence and abundance of the complementary receptor on the cell surface, and downstream signals activated by receptor binding; these last two factors can vary by cell type. Cytokines are characterized by considerable redundancy, in that many cytokines appear to share similar functions. It seems to be a paradox that cytokines binding to antibodies have a stronger immune effect than the cytokine alone. This may lead to lower therapeutic doses.

It has been shown that inflammatory cytokines cause an IL-10-dependent inhibition of T-cell expansion and function by up-regulating PD-1 levels on monocytes, which leads to IL-10 production by monocytes after binding of PD-1 by PD-L. Adverse reactions to cytokines are characterized by local inflammation and/or ulceration at the injection sites. Occasionally such reactions are seen with more widespread papular eruptions.

Roles in health and disease

Cytokines are involved in several developmental processes during embryonic development. Cytokines are released from the blastocyst, and are also expressed in the endometrium, and have critical roles in the stages of zona hatching, and implantation. Cytokines are crucial for fighting off infections and in other immune responses. However, they can become dysregulated and pathological in inflammation, trauma, sepsis, and hemorrhagic stroke. Dysregulated cytokine secretion in the aged population can lead to inflammaging, and render these individuals more vulnerable to age-related diseases like neurodegenerative diseases and type 2 diabetes.

A 2019 review was inconclusive as to whether cytokines play any definitive role in ME/CFS.

Adverse effects

Adverse effects of cytokines have been linked to many disease states and conditions ranging from schizophrenia, major depression and Alzheimer's disease to cancer. T regulatory cells (Tregs) and related-cytokines are effectively engaged in the process of tumor immune escape and functionally inhibit immune response against the tumor. Forkhead box protein 3 (Foxp3) as a transcription factor is an essential molecular marker of Treg cells. Foxp3 polymorphism (rs3761548) might be involved in cancer progression like gastric cancer through influencing Tregs function and the secretion of immunomodulatory cytokines such as IL-10, IL-35, and TGF-β. Normal tissue integrity is preserved by feedback interactions between diverse cell types mediated by adhesion molecules and secreted cytokines; disruption of normal feedback mechanisms in cancer threatens tissue integrity.

Over-secretion of cytokines can trigger a dangerous cytokine storm syndrome. Cytokine storms may have been the cause of severe adverse events during a clinical trial of TGN1412. Cytokine storms are also suspected to be the main cause of death in the 1918 "Spanish Flu" pandemic. Deaths were weighted more heavily towards people with healthy immune systems, because of their ability to produce stronger immune responses, with dramatic increases in cytokine levels. Another example of cytokine storm is seen in acute pancreatitis. Cytokines are integral and implicated in all angles of the cascade, resulting in the systemic inflammatory response syndrome and multi-organ failure associated with this intra-abdominal catastrophe. In the COVID-19 pandemic, some deaths from COVID-19 have been attributable to cytokine release storms. Current data suggest cytokine storms may be the source of extensive lung tissue damage and dysfunctional coagulation in COVID-19 infections.

Medical use as drugs

Some cytokines have been developed into protein therapeutics using recombinant DNA technology. Recombinant cytokines being used as drugs as of 2014 include:

• Bone morphogenetic protein (BMP), used to treat bone-related conditions
• Erythropoietin (EPO), used to treat anemia
• Granulocyte colony-stimulating factor (G-CSF), used to treat neutropenia in cancer patients
• Granulocyte macrophage colony-stimulating factor (GM-CSF), used to treat neutropenia and fungal infections in cancer patients
• Interferon alfa, used to treat hepatitis C and multiple sclerosis
• Interferon beta, used to treat multiple sclerosis
• Interleukin 2 (IL-2), used to treat cancer.
• Interleukin 11 (IL-11), used to treat thrombocytopenia in cancer patients.
• Interferon gamma is used to treat chronic granulomatous disease and osteopetrosis

Wood fuel

From Wikipedia, the free encyclopedia
https://en.wikipedia.org/wiki/Wood_fuel

Wood burning

Wood fuel (or fuelwood) is a fuel such as firewood, charcoal, chips, sheets, pellets, and sawdust. The particular form used depends upon factors such as source, quantity, quality and application. In many areas, wood is the most easily available form of fuel, requiring no tools in the case of picking up dead wood, or few tools, although as in any industry, specialized tools, such as skidders and hydraulic wood splitters, have been developed to mechanize production. Sawmill waste and construction industry by-products also include various forms of lumber tailings.

The discovery of how to make fire for the purpose of burning wood is regarded as one of humanity's most important advances. The use of wood as a fuel source for heating is much older than civilization and is assumed to have been used by Neanderthals. Today, burning of wood is the largest use of energy derived from a solid fuel biomass. Wood fuel can be used for cooking and heating, and occasionally for fueling steam engines and steam turbines that generate electricity. Wood may be used indoors in a furnace, stove, or fireplace, or outdoors in furnace, campfire, or bonfire.

Historical development

Further information: Firewood

Campfires have been used for ages: fires are integral to humanity.

Charcoal, a derivative of wood, was traditionally an important fuel in ironmaking and other processes

Wood has been used as fuel for millennia. Historically, it was limited in use only by the distribution of technology required to make a spark. Heat derived from wood is still common throughout much of the world. Early examples included a fire constructed inside a tent. Fires were constructed on the ground, and a smoke hole in the top of the tent allowed the smoke to escape by convection.

In permanent structures and in caves, hearths were constructed or established—surfaces of stone or another noncombustible material upon which a fire could be built. Smoke escaped through a smoke hole in the roof.

In contrast to civilizations in relatively arid regions (such as Mesopotamia and Egypt), the Greeks, Romans, Celts, Britons, and Gauls all had access to forests suitable for using as fuel. Over the centuries there was a partial deforestation of climax forests and the evolution of the remainder to coppice with standards woodland as the primary source of wood fuel. These woodlands involved a continuous cycle of new stems harvested from old stumps, on rotations between seven and thirty years.

One of the earliest printed books on woodland management, in English, was John Evelyn's "Sylva, or a discourse on forest trees" (1664) advising landowners on the proper management of forest estates. H. L. Edlin, in "Woodland Crafts in Britain", 1949 outlines the extraordinary techniques employed, and range of wood products that have been produced from these managed forests since pre-Roman times. And throughout this time the preferred form of wood fuel was the branches of cut coppice stems bundled into faggots. Larger, bent or deformed stems that were of no other use to the woodland craftsmen were converted to charcoal.

As with most of Europe, these managed woodlands continued to supply their markets right up to the end of World War Two. Since then much of these woodlands have been converted to broadscale agriculture. Total demand for fuel increased considerably with the industrial revolution but most of this increased demand was met by the new fuel source coal, which was more compact and more suited to the larger scale of the new industries.

During the Edo period of Japan, wood was used for many purposes, and the consumption of wood led Japan to develop a forest management policy during that era. Demand for timber resources was on the rise not only for fuel, but also for construction of ships and buildings, and consequently deforestation was widespread. As a result, forest fires occurred, along with floods and soil erosion. Around 1666, the shōgun made it a policy to reduce logging and increase the planting of trees. This policy decreed that only the shōgun, or a daimyō, could authorize the use of wood. By the 18th century, Japan had developed detailed scientific knowledge about silviculture and plantation forestry.

Fireplaces and stoves

Ceramic stoves are traditional in Northern Europe: an 18th-century faience stove at Łańcut Castle, Poland

The development of the chimney and the fireplace allowed for more effective exhaustion of the smoke. Masonry heaters or stoves went a step further by capturing much of the heat of the fire and exhaust in a large thermal mass, becoming much more efficient than a fireplace alone.

The metal stove was a technological development concurrent with the industrial revolution. Stoves were manufactured or constructed pieces of equipment that contained the fire on all sides and provided a means for controlling the draft—the amount of air allowed to reach the fire. Stoves have been made of a variety of materials. Cast iron is among the more common. Soapstone (talc), tile, and steel have all been used. Metal stoves are often lined with refractory materials such as firebrick, since the hottest part of a woodburning fire will burn away steel over the course of several years' use.

The Franklin stove was developed in the United States by Benjamin Franklin. More a manufactured fireplace than a stove, it had an open front and a heat exchanger in the back that was designed to draw air from the cellar and heat it before releasing it out the sides. The heat exchanger was never a popular feature and was omitted in later versions. So-called "Franklin" stoves today are made in a great variety of styles, though none resembles the original design.

Potbelly stove at the Museum of Appalachia

The 1800s became the high point of the cast iron stove. Each local foundry would make their own design, and stoves were built for myriads of purposes—parlour stoves, box stoves, camp stoves, railroad stoves, portable stoves, cooking stoves and so on. Elaborate nickel and chrome edged models took designs to the edge, with cast ornaments, feet and doors. Wood or coal could be burnt in the stoves and thus they were popular for over one hundred years. The action of the fire, combined with the causticity of the ash, ensured that the stove would eventually disintegrate or crack over time. Thus a steady supply of stoves was needed. The maintenance of stoves, needing to be blacked, their smokiness, and the need to split wood meant that oil or electric heat found favour.

The airtight stove, originally made of steel, allowed greater control of combustion, being more tightly fitted than other stoves of the day. Airtight stoves became common in the 19th century.

Use of wood heat declined in popularity with the growing availability of other, less labor-intensive fuels. Wood heat was gradually replaced by coal and later by fuel oil, natural gas and propane heating except in rural areas with available forests.

After the 1967 Oil Embargo, many people in the United States used wood as fuel for the first time. The EPA provided information on clean stoves, which burned much more efficiently.

1970s

A woman uses wood in a fireplace for heat. A newspaper headline before her tells of the community's lack of heating oil in 1973.

A brief resurgence in popularity occurred during and after the 1973 energy crisis, when some believed that fossil fuels would become so expensive as to preclude their use. A period of innovation followed, with many small manufacturers producing stoves based on designs old and new. Notable innovations from that era include the Ashley heater, a thermostatically controlled stove with an optional perforated steel enclosure that prevented accidental contact with hot surfaces. The decade also saw a number of dual-fuel furnaces and boilers made, which utilized ductwork and piping to deliver heat throughout a house or other building.

1980s

The growth in popularity of wood heat also led to the development and marketing of a greater variety of equipment for cutting, splitting and processing firewood. Consumer grade hydraulic log splitters were developed to be powered by electricity, gasoline, or PTO of farm tractors. In 1987 the US Department of Agriculture published a method for producing kiln dried firewood, on the basis that better heat output and increased combustion efficiency can be achieved with logs containing lower moisture content.

The magazine "Wood Burning Quarterly" was published for several years before changing its name to "Home Energy Digest" and, subsequently, disappearing.

Today

A wood pellet stove

A pellet stove is an appliance that burns compressed wood or biomass pellets. Wood heat continues to be used in areas where firewood is abundant. For serious attempts at heating, rather than mere ambience (open fireplaces), stoves, fireplace inserts, and furnaces are most commonly used today. In rural, forested parts of the U.S., freestanding boilers are increasingly common. They are installed outdoors, some distance from the house, and connected to a heat exchanger in the house using underground piping. The mess of wood, bark, smoke, and ashes is kept outside and the risk of fire is reduced. The boilers are large enough to hold a fire all night, and can burn larger pieces of wood, so that less cutting and splitting is required. There is no need to retrofit a chimney in the house. However, outdoor wood boilers emit more wood smoke and associated pollutants than other wood-burning appliances. This is due to design characteristics such as the water-filled jacket surrounding the firebox, which acts to cool the fire and leads to incomplete combustion. Outdoor wood boilers also typically have short stack heights in comparison to other wood-burning appliances, contributing to ambient levels of particulates at ground level. An alternative that is increasing in popularity are wood gasification boilers, which burn wood at very high efficiencies (85-91%) and can be placed indoors or in an outbuilding. There are plenty of ways to process wood fuel and the inventions today are maximizing by the minute.

Wood is still used today for cooking in many places, either in a stove or an open fire. It is also used as a fuel in many industrial processes, including smoking meat and making maple syrup.

As a sustainable energy source, wood fuel also remains viable for generating electricity in areas with easy access to forest products and by-products.

Measurement of firewood

Stapled birch wood

In the metric system, firewood is normally sold by the cubic metre or stere (1 m³ ≈ 0.276 cords).

In the United States and Canada, firewood is usually sold by the cord, 128 ft³ (3.62 m³), corresponding to a woodpile 8 ft wide × 4 ft high of 4 ft-long logs. The cord is legally defined by statute in most U.S. states. A "thrown cord" is firewood that has not been stacked and is defined as 4 ft wide x 4 ft tall x 10 ft long. The additional volume is to make it equivalent to a standard stacked cord, where there is less void space. It is also common to see wood sold by the "face cord", which is usually not legally defined, and varies from one area to another. For example, in one state a pile of wood 8 feet wide × 4 feet high of 16"-long logs will often be sold as a "face cord", though its volume is only one-third of a cord. In another state, or even another area of the same state, the volume of a face cord may be considerably different. Hence, it is risky to buy wood sold in this manner, as the transaction is not based on a legally enforceable unit of measure.

In Australia, it is normally sold by the tonne but is commonly advertised as sold by the barrowload (wheelbarrow), bucket (1/3 of a m3 bucket of a typical skid-steer), ute-load or bag (roughly 15–20 kg).

Energy content

A common hardwood, red oak, has an energy content (heat value) of 14.9 megajoules per kilogram (6,388 BTU per pound), and 10.4 megajoules recoverable if burned at 70% efficiency.

The Sustainable Energy Development Office (SEDO), part of the Government of Western Australia states that the energy content of wood is 16.2 megajoules per kilogram (4.5 kWh/kg).

According to The Bioenergy Knowledge Centre, the energy content of wood is more closely related to its moisture content than its species. The energy content improves as moisture content decreases.

In 2008, wood for fuel cost $15.15 per 1 million BTUs (0.041 EUR per kWh).

Environmental impacts

Combustion by-products

Fireplace and chimney after a wildfire, Witch Fire, California

As with any fire, burning wood fuel creates numerous by-products, some of which may be useful (heat and steam), and others that are undesirable, irritating or dangerous.

One by-product of wood burning is wood ash, which in moderate amounts is a fertilizer (mainly potash), contributing minerals, but is strongly alkaline as it contains potassium hydroxide (lye). Wood ash can also be used to manufacture soap.

Smoke, containing water vapor, carbon dioxide and other chemicals and aerosol particulates, including caustic alkali fly ash, which can be an irritating (and potentially dangerous) by-product of partially burnt wood fuel. A major component of wood smoke is fine particles that may account for a large portion of particulate air pollution in some regions. During cooler months, wood heating accounts for as much as 60% of fine particles in Melbourne, Australia.

The burning of fuel wood releases organic components over a wide volatility range. Here the organic components emitted from the combustion of fuel wood are measured with a range of state-of-the art analytical techniques including proton-transfer-reaction time-of-flight mass spectrometry, two-dimensional gas chromatography and two-dimensional gas chromatography coupled to time-of-flight mass spectrometry.

Significant quantities of volatile organic compounds are released from the combustion of fuel wood. Large quantities of smaller oxygenate species are released during the combustion process, as well as organics formed from the depolymerisation reaction of lignin such as phenolics, furans and furanones. The combustion of fuel wood has also been shown to release many organic compounds into the aerosol phase. The burning of fuel woods has been shown to release organic components over a range of volatilities, over effective saturation concentrations, C*, from 10¹-10¹¹ μg m⁻³. The emissions from fuel wood samples collected from the Delhi area of India were shown to be 30 times more reactive with the hydroxyl radical than emissions from liquefied petroleum gas. Furthermore, when comparing 21 polycyclic aromatic hydrocarbons emitted from the same fuel wood samples from Delhi, emissions from fuel wood were around 20 times more toxic than emissions from liquefied petroleum gas.

Slow combustion stoves increase efficiency of wood heaters burning logs, but also increase particulate production. Low pollution/slow combustion stoves are a current area of research. An alternative approach is to use pyrolysis to produce several useful biochemical byproducts, and clean burning charcoal, or to burn fuel extremely quickly inside a large thermal mass, such as a masonry heater. This has the effect of allowing the fuel to burn completely without producing particulates while maintaining the efficiency of the system.

In some of the most efficient burners, the temperature of the smoke is raised to a much higher temperature where the smoke will itself burn (e.g. 609 °C for igniting carbon monoxide gas). This may result in significant reduction of smoke hazards while also providing additional heat from the process. By using a catalytic converter, the temperature for obtaining cleaner smoke can be reduced. Some U.S. jurisdictions prohibit sale or installation of stoves that do not incorporate catalytic converters.

Combustion by-product effects on human health

Wood-burning fireplace with burning log

Depending on population density, topography, climatic conditions and combustion equipment used, wood heating may substantially contribute to air pollution, particularly particulates. The conditions in which wood is burnt will greatly influence the content of the emission. Particulate air pollution can contribute to human health problems and increased hospital admissions for asthma & heart diseases.

The technique of compressing wood pulp into pellets or artificial logs can reduce emissions. The combustion is cleaner, and the increased wood density and reduced water content can eliminate some of the transport bulk. The fossil energy consumed in transport is reduced and represents a small fraction of the fossil fuel consumed in producing and distributing heating oil or gas.

Harvesting operations

Much wood fuel comes from native forests around the world. Plantation wood is rarely used for firewood, as it is more valuable as timber or wood pulp, however, some wood fuel is gathered from trees planted amongst crops, also known as agroforestry. The collection or harvesting of this wood can have serious environmental implications for the collection area. The concerns are often specific to the particular area, but can include all the problems that regular logging create. The heavy removal of wood from forests can cause habitat destruction and soil erosion. However, in many countries, for example in Europe and Canada, the forest residues are being collected and turned into useful wood fuels with minimal impact on the environment. Consideration is given to soil nutrition as well as erosion. The environmental impact of using wood as a fuel depends on how it is burnt. Higher temperatures result in more complete combustion and less noxious gases as a result of pyrolysis. Some may regard the burning of wood from a sustainable source as carbon-neutral. A tree, over the course of its lifetime, absorbs as much carbon (or carbon dioxide) as it releases when burnt.

Some firewood is harvested in "woodlots" managed for that purpose, but in heavily wooded areas it is more often harvested as a byproduct of natural forests. Deadfall that has not started to rot is preferred, since it is already partly seasoned. Standing dead timber is considered better still, as it is both seasoned, and has less rot. Harvesting this form of timber reduces the speed and intensity of bushfires. Harvesting timber for firewood is normally carried out by hand with chainsaws. Thus, longer pieces - requiring less manual labor, and less chainsaw fuel - are less expensive and only limited by the size of their firebox. Prices also vary considerably with the distance from wood lots, and quality of the wood. Firewood usually relates to timber or trees unsuitable for building or construction. Firewood is a renewable resource provided the consumption rate is controlled to sustainable levels. The shortage of suitable firewood in some places has seen local populations damaging huge tracts of bush possibly leading to further desertification.

Greenhouse gases

Wood burning creates more atmospheric CO₂ than biodegradation of wood in a forest (in a given period of time) because by the time the bark of a dead tree has rotted, the log has already been occupied by other plants and micro-organisms which continue to sequester the CO₂ by integrating the hydrocarbons of the wood into their own life cycle. Wood harvesting and transport operations produce varying degrees of greenhouse gas pollution. Inefficient and incomplete combustion of wood can result in elevated levels of greenhouse gases other than CO₂, which may result in positive emissions where the byproducts have greater Carbon dioxide equivalent values. In an attempt to provide quantitative information about the relative output of CO₂ to produce electricity or domestic heating, the United Kingdom Department of Energy and Climate Change (DECC) has published a comprehensive model comparing the burning of wood (wood chip) and other fuels, based on 33 scenarios. The model's output is kilogram of CO₂ produced per Megawatt hour of delivered energy. Scenario 33 for example, which concerns the production of heat from wood chips produced from UK small roundwood produced from bringing neglected broadleaf forests back into production, shows that burning oil releases 377 kg of CO₂ while burning woodchip releases 1501 kg of CO₂ per MW h delivered energy. On the other hand, scenario 32 in that same reference, which concerns production of heat from wood chips that would otherwise be made into particleboard, releases only 239 kg of CO₂ per MW h delivered energy. Therefore, the relative greenhouse effects of biomass energy production very much depends on the usage model.

The intentional and controlled charring of wood and its incorporation into the soil is an effective method for carbon sequestration as well as an important technique to improve soil conditions for agriculture, particularly in heavily forested regions. It forms the basis of the rich soils known as Terra preta.

Regulation and Legislation

The environmental impact of burning wood fuel is debatable. Several cities have moved towards setting standards of use and/or bans of wood burning fireplaces. For example, the city of Montréal, Québec passed a resolution to ban wood fireplace installation in new construction.

Wood burning advocates claim that properly harvested wood is carbon-neutral, therefore off-setting the negative impact of by-product particles given off during the burning process. In the context of forest wildfires, wood removed from the forest setting for use as wood fuel can reduce overall emissions by decreasing the quantity of open burned wood and the severity of the burn while combusting the remaining material under regulated conditions. On March 7, 2018, the United States House of Representatives passed a bill that would delay for three years the implementation of more stringent emission standards for new residential wood heaters.

Potential use in renewable energy technologies

Sawmills create and burn sawdust: it can be pelletized and used at home.

• Efficient stove for developing nations
• Pellet stove
• Sawdust can be pelletized
• Wood pellets

Usage

World production of roundwood by type, comparison of wood fuel to other types

Some European countries produce a significant fraction of their electricity needs from wood or wood wastes. In Scandinavian countries the costs of manual labor to process firewood is very high. Therefore, it is common to import firewood from countries with cheap labor and natural resources. The main exporters to Scandinavia are the Baltic countries (Estonia, Lithuania, and Latvia). In Finland, there is a growing interest in using wood waste as fuel for home and industrial heating, in the form of compacted pellets.

Many lower- and middle-income countries rely on wood for energy purposes (notably cooking). The largest producers are all in these income groups and have large populations with a high reliance on wood for energy: in 2021, India ranked first with 300 million m³ (15 percent of total production), followed by China with 156 million m3 and Brazil with 129 million m³ (8 percent and 7 percent of global production).

In the United States, wood fuel is the second-leading form of renewable energy (behind hydro-electric).

Australia

A pile of firewood logged from the Barmah Forest in Victoria

About 1.5 million households in Australia use firewood as the main form of domestic heating. As of 1995, approximately 1.85 million cubic metres of firewood (1m³ equals approximately one car trailer load) was used in Victoria annually, with half being consumed in Melbourne. This amount is comparable to the wood consumed by all of Victoria's sawlog and pulplog forestry operations (1.9 million m³).

Species used as sources of firewood include:

• Red Gum, from forests along the Murray River (the Mid-Murray Forest Management Area, including the Barmah and Gunbower forests, provides about 80% of Victoria's red gum timber).
• Box and Messmate Stringybark, in southern Australia.
• Sugar gum, a wood with high thermal efficiency that usually comes from small plantations.
• Jarrah, in the Southwest of Western Australia. It generates a greater heat than most other available woods and is usually sold by the tonne.

Europe

In 2014, the construction of the biggest pellet plant in the Baltic region was started in Võrumaa, Sõmerpalu, Estonia, with an expected output of 110,000 tons of pellet / year. Different types of wood will be used in the process of pellet making (firewood, woodchips, shavings). The Warmeston OÜ plant started its activity by the end of 2014. In 2013, the main pellet consumers in Europe were the UK, Denmark, the Netherlands, Sweden, Germany and Belgium, as U.E.'s annual report on biofuels states. In Denmark and Sweden, pellets are used by power plants, households and medium scale consumers for district heating, compared to Austria and Italy, where pellets are mainly used as small - scale private residential and industrial boilers for heating. The UK is the single largest consuming market for industrial wood pellets, in large part due to its major biomass-fueled power stations such as Drax, MGT and Lynemouth.

Asia

Japan and South Korea are both growing markets for industrial wood pellets, and as of 2017, were expected to become the second and third largest global markets for wood pellets due to government policies favoring the use of biomass in power generation.

North America

Demand for wood fuel in the United States is principally driven by residential and commercial heating customers. Canada was not a major consumer of industrial wood pellets as of 2017, but has relatively aggressive de-carbonization policies and may become a significant consumer of industrial wood pellets by the 2020s.

Viral evolution

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Viral evolution is a subfield of evolutionary biology and virology that is specifically concerned with the evolution of viruses. Viruses have short generation times, and many—in particular RNA viruses—have relatively high mutation rates (on the order of one point mutation or more per genome per round of replication). Although most viral mutations confer no benefit and often even prove deleterious to viruses, the rapid rate of viral mutation combined with natural selection allows viruses to quickly adapt to changes in their host environment. In addition, because viruses typically produce many copies in an infected host, mutated genes can be passed on to many offspring quickly. Although the chance of mutations and evolution can change depending on the type of virus (e.g., double stranded DNA, double stranded RNA, single strand DNA), viruses overall have high chances for mutations.

Viral evolution is an important aspect of the epidemiology of viral diseases such as influenza (influenza virus), AIDS (HIV), and hepatitis (e.g. HCV). The rapidity of viral mutation also causes problems in the development of successful vaccines and antiviral drugs, as resistant mutations often appear within weeks or months after the beginning of a treatment. One of the main theoretical models applied to viral evolution is the quasispecies model, which defines a viral quasispecies as a group of closely related viral strains competing within an environment.

Origins

Three classical hypotheses

Viruses are ancient. Studies at the molecular level have revealed relationships between viruses infecting organisms from each of the three domains of life, suggesting viral proteins that pre-date the divergence of life and thus infecting the last universal common ancestor. This indicates that some viruses emerged early in the evolution of life, and that they have probably arisen multiple times. It has been suggested that new groups of viruses have repeatedly emerged at all stages of evolution, often through the displacement of ancestral structural and genome replication genes.

There are three classical hypotheses on the origins of viruses and how they evolved:

• Virus-first hypothesis: Viruses evolved from complex molecules of protein and nucleic acid before cells first appeared on earth. By this hypothesis, viruses contributed to the rise of cellular life. This is supported by the idea that all viral genomes encode proteins that do not have cellular homologs. The virus-first hypothesis has been dismissed by some scientists because it violates the definition of viruses, in that they require a host cell to replicate.
• Reduction hypothesis (degeneracy hypothesis): Viruses were once small cells that parasitized larger cells. This is supported by the discovery of giant viruses with similar genetic material to parasitic bacteria. However, the hypothesis does not explain why even the smallest of cellular parasites do not resemble viruses in any way.
• Escape hypothesis (vagrancy hypothesis): Some viruses evolved from bits of DNA or RNA that "escaped" from the genes of larger organisms. This does not explain the structures that are unique to viruses and are not seen anywhere in cells. It also does not explain the complex capsids and other structures of virus particles.

Virologists are in the process of re-evaluating these hypotheses.

Later hypotheses

• Coevolution hypothesis (Bubble Theory): At the beginning of life, a community of early replicons (pieces of genetic information capable of self-replication) existed in proximity to a food source such as a hot spring or hydrothermal vent. This food source also produced lipid-like molecules self-assembling into vesicles that could enclose replicons. Close to the food source replicons thrived, but further away the only non-diluted resources would be inside vesicles. Therefore, evolutionary pressure could push replicons along two paths of development: merging with a vesicle, giving rise to cells; and entering the vesicle, using its resources, multiplying and leaving for another vesicle, giving rise to viruses.
• Chimeric-origins hypothesis: Based on the analyses of the evolution of the replicative and structural modules of viruses, a chimeric scenario for the origin of viruses was proposed in 2019. According to this hypothesis, the replication modules of viruses originated from the primordial genetic pool, although the long course of their subsequent evolution involved many displacements by replicative genes from their cellular hosts. By contrast, the genes encoding major structural proteins evolved from functionally diverse host proteins throughout the evolution of the virosphere. This scenario is distinct from each of the three traditional scenarios but combines features of the Virus-first and Escape hypotheses.

One of the problems for studying viral origins and evolution is the high rate of viral mutation, particularly the case in RNA retroviruses like HIV/AIDS. A recent study based on comparisons of viral protein folding structures, however, is offering some new evidence. Fold Super Families (FSFs) are proteins that show similar folding structures independent of the actual sequence of amino acids, and have been found to show evidence of viral phylogeny. The proteome of a virus, the viral proteome, still contains traces of ancient evolutionary history that can be studied today. The study of protein FSFs suggests the existence of ancient cellular lineages common to both cells and viruses before the appearance of the 'last universal cellular ancestor' that gave rise to modern cells. Evolutionary pressure to reduce genome and particle size may have eventually reduced viro-cells into modern viruses, whereas other coexisting cellular lineages eventually evolved into modern cells. Furthermore, the long genetic distance between RNA and DNA FSFs suggests that the RNA world hypothesis may have new experimental evidence, with a long intermediary period in the evolution of cellular life.

Definitive exclusion of a hypothesis on the origin of viruses is difficult to make on Earth given the ubiquitous interactions between viruses and cells, and the lack of availability of rocks that are old enough to reveal traces of the earliest viruses on the planet. From an astrobiological perspective, it has therefore been proposed that on celestial bodies such as Mars not only cells but also traces of former virions or viroids should be actively searched for: possible findings of traces of virions in the apparent absence of cells could provide support for the virus-first hypothesis.

Evolution

Time-line of paleoviruses in the human lineage

Viruses do not form fossils in the traditional sense, because they are much smaller than the finest colloidal fragments forming sedimentary rocks that fossilize plants and animals. However, the genomes of many organisms contain endogenous viral elements (EVEs). These DNA sequences are the remnants of ancient virus genes and genomes that ancestrally 'invaded' the host germline. For example, the genomes of most vertebrate species contain hundreds to thousands of sequences derived from ancient retroviruses. These sequences are a valuable source of retrospective evidence about the evolutionary history of viruses, and have given birth to the science of paleovirology.

The evolutionary history of viruses can to some extent be inferred from analysis of contemporary viral genomes. The mutation rates for many viruses have been measured, and application of a molecular clock allows dates of divergence to be inferred.

Viruses evolve through changes in their RNA (or DNA), some quite rapidly, and the best adapted mutants quickly outnumber their less fit counterparts. In this sense their evolution is Darwinian. The way viruses reproduce in their host cells makes them particularly susceptible to the genetic changes that help to drive their evolution. The RNA viruses are especially prone to mutations. In host cells there are mechanisms for correcting mistakes when DNA replicates and these kick in whenever cells divide. These important mechanisms prevent potentially lethal mutations from being passed on to offspring. But these mechanisms do not work for RNA and when an RNA virus replicates in its host cell, changes in their genes are occasionally introduced in error, some of which are lethal. One virus particle can produce millions of progeny viruses in just one cycle of replication, therefore the production of a few "dud" viruses is not a problem. Most mutations are "silent" and do not result in any obvious changes to the progeny viruses, but others confer advantages that increase the fitness of the viruses in the environment. These could be changes to the virus particles that disguise them so they are not identified by the cells of the immune system or changes that make antiviral drugs less effective. Both of these changes occur frequently with HIV.

Phylogenetic tree showing the relationships of morbilliviruses of different species

Many viruses (for example, influenza A virus) can "shuffle" their genes with other viruses when two similar strains infect the same cell. This phenomenon is called genetic shift, and is often the cause of new and more virulent strains appearing. Other viruses change more slowly as mutations in their genes gradually accumulate over time, a process known as antigenic drift.

Through these mechanisms new viruses are constantly emerging and present a continuing challenge in attempts to control the diseases they cause. Most species of viruses are now known to have common ancestors, and although the "virus first" hypothesis has yet to gain full acceptance, there is little doubt that the thousands of species of modern viruses have evolved from less numerous ancient ones. The morbilliviruses, for example, are a group of closely related, but distinct viruses that infect a broad range of animals. The group includes measles virus, which infects humans and primates; canine distemper virus, which infects many animals including dogs, cats, bears, weasels and hyaenas; rinderpest, which infected cattle and buffalo; and other viruses of seals, porpoises and dolphins. Although it is not possible to prove which of these rapidly evolving viruses is the earliest, for such a closely related group of viruses to be found in such diverse hosts suggests the possibility that their common ancestor is ancient.

Bacteriophage

Escherichia virus T4 (phage T4) is a species of bacteriophage that infects Escherichia coli bacteria. It is a double-stranded DNA virus in the family Myoviridae. Phage T4 is an obligate intracellular parasite that reproduces within the host bacterial cell and its progeny are released when the host is destroyed by lysis. The complete genome sequence of phage T4 encodes about 300 gene products. These virulent viruses are among the largest, most complex viruses that are known and one of the best studied model organisms. They have played a key role in the development of virology and molecular biology. The numbers of reported genetic homologies between phage T4 and bacteria and between phage T4 and eukaryotes are similar suggesting that phage T4 shares ancestry with both bacteria and eukaryotes and has about equal similarity to each. Phage T4 may have diverged in evolution from a common ancestor of bacteria and eukaryotes or from an early evolved member of either lineage. Most of the phage genes showing homology with bacteria and eukaryotes encode enzymes acting in the ubiquitous processes of DNA replication, DNA repair, recombination and nucleotide synthesis. These processes likely evolved very early. The adaptive features of the enzymes catalyzing these early processes may have been maintained in the phage T4, bacterial, and eukaryotic lineages because they were established well-tested solutions to basic functional problems by the time these lineages diverged.

Transmission

Viruses have been able to continue their infectious existence due to evolution. Their rapid mutation rates and natural selection has given viruses the advantage to continue to spread. One way that viruses have been able to spread is with the evolution of virus transmission. The virus can find a new host through:

• Droplet transmission- passed on through body fluids (sneezing on someone)
  • An example is the influenza virus
• Airborne transmission- passed on through the air (brought in by breathing)
  • An example would be how viral meningitis is passed on
• Vector transmission- picked up by a carrier and brought to a new host
  • An example is viral encephalitis
• Waterborne transmission- leaving a host, infecting the water, and being consumed in a new host
  • Poliovirus is an example for this
• Sit-and-wait-transmission- the virus is living outside a host for long periods of time
  • The smallpox virus is also an example for this

Virulence, or the harm that the virus does on its host, depends on various factors. In particular, the method of transmission tends to affect how the level of virulence will change over time. Viruses that transmit through vertical transmission (transmission to the offspring of the host) will evolve to have lower levels of virulence. Viruses that transmit through horizontal transmission (transmission between members of the same species that don't have a parent-child relationship) will usually evolve to have a higher virulence.