VI. Risk Assessment

OSHA's approach to risk assessment is guided by Supreme Court interpretations of the OSH Act, namely decisions involving benzene (Industrial Union Department, AFL-CIO v. American Petroleum Institute, 448 U.S. 607 (1980)); and cotton dust (American Textile Manufacturers Institute v. Donovan, 452 U.S. 490 (1981)). The Court has ruled that OSHA may not promulgate a standard unless it has determined, based on substantial evidence in the record considered as a whole, that there is a significant risk of health impairment at existing permissible exposure levels and that issuance of a new standard is necessary to achieve a significant reduction in that risk. Although in the cotton dust case the Court rejected the use of cost-benefit analysis in setting OSHA standards, it reaffirmed its earlier holding in the benzene case that a risk assessment relating to worker health is not only appropriate, but is, in fact, required in order to identify a significant worker health risk and to determine whether a proposed standard will achieve a reduction in that risk. Although the Court did not require OSHA to perform a quantitative risk assessment in every case, the Court implied, and OSHA as a policy matter agrees, that such assessments should be put in quantitative terms to the extent possible (48 FR 17292).

Several approaches can be used to estimate cancer risk from exposure to toxic agents. A standard approach uses mathematical models to describe the relationship between dose (such as airborne concentration) and response (e.g., cancer). Generally, curves are fit to the data points observed at different exposure levels and these curves are used to predict the risk that would occur at exposure levels which were not observed. The shape of these curves is varied, ranging from linear extrapolations from the observed points through the origin (zero exposure and zero risk) to curves which may deviate far from linearity at the very highest and very lowest doses. The use of a particular model or curve can be justified in part by a statistical measure of "fit" to available data points, that is, a statistical test which measures how closely a predicted dose-response curve is to the actual observed data.

In all cases it is assumed that the mathematical curves are reflective of biological processes that control the biological fate and action of the toxic compound. To date, many of these factors have not been quantitatively linked to the mathematical models. Biological factors which may play important roles in the risk assessment are: (1) Dose of the material at the sensitive tissue; (2) the sensitive tissue(s) itself; (3) the nature of the response(s); (4) rates and sites of biotransformation; (5) toxicity of metabolites; (6) chronicity of the compound (cumulative nature of the material or its actions); (7) pharmacokinetic distribution of the material (especially effects of dose on the distribution); (8) the effect of biological variables such as age, sex, species and strain of test animal; and (9) the manner and method of dosing the test animals (48 FR 45969).

It is clear that all of these factors cannot be easily incorporated into a single mathematical model. Therefore, careful selection of the data and general assumptions necessary for evaluation in the model is important to the risk assessment in order to make use of as much information as possible.

In doing its risk assessment for MDA, OSHA has considered various assumptions that it believes to be the most reasonable. The risk estimates are found in Table 1 below (Table 1, Ex. 1-247). Some of the underlying assumptions used in predicting these risks are: (1) 100 percent GI absorption; (2) two 4-hour work shifts; (3) 2 percent dermal absorption rate; (4) body weight scaling factor; and (5) upper body absorption as set forth in Table 1. A body weight scaling factor is a quantative adjustment of the dose used in the NTP study to account for the differences in weight between humans and rodents.

Using these estimates of risk, approximately 6 to 30 per 1000 workers may be at risk of developing cancer when exposed at worst case existing conditions to MDA over a working lifetime (Table 1, Senario 1). OSHA also notes that these estimates of risk are not based on the application of a scaling factor based upon surface area. When this surface area scaling factor is applied, the estimates of risk significantly increase to ten times the risk levels shown in Table 1. OSHA did not adopt this scaling factor because there was no evidence that this was a more appropriate approach to use than the traditional body weight conversions used by OSHA.

In addition, OSHA notes that in making the estimates of risk, OSHA has gone beyond the traditional regulatory methodology and added to this assessment the estimates of risk which can be expected from dermal deposition. OSHA recognizes that substantial exposure may occur through deposition and subsequent absorption of MDA on the upper body, neck, etc., and has considered these confounding factors in assessing risk (in certain situations approximately 95 percent of exposure results from dermal absorption).

While OSHA was able to make estimates of risk which might result from dermal exposure, OSHA was unable to establish allowable dermal exposure limits. There are a number of reasons why this is impractical, among which are the difficulty of quantifying dermal exposures, the inability to select a reliable biological indicator, and finally the difficulty in correlating the amount absorbed with a precise adverse health affect. OSHA has not quantified risks resulting from dermal exposure in other toxic substance standards. In order, to adequately regulate dermal exposure to MDA, OSHA requires adherence to permissible exposure limits (which reduces surface contamination by MDA thereby reducing the opportunity for skin contact and reduces potential for re-entrainment into the air) and the use of personal protective clothing and equipment and the other standard provisions, all of which aid in preventing dermal exposure.

No evidence was provided subsequent to the issuance of the NPRM which would cause OSHA to change any of the findings herein stated.

[57 FR 35630, Aug. 10, 1992]