Biological Effects of Air Pollution
Man-made and naturally occurring air pollutants have immediate and long-term effects on plants and animals. Chemicals and particulates each have specific impacts on the environment. Further, secondary effects of air pollution may be seen in animals that eat affected plants. Air pollution effects on humans are harder to determine because of multiple exposures and early medical intervention. The impact of air pollution can be observed on local, regional or global scales.
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Types of Air Pollutants
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Air pollutants can be solids, liquids or gases. Carbon and sulfates are the significant solid particulate air pollutants observed at local, regional and global scales. Water, acid and oil droplets are liquid pollutants often seen on a local or regional level. Ozone (O3), sulfur dioxide (SO2), carbon dioxide (CO2), nitrogen oxides (NOx), carbon monoxide (CO) and volatile organic carbon (VOC) compounds are gaseous pollutants that can be seen at all levels of distribution. The U.S. EPA has classified six contaminants as "criteria pollutants" that have set regulatory ambient air standards. They are: ozone, carbon monoxide, sulfur dioxide, nitrogen oxides, lead (Pb) and particulates (PM10 or particulates under 10 micrometers in diameter).
Sources of Air Pollutants
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Primary pollutants are issued directly from the generating sources. Secondary pollutants are formed as a result of chemical interactions between primary pollutants and normal atmospheric constituents or conditions. Ozone is a secondary pollutant generated when VOCs, NOx and other precursors react in the atmosphere. Another harmful secondary pollutant is peroxyacyl nitrate (PAN). PAN is a secondary pollutant generated by the interaction of VOCs, NOx, O3 and sunlight. Nature produces more pollution than humans through volcanic activity, forest fires and dust storms. Natural releases generally occur episodically with lower concentration exposures to humans than anthropogenic (man-made) contaminants. Anthropogenic pollutants are of most concern because they are generated more consistently in areas inhabited by humans. This poses a higher potential for human exposure and impacts on human health and welfare.
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Air Pollution Impacts on Plants
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Phytotoxic air pollutants (plant toxins) include O3, PAN, fluorides, SO2 and ethylene. Nitrogen dioxide (NO2), chlorine (Cl2), hydrogen chloride (HCl), ammonia (NH3) and particulates are minor pollutants that can cause harm to plants. The injuries caused to plants by pollution can be visible or subtle. Visible damage is seen as a loss of chlorophyll (chlorosis), death of plant tissue (necrosis) and the formation of unusual pigment patterns. Chlorosis occurs after a chronic exposure to specific pollutants. Loss of chlorophyll also occurs with natural aging, nutritional deficiencies and physical injuries. The pattern of chlorophyll loss usually is indicative of the pollutant causing the damage. Necrotic tissue generally is an indication of an acute exposure to high concentrations of plant toxins (phytotoxins). Necrosis can be observed as flecks on leaf surfaces, leaf spotting and discoloration of leaves.
Measurement and physiological testing are required to determine subtle effects. The subtle effects may be manifested as genetic changes, changes in photosynthetic process, and reduction in growth. Subtle effects generally occur as a result of long-term (chronic) exposure to lower concentrations of pollutants.
Air Pollution Impacts on Animals
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Air pollution can cause a variety of illnesses and injuries to animals. The highest number of injuries caused by air pollutants in domestic animals results from exposure to fluoride. Fluorosis (fluoride toxicity) usually results from animal consumption of plants that have been exposed to gaseous fluoride pollution. Fluorosis in animals can show up as weakening of teeth, thickening of bones and calcification of ligaments. These injuries can lead to lameness, reduced food intake, weight loss and reduced milk production in dairy cows. Lead is the second most common air pollutant affecting animals. Animals generally are exposed to lead when they eat foliage contaminated with lead dust. High lead exposure can result in death. A variety of symptoms are seen when acute or chronic lead exposure occurs. Animals with acute lead exposure may exhibit unusual thirst, delirium, loss of appetite, constipation and lower milk production. Chronic lead poisoning can lead to symptoms that include nervous system damage, diarrhea, lethargy, loss of coordination, constipation, reduced coat quality and weight loss. Poisoning with other elemental air pollutants such as arsenic and selenium will produce similar symptoms to lead poisoning.
Air Pollution Impacts on Humans
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Human biological response to air pollution can range from irritation to death. It is hard to determine what illnesses and injuries are the sole result of exposure to air pollutants. Human studies are complicated by other exposures and the fact that human illnesses and injuries are treated before terminal results are observed. It is known that air pollution can cause eye and skin irritation, respiratory distress and cardiac disease. It is also thought that air pollutants can contribute to lung cancer, asthma, emphysema and even deafness. Much more study is required to determine the impact of air pollutants on humans because of the difficulty in the collection and analysis of exposure data.
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