Current assessment practices for noncancer end points.

The need for assessing noncancer risks for agents to which humans are routinely exposed indoors arises from the large amount of time spent indoors (i.e., employed persons spend about 60% of their time at home indoors, 30% at work indoors, and 5% in transit). Sources of air pollutants include heating and cooling systems, combustion appliances, personal use products, furnishings, tobacco products, pesticides, bioeffluents from humans and animals, and other microbial contamination such as toxins from molds. The purpose of this paper is to describe current dose-response assessment methods applicable to assessing risk following exposure to indoor air pollutants. The role of structure-activity relationships in hazard identification is also described. Risk assessments from exposure to indoor air pollutants require exposure assessments and dose-response assessments. Dose-response assessment methodologies include the inhalation reference concentration (RfC), structure-activity relationships, dose-response models, and the decision analytic approach. The RfC is an estimate (with uncertainty spanning perhaps an order of magnitude) of a daily exposure to the human population (including sensitive subgroups) that is likely to be without an appreciable risk of deleterious effects during a lifetime. The current RfC method provides guidelines for making the necessary dosimetric adjustments for gases and aerosols. Human equivalent concentrations for no-observed-adverse-effect levels in animals are determined by using mathematical relationships that adjust for regional deposition, solubility, ventilation rate, and blood:air partition coefficients. The RfC methodology exists as an interim methodology. Future scientific advancements are expected to further refine the approach.