Toxic Tuesdays

CHEJ highlights several toxic chemicals and the communities fighting to keep their citizens safe from harm.

How the Duration of Exposure Affects Toxicity

CHEJ has previously written about the importance of considering multiple chemical exposures when assessing the toxicity of exposure to toxic chemicals. In addition, it is also important to consider the duration of exposure. How long was a person exposed? What was the concentration of the substance(s) during this period of time are critical to truly understanding the cumulative effects that a person has suffered? Without this information, we can only partially understand the risks of exposure to toxic chemicals. 

Yet when evaluating whether exposures to toxic chemicals pose risks to human health, the government’s approach is usually very narrow: it assumes there is a single chemical from a single source at a single point in time with a single exposure pathway causing a single health effect. This approach makes risk assessment more feasible and understandable, but it does not reflect the reality of our lives.

In reality, we are exposed to multiple chemicals at a time and exposures can happen over a long period of time. This means that considering the potential effects of a single exposure to a single chemical isn’t sufficient for evaluating public health risks. We need to include cumulative risks that account for both multiple chemical exposures and exposure over time in order to begin to understand the risks to public health. But incorporating these parameters into a risk assessment poses significant new challenges that  requires more estimates and generates additional uncertainty than the traditional risk assessment approach.

In many cases, exposure assessments assume that exposure to a chemical happens in a single instant in time. In some cases – like cancer risk – EPA assumes that risk is proportional to the lifetime dose. In general, longer exposure means greater risk, but the relationship between duration of exposure and health risk is complicated. The risk depends on the effects an exposure has on the body and the body’s response to it. In some cases, the body may adapt to exposure and the threat over time may be less than additive. In other cases, the body may become more sensitive and the threat over time may be more than additive. Repeated exposure can also influence health risk: past exposure to some chemicals can make us more vulnerable to subsequent exposure. And how do you consider the effects caused by exposure to multiple chemicals that target the same organ in the body can cause more damage than exposure to any of those chemicals individually?

The effect of exposure over time is important to consider in risk assessments, but agencies like EPA and ATSDR do not have comprehensive frameworks for how to assess this cumulative risk. Part of the reason for this is a lack of data. The guideline values we use to evaluate risks are driven by data generated from exposures to a single chemical for a defined period of time. For common chemicals and chemical mixtures that people are exposed to, we need to know how different concentrations and durations of exposure affect health. There is a need for more scientific study on how exposures to chemicals over long periods of time can impact our risk for adverse health effects. Once people have been exposed to chemicals, we also need better tools to measure their past exposure so it can be accounted for in risk assessments.

While more research and data is crucial, there will always be uncertainty in science and data, and we cannot let uncertainty stop us from taking action to protect health. In addition to more scientific study, we need clearer guidelines from federal agencies regarding how to consider cumulative risk – both from multiple chemical exposures and exposures over time – in evaluating if an environmental hazard is a threat to human health. We also need to acknowledge how poor the tools we have are at considering cumulative risks caused by exposures to multiple chemicals over time.

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