New Mexico Citizens
for Clean Air & Water
Like most things, words keep evolving. Today, “synergism” describes how some pollutants do more harm mixed together than the sum of their harms alone. We breathe pollutants mixed together much more than we get them one at a time. A bad gas alone might be breathed in and out with little harm, but if the gas comes adsorbed on fine particles, it can remain in the lungs and do damage. The term is synergism.
Mixtures are everywhere. What can be brewed from a mix of emissions from cars, trucks, factories, businesses (dry cleaners, paint shops, gas pumps), fires for every use and no use, plus air, dew and sunlight? The mixtures are as varied as germs.
Sorting out how groups of pollutants affect people is an infant science. So for now, the legal limit for each pollutant in the air is set without regard for any other pollutant.
The problem is the vastness of the subject and the early stage of the science. This fact is recognized by the U.S. Environmental Protection Agency and the U.S. Agency for Toxic Substances and Disease Registry. Their guidance can be found on the Internet, search at “EPA, risk assessment, synergism” or “ATSDR, risk assessment, synergism.”
To start, try the link to “Basic Concepts for Mixtures Risk Assessment.” The slowly growing understanding of risk is discussed. The EPA first issued risk assessment guidelines for mixtures in 1989, with an update in 2000 and a formal framework for assessing cumulative risk in 2003.
The EPA primer says, where data for a mixture are lacking, possible synergistic effects are not included. The text says, “We usually assume each chemical would cause the same harm regardless of whether it is in the mixture or by itself.” The EPA’s issuing guidelines for treating mixtures is part of its federal role to standardize methods of analysis. The benefits of standardizing get little attention in the public forum.
Standard methods have two strong points and a weakness. A great strength is they provide a basis for comparing public risks from various sources. To the extent that different methods are used in assessing two cases, the results tell us nothing about the relative risk in each situation. Comparative risk, or relative risk, as a rule is more meaningful and accurate than absolute risk.
The point against standard methods is time—the time EPA requires to review all the studies on a particular pollutant or mixture. No single study ever answers all the fair questions. Judging sufficient studies takes forever to finish.