Contamination

An enormous variety of chemicals occurs naturally in the environment. For example, all of the 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 non-living environment with all of the natural elements. This includes all of the 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 non-organic 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 of these top predators, such as raptorial birds and some marine mammals, have suffered toxicity 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 a trace contamination. 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.