Indirect Effects of Toxicants - Why Contaminants Shown to Be Nontoxic to Animal Cells Can Cause Harm?

One occasionally sees reports, which clearly show that the concentrations of toxicants in the environment are a million times smaller than those required to cause any effects in animal cells. Yet, other equally carefully done studies show that animals are affected by the concentrations occurring in the environment. This discrepancy often occurs in the case of herbicides. Nowadays they are usually molecules, which do not take part in any metabolic pathways of animal cells.

The simplest explanation for indirect effects in the case of herbicides is that changes in vegetation affect the well-being of animals by affecting the possibilities for hiding, by changing visibility etc. However, occasionally such effects are not adequate to explain  observations.

In such cases it is possible that the toxicant affects the microbiota of animals, and the changes occurring are such that the overall health of the animal is affected. We and all animals have billions of micro-organisms in our body. Some of the organisms have the pathways targeted, e.g., by herbicides. Consequently the microbial communities in animal body will be modified. Research has already indicated that changes in microbiota can influence immunity, digestion, nervous system, affect allergies etc...

An important point to note here is that if only cellular toxicicity is studied, the results indicating that the toxicant is non-toxic to animal cells is correct. It is only, because the microbiota of the animal is affected, and the poorly known signals, required for good animal health, influenced that adverse, indirect effects on animals are seen. This being the case, cell toxicological studies cannot replace studies on intact animals, when trying to uncover the risks of a chemical to the environment.