Tales of Our Times
 

 

 

“Synergism” has a history that stretches from the Church of 150 AD to news about pollutants. The word comes from the Greek “synergos,” or “working together.” “Synergistic” describes medicines that do more good together than the sum of their separate effects.

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.

 

But there is more to say. Most data on the health effects of pollutants—the data used to set legal limits—come from urban areas. In cities, pollutants are all mixed together. By this circumstance, synergism among mixed pollutants affects the limits set for separate pollutants in the air. In effect, the set limits provide more safety than if pollutants really were judged separately. It is a nice gain, but does not come by intent. We have a poor idea of how much synergistic effect is included in the limits set today.

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 second strength of standard methods is they prevent any particular side of an issue from steering a risk assessment to suit their interests. Through the years, examples of such steering are plentiful on all sides of issues. The EPA’s guidelines guard against self-serving answers.

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.

 

To sum up where things stand: Risk assessments typically evaluate risks from distinct chemicals. As the science continues to grow, interactions among chemicals will be used to make better assessments.