Table 2.

Tiered approach for selecting acute and chronic non-cancer health guidance values (HGVs).

Tier	Source	Health Guidance Value
Tier I	U.S. EPA’s Integrated Risk Information System (IRIS)	Chronic: Reference Concentration (RfC) Cancer: Inhalation Unit Risk (IUR)
Tier II	Center for Disease Control—Agency For Toxic Substances and Disease Registry	Acute & Chronic: Minimal Risk Level (MRL)
Tier III	US EPA Peer-Reviewed Toxicity Values (PPRTV’s)	Chronic: PPRTV
Tier IV	California EPA (Cal EPA)	Acute and Chronic: Reference Exposure Level (REL) Cancer: Inhalation Unit Risk (IUR)
Tier V	Texas Commission on Environmental Quality (TCEQ)	Short & Long-Term: Air Monitoring Comparison Value (AMCV)
Tier VI	Surrogate approach	Not applicable