Estimates of exposure levels posing minimal risk to humans (MRLs) have
been made, where data were believed reliable, for the most sensitive noncancer
effect for each exposure duration. MRLs include adjustments to reflect human
variability from laboratory animal data to humans.
Although methods have been established to derive these levels (Barnes et
al. 1988; EPA 1989a), uncertainties are associated with these techniques.
Furthermore, ATSDR acknowledges additional uncertainties inherent in the
application of the procedures to derive less than lifetime MRLs. As an
example, acute inhalation MRLs may not be protective for health effects that
are delayed in development or are acquired followLng repeated acute insults,
such as hypersensitivity reactions, asthma, or chronic bronchitis. As these
kinds of health effects data become available and methods to assess levels of
significant human exposure improve, these MRLs will be revised.