Threshold (in health science)

The lowest dose of a chemical at which a specified measurable effect is observed and below which it is not observed.

Source: National Safety Council Environmental Glossary  

Examples include LOAEL (Lowest Observed Adverse Effect Level) and NOAEL (No Observed Adverse Effect Level).

When such thresholds are identified, they are usually used to set health standards.

Source: National Safety Council Environmental Glossary  

Ozone

Particulate Matter

Nitrogen Dioxide

Evaluation of endocrine disruptors