Minimal risk level (mrl), английский An atsdr estimate of daily human exposure to a hazardous substance at or below which that substance is unlikely to pose a measurable risk of harmful (adverse), noncancerous effects. mrls are calculated for a route of exposure (inhalation or oral) over a specified time period (acute, intermediate, or chronic). mrls should not be used as predictors of harmful (adverse) health effects [see reference dose].