The Public Health Challenge

People exposed to toxic substances may develop respiratory illnesses, immune system disorders, nervous system damage, cancer, birth defects, or other adverse health effects. However, in environmental health, cause and effect relationships are difficult, if not impossible, to prove. Uncertainties about research findings may occur because of long latency periods, multiple exposures, and multiple causes of the diseases or disorders. Establishing a link between an exposure and a health effect is therefore challenging, and planning appropriate public health actions becomes equally challenging.

Health Effects and Hazardous Substance Exposure

The relationship between hazardous substance exposure and the occurrence of adverse health effects depends on the sequence of events depicted in the following Environmental Health Model. Adverse health effects may occur when a hazardous substance is taken into the body through the mouth, eyes, respiratory tract, or skin, and the hazardous substance or its metabolites are delivered to a target organ where a physiologic change(s) occurs that leads to disease.

Providing Accurate Health Risk Assessments

The ability of a chemical to cause adverse health effects depends on a number of factors. Determining how much harm is likely to result from a given level of exposure over a given amount of time is not an easy task. The health effects associated with a chemical may differ depending on

• the level or “dose” of exposure
• the time of exposure
• age
• sex, or
• current health status

Determining toxicity is made even more complicated because many chemicals are only tested in animals, and the toxicity of a substance in animals may not accurately predict the effects in humans.

Additional Considerations

In addition, assessing the health effects of long-term, low-level exposure to a substance is more complex than assessing the health effects of short-term, high-level exposure. With chronic exposure, there is often a long latency period between one’s initial exposure and the onset of disease - signs and symptoms of illness may not occur for years. Moreover, the signs and symptoms may resemble other diseases and be the result from multiple environmental exposures. As a further complication, compounds (a combination of chemicals) may interact with each other and cause harm only when both chemicals are present. In addition, a compound may be harmful only at a specific life stage.