Table 1.

Common toxicity values used in risk assessment for dose-response assessment

Toxicity Valuesa	Definition	Population Variability Addressed?
NOAEL:  No Observed Adverse Effect Level	The highest exposure level at which there are no biologically significant increases in the frequency or severity of adverse effect between the exposed population and its appropriate control.	No.
BMD(L):  Benchmark Dose (Lower confidence limit)	A dose of a substance that when ingested produces a predetermined change (“benchmark response”) in the response rate of an adverse effect relative to the background response rate of this effect.	No.
RfD:  Reference Dose	An estimate of the dose of a substance (with uncertainty spanning perhaps an order of magnitude) to which a human population can be exposed (including sensitive subgroups) that is likely to be without an appreciable risk of deleterious effects during a lifetime.	Uncertainty factor (10-fold) for human variability.
MRL:  Minimal Risk Level	An estimate of the daily human exposure to a hazardous substance that is likely to be without appreciable risk of adverse non-cancer health effects over a specified duration of exposure.	Uncertainty factor (10-fold) for human variability.
OSF:  Oral Slope Factor	An upper-bound estimate of risk per increment of oral dose that can be used to estimate risk probabilities for different exposure levels.	Increased risk at early life stages for mutagenic compounds.
OED(L):  Oral Equivalent Dose (Lower confidence limit)	Daily oral dose necessary to produce steady-state in vivo blood concentrations equivalent to the AC50 (concentration at 50% of maximum activity) or LEC (lowest effective concentration) values in the in vitro assays.	Modeled toxicokinetic variability.
HDMI:  “Target Human Dose” for a specific Magnitude and Incidence	The human dose at which a fraction I of the population shows an effect of magnitude (or severity) M or greater (for the critical effect considered).	Probabilistic factor separately addressing variability and uncertainty.

Sources: (ATSDR 2013; U.S. EPA 1989, 2002., 2005., 2012; Wetmore 2015; WHO/IPCS 2014)

^aA numerical expression of the dose-response relationship that, when combined with exposure, gives information to characterize risk.