NOAEL and LOAEL are used in deriving reference doses. Which statement is correct?

• A. NOAEL is the highest exposure level with no observed adverse effects; LOAEL is the lowest exposure level with adverse effects; they are used with uncertainty factors to derive reference doses.
• B. NOAEL is the lowest exposure with no adverse effects; LOAEL is the highest exposure with adverse effects; they are used for cancer risk estimation.
• C. LOAEL is the highest exposure level with no adverse effects; NOAEL is the lowest exposure with adverse effects; they are not used in risk assessment.
• D. NOAEL and LOAEL are only applicable to acute, not chronic, risk assessment.

Answer: (A)

Understanding how NOAEL and LOAEL are used in risk assessment starts with their definitions. NOAEL is the highest dose at which no adverse effects are observed in the studied population, while LOAEL is the lowest dose at which adverse effects are observed. In deriving a reference dose for non-cancer risk, these points of departure are adjusted with uncertainty factors to account for differences between animals and humans, variability within human populations, and data gaps, yielding a safe daily exposure level. If only a LOAEL is available, additional uncertainty—often an extra factor—is applied to account for extrapolating from a dose where effects are already observed, which typically results in a more conservative (lower) reference dose. The concept aligns with non-cancer risk assessment; cancer risk uses different metrics (like slope factors) rather than NOAEL/LOAEL-based reference doses.