Could the contaminant cause the health outcome?
Different chemicals may cause very different health outcomes. In an environmental study, it must be scientifically reasonable to think that the exposure to the contaminants could cause the health problems that a community is concerned about.
If exposure to a particular exposure is consistent with what we know about how a particular disease develops, then the connection between the exposure and the disease is said to be biologically plausible. To determine biological plausibility, scientists explore past research findings looking for evidence of a biological mechanism connecting the exposure to the disease. So if there was contamination, and people were actually exposed to it, then we must also ask: is the association between the exposure and the health outcome biologically plausible?
Suppose a community is concerned about reproductive health effects resulting from exposure to the pesticide atrazine, which is commonly used in the U.S. Suppose atrazine was detected in the community's drinking water and people drank the water. Before proceeding with a health study, it is important to know if it is biologically plausible that exposure to atrazine would result in reproductive health effects. To investigate the possible connection, one would look at published research findings.
Based on animal studies, scientists suspect that atrazine may disrupt levels of human sex hormones. Animals exposed to atrazine had imbalanced levels of sex hormones. In addition, experiments on animal cells have shown that atrazine increases the production of an enzyme called aromatase. Aromatase is responsible for converting androgens to estrogen. In one study, researchers observed that atrazine resulted in the male frogs developing female characteristics. Researchers believe that atrazine is increasing aromatase levels, and thus increasing estrogen levels in male frogs.
Scientists do not necessarily suspect that atrazine is causing such extreme effects on sexual differentiation and development on humans as was seen in animals. However, there is continued concern about the nature of atrazine's impact on human reproductive health. In terms of biological plausibility, it would be reasonable to carry out a health study further investigating the link between atrazine in the drinking water and reproductive health of the community.