Contamination refers to the occurrence of some substance in an environment. It may be present in a larger than normal concentration, but contamination is usually said to occur when the concentration is smaller than that at which measurable biological or ecological damage can be demonstrated. Contamination is different from pollution, which is judged to occur when a chemical is present in the environment at a concentration greater than that required to cause damage to organisms. Pollution results in toxicity and ecological change, but contamination does not cause these effects because it involves sub-toxic exposures.
Chemicals that are commonly involved in toxic pollution include the gases sulfur dioxide and ozone, elements such as arsenic, copper, mercury, and nickel, pesticides, and some naturally occurring biochemicals. In addition, large concentrations of nutrients such as phosphates and nitrates can cause eutrophication (over-enrichment), another type of pollution. All these pollution-causing chemicals can occur in the environment in concentrations that are smaller than those required to cause toxicity or other ecological damage. Under these circumstances, the chemicals would be regarded as contaminants.
Modern analytical chemistry has become extraordinarily sophisticated. As a result, trace contamination by potentially toxic chemicals can often be measured in amounts that are much smaller than the thresholds of exposure, or dose, that are required to demonstrate physiological or ecological damage.
An important notion in toxicology is that any chemical can poison any organism, as long as a sufficiently large dose is administered. In other words, all chemicals are potentially toxic, even water, carbondioxide, sucrose (table sugar), sodium chloride (table salt), and other substances that are routinely encountered. However, exposures to these chemicals, or to much more toxic substances, do not necessarily result in a measurable poisonous response, if the dose is small enough. Toxicity is only caused if the exposure exceeds physiological thresholds of tolerance. According to this interpretation of toxicology, it is best to refer to “potentially toxic chemicals” in any context in which the actual environmental exposure to chemicals is unclear, or when the effects of small doses of particular chemicals are not known.
However, it is important to understand that there is scientific controversy about this interpretation. Some scientists believe that even exposures to single molecules of certain chemicals are toxicologically significant, and that dose-response relationships can therefore be extrapolated in a linear fashion to a zero dosage. This might be especially relevant to some types of cancers, which could theoretically be induced by genetic damage caused by a single cell and potentially caused by a single molecule of a carcinogen. This is a very different view from that expressed above, which suggests that there are thresholds of physiological tolerance that must be exceeded for toxicity to occur.
The notion of thresholds of tolerance is supported by several lines of scientific evidence. It is known, for example, that cells have some capability to repair damage to DNA (deoxyribonucleic acid), suggesting that minor damage caused by toxic chemicals might be tolerated because it could be repaired. However, major damage could overwhelm the physiological repair function, indicating a threshold of tolerance.
In addition, organisms have physiological mechanisms for detoxifying many types of poisonous chemicals. Mixed-function oxidases (MFOs), for example, are a class of enzymes that are especially abundant in the liver and, to a lesser degree, in the bloodstream of vertebrate animals. Within limits, these enzymes can detoxify certain potentially toxic chemicals, such as chlorinated hydrocarbons, by breaking them into simpler, less toxic substances. MFOs are inducible enzymes, meaning that they are synthesized in relatively large quantities when there is an increased demand for their metabolic services, as when an organism is exposed to a large concentration of toxic chemicals. However, the MFO system’s ability to deal with toxic chemicals can be overwhelmed if the exposure is too intense; this represents a toxicological threshold.
Organisms also have some ability to deal with limited exposures to potentially toxic chemicals by partitioning them within tissues that are not vulnerable to their poisonous influence. For example, chlorinated hydrocarbons such as the insecticides DDT and dieldrin, the industrial fluids known as polychlorinated biphenyls (PCBs), and the dioxin TCDD are all soluble in fats, and therefore are mostly found in the fatty tissues of animals.
Within limits, organisms can tolerate exposures to these chemicals by immobilizing them in fatty tissues. However, toxicity may still result if the exposure is too great, or if the fat reserves must be mobilized to deal with large metabolic demands, as might occur during migration or breeding. Similarly, plants have some ability to deal with limited exposures to toxic metals by synthesizing certain proteins, organic acids, or other biochemicals that bind with the ionic forms of metals, rendering them much less toxic.
Moreover, all of the chemicals required by organisms as essential nutrients are toxic at large exposures. For example, the metals copper, iron, molybdenum, and zinc are required by plants and animals as micronutrients. However, exposures that exceed the therapeutic levels of these metals are poisonous to these same organisms. The smaller, sub-toxic exposures would represent a type of contamination.
An enormous variety of chemicals occurs naturally in the environment. For example, all natural elements are ubiquitous, occurring in all aqueous, soil, atmospheric, and biological samples in at least a trace concentration. If the methodology of analytical chemistry has sufficiently small detection limits, this ubiquitous presence of all of the elements will always be demonstrable. In other words, there is a universal contamination of the living and nonliving environment with all of the natural elements. This includes all potentially toxic metals, most of which occur in trace concentrations.
Similarly, the organic environment is ubiquitously contaminated by a class of synthetic, persistent chemicals known as chlorinated hydrocarbons, including such chemicals as DDT, PCBs, and TCDD. These chemicals are virtually insoluble in water, but they are very soluble in fats. In the environment, almost all fats occur in the biomass of living or dead organisms, and, as a result, chlorinated hydrocarbons have a strong tendency to bioaccumulate in organisms, in strong preference to the nonorganic environment. Because these chemicals are persistent and bioaccumulating, they have become very widespread in the biosphere. All organisms contain their residues, even in remote places such as Antarctica.
The largest residues of chlorinated hydrocarbons occur in predators at the top of the ecological food web. Some top predators, such as raptorial birds and some marine mammals, have suffered as a result of their exposures to chlorinated hydrocarbons. However, toxicity has not been demonstrated for most other species, even though all are contaminated by various of the persistent chlorinated hydrocarbons.
One last example concerns some very toxic biochemicals that are synthesized by wild organisms, and are therefore naturally occurring substances. Saxitoxin, for example, is a biochemical produced by a few species of marine dinoflagellates, a group of unicellular algae. Saxitoxin is an extremely potent toxin of the vertebrate nervous system. When these dinoflagellates are abundant, filter feeders such as mollusks can accumulate saxitoxin to a large concentration, and these can then be poisonous to birds and mammals, including humans, that eat the shellfish. This toxic syndrome is known as paralytic shellfish poisoning. Other species of dinoflagellates synthesize the biochemicals responsible for diarrhetic shellfish poisoning, while certain diatoms produce domoic acid, which causes amnesic shellfish poisoning. The poisons produced by these marine phytoplankton are commonly present in the marine environment as trace contaminations. However, when the algae are very abundant, the toxins occur in large enough amounts to poison animals in their food web, causing a type of natural, toxic pollution.
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