Toxicology, a critical science in environmental health, is the study of the negative effect of a physical stressor (chemical, biological, or radioactive) on a biological system (a cell, tissue, organ, organ system, or organism). The key variables in determining the relationship between an exposure to a stressor and a health effect are: 1) the "dose" of the exposure; 2) the "duration" of the exposure; 3) the "toxicity" or strength of the toxin; and 4) a variety of "host factors" (such as age, sex, health status, other exposures). Environmental toxins can enter the human body by ingestion, inhalation, and dermal exposure. People may ingest toxic chemicals in their drinking water, as well as foods and beverages. Air pollutants can affect many body systems. Some toxic exposures, such as solvents and some pesticides, can be absorbed through the skin.

In the same way that the desired effects of pharmacological agents are dose-dependent and depend on host factors, the effects elicited by toxic chemicals in our environment are dose-dependent and dependent on host factors. This concept is extremely important when discussing children's special vulnerabilities to environmental exposures because a number of variables influence the dose of toxic chemicals to which children are exposed.