I-LJ.Jd
GUEST EDITORIAL
Toxicology of acid rain Are we overlooking the obvious?
In recent years, scientists and politicians from many countries have voiced their concern regarding the deleterious environmental consequences of acid rain. Current research efforts in North America and Europe are directed toward determining both the direct and the indirect effects of increased environmental acidification on various aquatic and terrestrial species. Substantial increases in acidity per se (the direct effect) are obviously injurious to many life forms. But the indirect or secondary toxicological manifestations of acid deposition by their very nature tend to be more insidious and therefore may escape our recognition and investigation. It seems that the only indirect toxicological effects of environmental acidification receiving any attention to date are those caused by higher concentrations in acidified water of certain metals that have leached out of surrounding geological formations or conducting pipes made of aluminum or copper, for example. Although the potential toxicological problems caused by increased aqueous concentrations of inorganic substances merit extensive study, an area of possibly even greater importance has not received proper consideration. Specifically, I refer to the predictable changes in the toxicity of many lipophilic organic compounds concomitant with changes in hydrogen ion concentration. A fundamental tenet of pharmacology and toxicology states that only the un-ionized form of a lipid-soluble organic (xenobiotic) molecule will diffuse across biological membranes. Therefore, of the many lipophilic organic compounds-whether they are anthropogenic or natural in origin-those that are weak acids will be less ionized and as a result capable of an enhanced bioavailability resulting in relatively greater toxicity in an acidified environment. In contrast, the toxicity of weak organic bases should theoretically diminish as the pH decreases, whereas neutral organic compounds will be unaffected by changes in pH. 0013-936X/8610920-0211$01.5010
I 1986 American
Chemical Society
The implications of increased acidification of the environment on the toxicity of many organic compounds to be found therein need to be seriously considered. Potential pH-dependent changes in xenobiotic toxicity may suggest alternative explanations for those detrimental effects on our forests and fish species thought to be attributable to acid deposition that have not been fully elucidated by existing theoretical and experimental approaches to the problem. Moreover, because of regional differences in the extent of environmental acidification, it is reasonable to assume that certain organic pollutants may require geographically differing regulatory standards with respect to their levels of containment and use. Pesticide application rates are one obvious example in this regard. The relationship between the potential toxicity of a weakly acidic or basic lipophilic xenobiotic species and the pH of its milieu is well established in the toxicology laboratory. Although complicated when considered outside the laboratory setting because of the association of lipophilic organic substances with particulate matter, it is high time, nevertheless, that we recognize that this basic phenomenon may be of major relevance to a more complete understanding of the toxicology of acid rain.
Acudiu Uniwr.sir? in Now Scofiu and ir editor o f f h e Inlerna~ionelJournal of
Immuoopharmacology. Environ. Sci. Technol., Vol. 20,NO.3. 1986 211
