Biological decomposition of organic matter, natural gas, volatile emissions, incomplete combustion of fossil fuels and biomass, automobile exhausts etc. are some of the sources of hydrocarbons in the atmosphere. Harmful effects caused by these substances vary from compound to compound.
However, some of these in general cause necrosis of leaves, choruses of flower buds, growth inhibition etc. in plants. In animals hydrocarbons may cause irritation of mucous membrane and bronchial constriction. Many hydrocarbons are known to have carcinogenic properties.
Methane, a gaseous hydrocarbon is a greenhouse gas. Like carbon dioxide it is capable of blocking infra-red and heat waves from escaping earth’s surface, thereby causing a heating effect. Human activity has caused an increase in the production of this gas, the main sources of which are decomposition of organic matter under anaerobic conditions, burning of biomass and excessive use of fossil fuels.
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About 360 million hectares of rice fields and about 1.2 billion odd heads of ruminating livestock which release methane when they belch or break wind, collectively inject an enormous quantity of methane into the atmosphere. About 400-765 x 1012 gems per year of methane have been estimated to be produced on global scale (Hoffman and Wells, 1987). Methane is oxidized to carbon dioxide and water which adds to carbon dioxide and water content of atmospheric air. Recent studies suggest that methane reacts with stratospheric chlorine to form hydrochloric acid and thus exerts a protective influence on ozone concentration by eliminating the ozone depleting chlorine content from the medium.
2. Chlorofluorocarbons (CFCs):
Another group of hydrocarbons which is causing great concern these days is that of Chloral- flour-carbons (CFCs). These are non-toxic, colorless, odorless inert chemicals which persist in the atmosphere for long durations. They can easily be liquefied. These inert chemicals find widespread use in refrigeration, air-conditioners, foam blowing, spray cans and as solvents etc. Though highly persistent in the lower atmosphere, high up in the stratosphere they undergo dissociate under the influence of ultraviolet radiations to yield chlorine atoms which catalytically decompose ozone molecules.
The total world production of these halogenated hydrocarbons has increased from 41 million kg in 1950 to 700 million kg in the year 1980. The use of CFCs in refrigerators and air conditioners involves circulation in a close circuit from where their release is only accidental. However, use of these chemicals in spray cans or aerosol sprays, plastic foam blowing, instant foam shaving creams and as cleaning solvents etc. causes the release of substantial amount of CFCs in the atmosphere on global scale.
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3. Aldehydes:
Exhausts from automobiles and incomplete combustion of fossil fuels, biomass, plywood etc. generate a good amount of aldehydes most of which are toxic chemicals. Formaldehyde, acetaldehyde and caroling, an unsaturated aldehyde, probably contribute much to the odor, eye and lung irritation produced by the photochemical smog. Formaldehyde usually accounts for about 50% while caroling accounts for about 5% of the total estimated aldehydes present in polluted air.
Two important aspects of formaldehyde have rather recently attracted much public attention Firstly, their presence in indoor atmosphere where it comes from incomplete combustion of wood and fuels, from plywood, from foam insulators and a number of objects in which formaldehyde containing adhesives are used. Many pesticides also contain formaldehyde. Secondly, the finding of nasal cancers in rodents chronically exposed to 3-15 pap of formaldehyde has caused muck concern.
However, this aspect has not fully been investigated and many workers argue that formaldehyde has been in use since very long time. If it has any such properties there should have been complaints of increased nasal carcinoma in at least occupationally involved workers. Being highly reactive chemicals these are rather rapidly degraded in the atmosphere.
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Reactions involving hydrocarbons, particularly aldehydes and oxides of nitrogen in the atmosphere at times result in the formation of compounds like Peroxyacetyl nitrate (PAN) and Peroxybenzonyl nitrate (PBN). These compounds cause acute irritation of eyes-a feature often met with in photochemical or oxidizing type of air pollution. These chemicals have also been implicated in causing various respiratory disorders and in plants they are capable of suppressing photosynthesis and thereby the plant yield.
Though formaldehyde, the simplest member of aldehyde group also forms peroxyformyl nitrate, it is the presence of peroxyacetyl nitrate, formed by acetaldehyde which causes more problems. Peroxybenzonyl nitrate derived from aromatic aldehydes is also of considerable importance in many cases.
A number of workers have recorded glazed appearance on the lower side of bean and lettuce plants sensitive to these chemicals. PAN and PBN are also capable of damaging chloroplasts and thus causing drastic reduction in photosynthetic efficiency, inhibition of electron transport system and interference with a number of enzyme systems which play an important role in cellular metabolism,
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In general aromatic hydrocarbons are considered to be more toxic than simple aliphatic compounds. Much of aromatic hydrocarbons present in the atmosphere are derived from combustion of fossil fuels such as coal, oil, tar and biomass etc. Edible material such as fish and meat smoked for preservation may contain appreciable quantities of these aromatic compounds. Benzene (C6H6) is the simplest aromatic hydrocarbon used in various industries as solvent and in fuel oils because of its antiknock properties. Of the total amount of hydrocarbons emitted with exhausts of internal combustion engines, benzene may constitute almost 2.0 – 2.5% by volume. About 470,000 tons of benzene was estimated to have escaped into the environment in 1977 alone.
Total daily intake of benzene by humans on an average basis is 1.1 – 1.3 mg of which about 80% comes from air which we breathe. Benzene toxicity is characterised by reduction in white cell counts in laboratory animals and leukemia in humans. Benzene has also been shown to be carcinogenic.
6. Poly-Nuclear Aromatic Compounds :
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Polynuclear aromatic hydrocarbons (PAHs) are organic chemicals which possess two or more benzene rings fused together. More than hundred of these compounds have been identified in the environment of which 11 have been shown to be carcinogenic. Three important carcinogenic polynuclear hydrocarbons are: benzopyrene, benzoanthracine and dibenzine. Many of these compounds are produced naturally.
However, much of PAHs present in the atmosphere is derived from pyrolysis of organic matter. Polynuclear aromatic hydrocarbons are lipophilic compounds which have a low solubility in water. Metabolism of these compounds involves the formation of epoxides which are finally converted to phenols.
The carcinogenic action of PAHs is due to the formation of strongly electrophilic epoxides which bind to various macro-molecules such as DNA and RNA within the cell. Lung cancer risk multiplies if plenty of PAHs is present in the air which we breathe. Ophthalmic preparation like Kajal and Surma common in many Indian households, contains a significant amount of PAHs (230-250 µg/gm) of which Pyrene is most abundant.