Life at the Intersection of Engineering and Public Health: Bioaccumulation of Chemical Contaminants in our Ecosystems

Have you seen the warnings on PFAS in drinking water? If you haven’t yet, you probably will in the future. PFAS (polyfluoroalkyl/perfluoroalkyl substances) encompass a class of chemicals (around 4700 actually) that are broadly used in such things as commercial products, food storage and firefighting foams. The chemicals have been used for decades and are extremely durable with many features that make them attractive for a wide range of products. Consequently, they are widely distributed in society creating exposures from many different sources. This was a primary reason that concerns were raised. Teflon and Scotchgard are well known examples.

Unfortunately, many of these same properties create concerns from both a public health and environmental health perspective. While production of many (but not all) of these chemicals has been curtailed in the last 10 years, the potential health impacts of the residuals remain. The fact that it is not unusual to be found in human blood serum samples and drinking water supplies raised concerns. Various monitoring reports, toxicological studies and risk assessments suggest that the broad exposure to PFAS should be controlled, especially in drinking water. To that end the EPA has set a non-enforceable target limit of 70 ng/L. For reference, one nanogram per liter (ng/L) is equivalent to 1 part per trillion. That’s not very much…roughly one grain of sugar in an Olympic sized swimming pool. You might ask, ‘how can chemicals measured in ‘parts per trillion’ be that much of a problem’?

The answer lies in how a chemical moves through the environment and/or your body. A key question involves considering whether the chemical bioaccumulates in the ecosystem. In other words, does the chemical remain in the environment and move through the food chain, or does it naturally degrade and decompose? Also, are the residual products harmful or benign? While short term exposure may not be a problem, the long-term buildup in the environment (or your body) can increase concentrations over time to the point that they are an issue.

We might find that interesting, but what does that have to do with harm to our personal health or damage to the environment? By looking at chemicals this way we can begin to understand why certain chemicals in the environment, even in seemingly small amounts, can actually result in harming us.

Biologists and toxicologists have identified at least four factors which help provide clues as to what contaminants have the most potential for causing harm, especially through our food. Generally, it must have the following characteristics:

If the chemical contaminant is short-lived, it will degrade before it can become dangerous because it won’t be able to interact with a large portion of the population. If it’s not mobile, unless there is just a massive amount of the material in the environment, it will be too dispersed for a population of organisms to efficiently consume it. If the contaminant is water soluble it will tend to be routinely excreted by the organism waste functions and not able to accumulate or concentrate. However, if the chemical is soluble in fat, it can be stored for long periods of time in the fatty tissue of the organism. As a side not this is one reason that environmental toxicants can show up in female milk and thus affect the very young who often are more susceptible to damage by toxins. Finally, a contaminant must be biologically active (can cause changes) for it to be significant from a health perspective. With that background, we’re able to search for where chemicals go (i.e. the fate of chemicals) and how they can enter the food chain to get a sense of the problem.

As organisms which are lower in the food chain metabolize the chemical, it is absorbed in the fat tissue and begins to concentrate. Thus, it is said that the body burden of the chemical increases. If they continue their intake of the chemical it will begin to bioconcentrate because the body can’t eject it efficiently from the fatty tissue. Now the concentration of the chemical may not harm the organism at all because it is stable in the fatty tissue and doesn’t interfere with the normal body functions, but it exists as a body burden.

At this point a very significant thing happens. As one moves up the trophic levels (food chain) the new organism will be consuming organisms (food) that has the chemical bioconcentrated in the fatty tissue. This means that in addition to any intake that may occur directly from the environment, the new organism receives a more concentrated dose from the lower trophic level. One rule of thumb is that the concentration increases in the fat 10-fold for each rise in trophic level of the food chain.

There is much that is not understood in these extremely complex interactions. However, the basic concepts have been recognized and on-going research may help untangle the fate of each chemical in its path through the environment. In the interim regulatory agencies must attempt to develop regulations that balance the beneficial use of chemicals with their adverse and potentially long-term reactions. This is a difficult task in the face of the myriad of unknown factors in each case. In the face of this uncertainty the so called “precautionary principle” is invoked and a conservative ruling is passed based of expert assessments. Currently this is where PFAS regulation development is positioned.

Stayed tuned for further communications on the status of PFAS impacts and regulation as it evolves.