The concentration of no toxicological concern (CoNTC): a risk assessment screening tool for air toxics.

Although numerous chemicals might occur in ambient air as a result of natural or anthropogenic activity (primarily through vehicle exhaust and industrial emissions), not all are necessarily of concern for public health even if they are classified as hazardous. There are many minor components in emissions that are predicted to be present at small concentrations. For the majority of these chemicals a health-based guideline does not exist to facilitate risk assessment. Furthermore, there are no appropriate toxicological or health data to enable health-based guidelines to be established. Consequently in most risk assessments these substances are usually, and conveniently, ignored. The tacit justification is that concentrations in ambient air are small and thus insignificant. For many stakeholders this is an inadequate explanation, and the justifiable question of how it is known exposures are insignificant for health is often asked. The concept of a "concentration of no toxicological concern" (CoNTC) was developed for air toxics and can be applied as a risk assessment screening tool to legitimately dismiss substances whose ground-level concentrations are predicted to be trivial. The CoNTC helps define trivial and is grounded in regulatory and scientific deliberations of the U.S. Food and Drug Administration (FDA) and the European Commission for developing concentrations of no toxicological or regulatory concern for contaminants in food. The suggested conservative generic CoNTC value that can be applied to most organic chemicals in air is 0.03 microg/m3. The derivation of the CoNTC and its validation and limitations are discussed, and its utility as a screening tool is presented.