We previously addressed individual variability and how it affects a person’s response to toxic chemicals. Another important factor in toxicology is a person’s individual sensitivity to chemicals. How sensitive a person is to chemical exposure helps determine how susceptible or vulnerable they are to toxic chemicals. Several factors determine how sensitive a person is including age, sex, health, genetics, diet, lifestyle, preexisting conditions and previous environmental exposures. While some people are more sensitive to chemical exposures than others, there is no clear definition of what sensitivity is or what it means. This is partially since so little is understood about the human response to toxic chemicals, especially to low level mixtures of chemicals.

Because of this uncertainty, there is no generally accepted definition of sensitivity. Nicholas Ashford and Claudia Miller describe the various meanings of the term. In traditional toxicology, sensitivity has been defined as individuals who require relatively lower doses to induce a particular response. These individuals are considered more sensitive than people who require relatively higher doses to experience the same response. The distribution of this population is described by the classic bell curve where the sensitive and resilient populations are found in the tails of the curve. Most people fall into this response category. In traditional medicine, sensitivity has been defined as individuals who have a significant and rapid immune-mediated response to an allergen or agent. In this population, some individuals, described as chemically sensitive, have a striking immune response to an allergen or agent, while non-allergic individuals do not, even at high doses. Classic allergens include ragweed or bee venom, but also include chemicals such as nickel or toluene diisocyanate (TDI).

In recent years, a growing population of people have expressed an entirely different sensitivity response. These are people who have developed multiple chemical sensitivities. Ashford and Miller found that people who have developed multiple chemical sensitivities may exhibit a third and entirely different type of sensitivity. These authors stated this about people with multiple chemical sensitivities (MCS): “Their health problems often (but not always) appear to originate with some acute or traumatic exposure, after which the triggering of symptoms and observed sensitivities occur at very low levels of chemical exposure. The inducing chemical or substances may or may not be the same as the substances that thereafter provoke or ‘trigger’ responses.” Unlike classical toxicological or immune mediated responses, people with MCS sensitivity respond in a two-step process of an initial exposure event followed by a second triggering exposure. Much still needs to be understood about this third wave of sensitivity.  

Another factor that influences a person’s sensitivity is the body’s reserve capacity. Researchers have speculated that a chemical exposure may affect the reserve capacity of the body without causing an immediate adverse effect. However, when there are subsequent exposures, the body becomes unable to compensate for the additional stress and toxicity develops.

The science behind what is known about how people respond to chemical exposures, especially to low level mixtures of chemicals, is highly complex and not well understood. We know that people exposed to low level mixtures of toxic chemicals, like the people in East Palestine, OH, the site of that horrific train derailment and subsequent intentional burn of vinyl chloride, continue to suffer adverse health effects despite reassurances from EPA and public health agencies who are relying on traditional toxicology and risk assessments. Perhaps the people in East Palestine have developed a unique chemical sensitivity much like the third wave described by Ashford and Miller. So as their exposures continue during the ongoing cleanup, their chemical sensitivity and the subsequent adverse health responses are not what would be predicted by traditional toxicology or medical models. 

This is an important consideration to consider in East Palestine because it is clear that we do not understand what is happening to the health of the people there. It’s time to recognize that we cannot rely solely on traditional toxicology to address the questions people have about exposures to low level chemical mixtures.