What is the primary purpose of a reference concentration (RfC) in toxicology?

• A. A measure of dermal absorption efficiency
• B. A threshold inhalation exposure level considered safe for chronic exposure used to assess non-cancer risk via inhalation
• C. A regulatory limit enforced in workplaces
• D. A maximum concentration used for cancer risk assessment

Answer: (B)

RfC is a reference concentration for inhalation: a long-term safety threshold for airborne contaminants that represents a level unlikely to cause non-cancer health effects from lifetime exposure. It guides non-cancer risk assessment by providing a conservative air concentration against which measured or modeled concentrations can be compared. Values are derived from toxicology data (such as NOAEL/LOAEL) using uncertainty factors to protect sensitive populations, and are expressed in mg/m3 (or ppm). It is not a regulatory limit and not used for cancer risk assessment (that uses inhalation unit risk or cancer slope factors) or a measure of dermal absorption. If ambient concentrations stay below the RfC, non-cancer risk is considered low; if they approach or exceed it, further evaluation or controls may be needed.