Several factors must be in place and connected for persons to be harmed by something in the environment.

1. A source of harm must exist, which has chemical and/or physical properties that make it persistent and/or transportable (volatile, water soluble) in the environment.

2. An environmental medium is necessary for transport: air, surface water or groundwater, soil, or combinations of the three.

3. A receptor population must be within the exposure pathway for harm to human health. Potential population exposures are important if the contaminant is persistent and land/property use changes over time.

4. Route of exposure.. Humans can be exposed to environmental contaminants via three routes: inhalation, ingestion, and dermal absorption. Inhalation requires the contaminant to be within the breathing space. Ingestion may occur through drinking contaminated water or milk, eating contaminated plants, animals, or soil. Dermal absorption requires direct contact with a contaminant that is capable of penetrating the skin.

5. The final ingredient for exposure is dose. There must be an adequate amount of the chemical to result in human harm. Almost any chemical can be harmful in large enough doses. We practice the dosing principle by careful instructions to patients regarding prescribed medications. The converse is also true; very small amounts of even the most toxic materials can do no or immeasurably small harm.