Toxic exposure to the environment primarily arises from industrial or agricultural sources, contaminating air, water, and soil.

Air Pollution
Smoke is the most common form of air pollution.  It presents complex toxicologic problems, given its numerous constituents.
One of the most common sources of air pollution in the U.S.A. is the incomplete combustion of fossil fuels arising from power stations, automobiles, and industry.  The primary constituents include carbon monoxide (52%), sulphur oxides (18%), hydrocarbons (12%), particulate carbon (10%), nitrogen oxides (6%), plus carbon dioxide and hydrogen sulphide.

The specific type and severity of air pollution is not only dependent upon the particular pollutant, but also the climate.  High levels of humidity, combined with temperature inversions and air pollution give rise to smog (smoke + fog = "smog"), which may present as one of two types.

Reducing Smog -- High in particulates and sulphur oxides.  Reducing smog arises from a combination of incomplete fossil fuel combustion, fog, and cool temperatures.  It is capable of reducing other chemicals, hence the name.

Photochemical Oxidant (Oxidising) Smog -- This is the type of smog commonly associated with the Los Angeles Basin.  It is high in ozone, nitrogen oxides, and hydrocarbons and arises from automobile exhaust, sunlight, and temperature inversion.  With the hydrocarbons acting as a catalyst, it results in the formation of ozone by converting NO₂ ------> NO + O and then O + O₂ ---------> O₃.  As the name implies, it is capable of oxidising chemicals, including components of the respiratory tract.

Either type of smog will have effects on humans, other animals, and plants, with the potential for causing death.  Some of the most lethal cases of smog include the Meuse Valley of Belgium, where in 1930, 65 people died as a result of pollution; Donora, Pennsylvania (1948) with 20 deaths, and London, England, where in 1952, a persistant smog resulted in 4000 deaths.  A second London fog ten years later caused an additional 400 deaths.  In each of these cases, there was a combination of high levels of pollution and temperature inversion, resulting in stagnant air and persistant smog.

Deaths may result from acute toxicity or from complications of existing conditions such as asthma, emphysema, and chronic obstructive pulmonary disease.

Epidemiological data indicates that chronic exposure to air pollution results in increased risk and incidence of pulmonary disease, cardiovascular disease, and lung cancer.  Additionally, it worsens pre-existing conditions (reducing smog has been positively correlated with chronic bronchitis) and decreases pulmonary function (oxidising smog has been shown to decrease lung function, using the FEVT test).

Specific air pollutants and their effects include:
Sulphur oxides -- Common Exposure Levels, 0.05-0.2 ppm; 1-5 ppm will cause upper respiratory tract irritation.

Nitrogen oxides -- Common Exposure Levels, 0.7 ppm;  5-10 ppm will penetrate deep within the respiratory tract, causing irritation.

Ozone -- Exposure to as little as 50 ppb will cause free radical formation.

Carbon monoxide -- Common Exposure Levels, 10-20 ppm; Exposure to 120 ppm for 1 hr or 30 ppm for 8 hr is considered dangerous and acute exposure to 75 ppm will alter cardiovascular function.  Some occupational exposure (automobile mechanics, traffic wardens) may be exposed to levels as high as 100 ppm.  Acute, high dose exposure will cause carboxyhæmoglobinæmia leading to tissue anoxia.  Chronic exposure will increase the incidence of cardiovascular disease, atherosclerosis, and hypertension.

 
Particulates -- Specific particles (especially those smaller that 10 microns) have been correlated with an increase in illness and hospital admissions as well as increases in overall morbidity and mortality.
 
Acid Rain
The term acid rain is most often used to refer to precipitation containing sulphuric and nitric acids.  However, there are other forms of "acid rain".  It is most often formed by a chemical reaction, as follows:
H₂O + SO₂ -----> H₂SO₄, with oxygen, ultraviolet light, and hydrocarbons as catalysts, and
H₂O + NO₂ -----> HNO₃, with oxygen, UV light, and ozone as catalysts.
As the acid is formed in the atmosphere, it may become solubilized in clouds, a process referred to as "washout".  As the acid is removed from the clouds with rain, it is called "rain out".  Other forms of "acid rain" include the direct deposit of SO₂ and NO₂ (which may then be converted to the acids) and the acids themselves on plants and soil.  This often occurs at higher elevations that may be enshrouded with clouds containing the pollutants (common examples of damage due to deposition of this type may be seen on Mt. Mitchell in North Carolina and in the mountains of upstate New York).  Acid rain is a world wide problem, with the pollutants often bourne by winds far from the point-source of pollution (deforestation by acid rain in Scandanavia from pollution arising in Great Britain).
Acid "snow" presents a another type of problem with acid damage.  During the winter, snow laden with acid precipitation may accumulate.  The onset of spring and melting snow allows the acid to be more concentrated in the runoff, increasing the amount of acid that enters streams, rivers, and soil.  In addition to the damage that occurs directly from acidic conditions, lower pH in the water may result in leaching of metals, thus increasing the risk of metal toxicity.
 
Deposition on soil may be buffered by certain soil types.  However, when the buffering capacity is exceeded or when the soil buffers poorly, the growth of plants may be interfered with or even inhibited.  Agricultural contribution to this type of acid damage may be substantial.

Lead
Environmental sources of lead include car exhaust, industry, batteries, cigarette smoke, and paint.  Permanent damage may result from one acute exposure or from chronic, low level exposure to lead.  The removal of lead from gasoline fuel in the U.S.A. has removed one of the primary sources of lead exposure.  However, the average body burden of lead in the U.S.A. is 0.3 mcg/ml of blood (0.15-0.7 mcg/ml range).  The threshold for toxicity is 0.8 mcg/ml and encephalopathic CNS damage may occur with blood levels as low as 1-2 mcg/ml.  For the physiologic effects of lead, refer to previous sections.
 

Water Pollution
The most common sources of water pollution are the effluent discharges of factories, industry, and domestic waste; contamination of soil with pesticides and fertilisers that may enter rain runoff into groundwater, streams, rivers, and ultimately the sea.  (An interesting aspect of this is the fact that DDT may be found in the Arctic Ocean.  Polar bears that have never lived in an area where DDT was used have bioaccumulated the pesticide simply from feeding on fish) Additional sources of water pollution include contaminated rain, air pollution; and stored waste which may leak into the water table.
 

Eutrophication
Eutrophication is a worst-case scenario that may result from water contamination with fertilisers, sewage, and organic waste products.  The initial exposure to these pollutants will provide a nutrient source for algae and other aquatic plants, resulting in their overgrowth.  This results in the "choking" of the local environment, using up all the available nutrients.  As the nutrients are depleted, the algae die off and begin to decay.  This process is in part accomplished by ærobic  bacteria which will deplete the dissolved oxygen of the water, allowing anærobic bacteria to continue the process.  These anærobic bacteria will produce toxins that, when released into the water, cause additional aquatic death.  The final result is a stagnant, lifeless body of water.

Other water contamination may occur with heavy metals, that are often not removed by standard purification techniques and flooding which may introduce pathogenic bacteria into drinking water supplies.  Also of primary concern following water contamination with environmental œstrogens are the reproductive effects on indigenous organisms (seen with the PCBs -- refer to previous sections including the use of vitellogenin as a biomarker of aquatic reproductive toxicity).

NOTE that biotransformation may occur in aquatic environments, resulting in both detoxification and bioactivation of pollutants.

Food Chains
There are two major classifications of food chains.
Grazing -- The grazing food chain has been alluded to in the section on food contaminants.  It is typified by the "standard" food chain -- i.e. bears eat fish that eat daphnids that eat contaminated plants.

Detritovore Food Chain -- This chain most often involves decaying organisms and usually refers to bacterial food chains.  It is typically limited to micro-organisms, but may contribute to the bioactivation of certain toxicants (i.e. the conversion of DDT to DDE).  Refer to the case study below for an example of the importance of the detritovore food chain in bioactivation of certain toxicants.

The primary factors of persistance of a toxicant in the food chain is high degrees of lipophilicity and metabolic preservation of the toxic effects.

The safety level of mercury ingestion is 0.1 mg/day, which would equate to approximately 200 Gm (7 oz.) fish with a mercury level of 0.5 ppm.  Some fish in Lake Michigan and of the coast of Sweden have mercury body burdens of 2000 ppm.  Additionally, the allowable water level of mercury is 5 ppb.  Water near the effluent of a battery plant in Michigan has tested for mercury at levels of 1000 ppm.  It is, therefore, possible to suffer severe mercury toxicity if fish from contaminated waters are eaten.

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