How do researchers determine how toxic a chemical is? A toxicologist explains alternatives to animal testing

A vast number of chemicals are registered for production and use around the world. But only a portion have been thoroughly evaluated for their toxicity due to time, cost, ethical concerns and regulatory limitations.

To safeguard public health, researchers at organizations such as the U.S. Environmental Protection Agency, U.S. Food and Drug Administration and European Chemicals Agency evaluate the safety of the potentially hazardous chemicals people are likely to come into substantial contact with. These include volatile organic compounds such as formaldehyde, air pollutants such as nitrogen dioxide, consumer chemicals such as bisphenol A, and herbicides such as atrazine. Recently, “forever chemicals” that persist in the environment, such as perfluorooctanesulfonic acid (PFOS) and perfluorobutane sulfonic acid (PFBS), have been the focus of human toxicity assessments.

There are thousands of other chemicals used by industry that haven’t been thoroughly tested. To be efficient and cost-effective, these chemicals are prioritized for targeted testing. I am a toxicologist who studies how chemicals affect human health, particularly when they cause harmful effects. Better understanding the process of determining the toxicity of chemicals could help make them safer.

Chemical safety and toxicity testing

Historically, researchers have tested the safety and toxicity of chemicals by using biological assays, or bioassays. These tests involve exposing nonhuman animals – often rodents such as rats or mice – to a substance in controlled conditions to study its biological effects, including its potential harms.

Different types of studies are designed to analyze different effects from chemicals. These include immediate effects, effects from both short-term and long-term exposure, and reproductive or developmental effects. The key premise in using bioassays in animals is that researchers can use the results to help understand the chemical’s safety for people.

There are, however, significant limitations in using animals to conduct these studies.

First, it can be difficult to extrapolate results obtained from lab animals to humans. There are notable differences in anatomy, physiology, biochemistry and genetics between laboratory animals and people. In some cases, a chemical that is highly toxic to humans may be relatively harmless to other species. Moreover, even within a given species, there can be significant differences in how the body breaks down molecules, a process critical to determining a chemical’s toxicity.