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(polyaromatic hydrocarbons). Potential cancer risk factors are determined by both
approaches and both types of damage functions are important sources of uncertain-
ties — especially the question of extrapolation to lower doses and the correlations
made implying insecurity.

4.6.1 HUMAN HEALTH DOSE–RESPONSE AND EXPOSURE RESPONSE

Government agencies are charged with the protection of the public health and
ecology; however, the U.S. Environmental Protection Agency (USEPA), the Agency
for Toxic Substances and Disease Registry (ATSDR), and the U.S. Food and Drug
Administration (FDA) must have a reference, or comparison value, upon which to
base an evaluation of potential health threats posed by any substances or chemicals.
The bases, or starting points, for such estimations may have different names or
acronyms but they represent more or less the same thing. The risk of carcinogenic
pollutants is estimated with a factor of potential cancer; however, the risk of non-
carcinogenic pollutants is characterized by a reference dose. These values for non-
carcinogenic endpoints are called oral reference doses (RfDs) and inhalation refer-
ence concentrations (RfCs) by the USEPA, acceptable daily intakes (ADIs) by the
FDA, and oral and inhalation minimal risk levels (MRLs) by ATSDR (Table 4.2).

TABLE 4.2
Oral RfDs/ADIs/MRLs for Noncarcinogenic Effects of Some Different
Chemicals

Regulated substance RfDs (mg/kg-day) ADIs (mg/kg-day) MRLs (mg/kg-day)
Acetone 0.100 0.1 Oral int. 2
Cadmium 0.0005 5.10 –4 Oral chr. 0.0002
Chloroform 0.010 0.01 Oral acute 0.3
Oral int. 0.1
Oral chr. 0.01
1,1-Dichloroethylene 0.009 0.009 Oral chr. 0.009
Cis-1,2-Dichloroethylene 0.010 0.01 Oral acute 1
Oral int. 0.3
Methylene chloride 0.060 0.06 Oral acute 0.2
Oral chr. 0.06
Tetrachloroethylene 0.010 0.01 Oral acute 0.2
Oral chr. 0.06
Toluene 0.200 0.3 Oral acute 0.8
Oral int. 0.02
1,1,1-Trichloroethane 0.035 0.09 Inhalation acute 2 ppm a
Inhalation int. 0.7 ppm
Xylene 2.000 2.0 Oral int. 0.2
a Just available inhalation effects.
Note: Oral RfDs and ADIs refer to chronic effects. MRL values refer to intense, acute and chronic
effects.

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