Public Citizen Health Research Group v. Patrick R. Tyson, Acting Assistant Secretary, Occupational Safety & Health Administration, and Raymond J. Donovan, Secretary of Labor, Association of Ethylene Oxide Users, Intervenor. Association of Ethylene Oxide Users v. Patrick R. Tyson, Acting Assistant Secretary, Occupational Safety & Health Administration, and Raymond J. Donovan, Secretary of Labor, Public Citizen Health Research Group v. Robert A. Rowland, Assistant Secretary, Occupational Safety and Health Administration

In these consolidated cases, we review the Occupational Safety and Health Administration's rule limiting exposure to ethylene oxide, a chemical widely used in manufacturing and in hospital instrument sterilization. The Administration's rule sets a long-term exposure limit but does not set a short-term limit. We find substantial evidence in the record to support OSHA's long-term exposure limit. We thus affirm that aspect of the rule. We also find, however, that OSHA's decision not to issue a short-term exposure limit is not supported by the record, and we remand for further consideration on that point. Since we remand for further proceedings, we do not reach the constitutional questions raised by several petitioners and amici.

Congress enacted the Occupational Safety and Health Act (OSH Act) "to assure ... every working man and woman in the Nation safe and healthful working conditions." 29 U.S.C. Sec. 651(b) (1982). To achieve this goal, the Secretary of Labor may hold hearings, id. Sec. 655(b)(3), promulgate standards, id. Secs. 655(b)(2), 655(b)(4), conduct inspections, id. Sec. 657, issue citations, id. Sec. 658, and pursue enforcement proceedings, id. Secs. 659, 662, 666. The Occupational Safety and Health Administration (OSHA) assists the Secretary in accomplishing these tasks.

In this case, the litigants challenge a standard issued by the Secretary regulating permissible exposure to ethylene oxide (EtO), a chemical of major significance in American industry. Occupational Exposure to Ethylene Oxide, Final Standard, 49 Fed.Reg. 25,734, 25,734-35 (1984). The bulk of EtO usage occurs in manufacturing, and a small fraction is consumed in the sterilization of hospital instruments. EtO is a highly reactive gas. Thus, in manufacturing processes employing EtO, tightly closed and highly automated systems are used. Nevertheless, workers can be exposed to the chemical during both shipping and equipment maintenance. While hospital sterilization accounts for only a small portion of the nation's EtO usage, it is far more hazardous to workers. Workers must enter areas where EtO is, or has been, present. For example, instruments are typically placed in a sterilization chamber which is then flooded with EtO. These sterilization chambers must be purged of the gas before workers can enter. The process is imperfect, and workers risk exposure to the chemical. Id.

In 1968, the American Conference of Governmental Industrial Hygenists (ACGIH) recommended a long-term permissible exposure limit (PEL) of 50 parts EtO per million parts air (ppm) as a time-weighted average (TWA) exposure level over eight hours. ACGIH based this recommendation on two types of studies. Six-month animal inhalation studies found ill effects only at levels at or above 50 ppm. A study of employees exposed for at least a decade to levels of 5 to 10 ppm reported no adverse effects. At the time ACGIH recommended the 50 ppm PEL, researchers had not yet discovered the carcinogenic properties of EtO. Id. at 25,736. In 1971, pursuant to 29 U.S.C. Sec. 655(a) (1982), which permits the promulgation of "national consensus standards," the Secretary of Labor adopted the 50 ppm PEL as a limit on workers' exposure.

Soon, however, the body of knowledge surrounding EtO began to grow, and researchers discovered more alarming health effects. In 1977, the National Institute for Occupational Safety and Health (NIOSH)1 concluded that a 75 ppm short-term exposure limit (STEL), in addition to the long-term 50 ppm PEL, was necessary to protect workers. A STEL regulates the maximum average exposure over a shorter period--usually fifteen minutes. NIOSH based its conclusion on studies showing that EtO exposure alters cell genetic material. In 1979, ACGIH began considering a PEL of 10 ppm, based on a number of more recent studies, including a study of actual worker exposure to EtO. These studies reinforced the belief that EtO exposure effects cell genetics. Moreover, a study of workers exposed to the chemical revealed a possible connection between EtO and leukemia, a cancer of the blood.

In 1981, ACGIH lowered its recommended PEL to 10 ppm. Moreover, the Conference designated EtO as a suspected carcinogen and proposed an even lower PEL of 5 ppm. Id. ACGIH based this reconsideration of the PEL on a two-year study of rats exposed to EtO. NIOSH also responded in 1981 to the new data on EtO by issuing a bulletin to employers and employees about the potential carcinogenic hazards posed by EtO. NIOSH recommended a revision in the then-current OSHA standard. Id. at 25,737. In June of 1982, ACGIH recommended a PEL of 1 ppm, to take effect in 1984.

In January of 1982, OSHA issued an advance notice of proposed rulemaking, inviting interested individuals to submit data or comments on revising the OSHA EtO standard. See Advance Notice of Proposed Rulemaking, 47 Fed.Reg. 3566 (1982). After some delay, petitioner Public Citizen Health Research Group (Public Citizen) objected to OSHA's lack of progress on promulgating an EtO standard. A year of litigation over OSHA's progress ensued, and this court ordered OSHA to propose a rule by April, 1983. Public Citizen Health Research Group v. Auchter, 702 F.2d 1150 (D.C.Cir.1983) (per curiam). The court also expected that the agency would "bring this matter to a close within a year." Id. at 1154 n. 12. OSHA complied by publishing a proposed rule, suggesting a PEL of 1 ppm (replacing the 1971 standard of 50 ppm) and a short-term exposure limit (STEL) "ranging from 5 to 50 ppm for 30 minutes or less." Occupational Exposure to Ethylene Oxide, Proposed Rule, 48 Fed.Reg. 17,283, 17,284 (1983).

OSHA held public hearings on the proposed rule in July of 1983. Each aspect of the proposed rule was supported and opposed by a wide range of witnesses. The hearing produced 1600 pages of transcript and over 300 exhibits. 49 Fed.Reg. at 25,737. Despite this court's ruling, however, OSHA did not issue a final rule by March of 1984. After some prodding by Public Citizen, OSHA stipulated that it would complete the rulemaking by June 15, 1984. On June 14, OSHA had a final rule in hand. It provided for a 1 ppm PEL eight-hour TWA and a 10 ppm STEL fifteen-minute TWA. See Proposed Final Standard, reprinted in Joint Appendix (J.A.) at 12. OSHA did not publish that final rule.

Pursuant to Executive Order No. 12,291, 3 C.F.R. 127 (1981), reprinted in 5 U.S.C. Sec. 601 note, at 431 (1982), OSHA transmitted its final rule to the Office of Management and Budget (OMB) for review. Executive Order 12,291 provides that to the extent permissible by law, regulations of agencies within the executive branch must comply with certain substantive requirements. The Order requires agencies to consider cost/benefit analysis, cost effectiveness, maximization of benefit to society, condition of the regulated industries, and conditions of the national economy. Exec. Order No. 12,291, Sec. 2. Moreover, the Order requires agencies to prepare a "Regulatory Impact Analysis," a document evaluating the proposed regulations in light of the above substantive requirements. This analysis must be sent to OMB. Id. Sec. 3. Finally, agencies must defer final action until they have responded to OMB's view. Id. Sec. 3(f)(1). In this case, OMB questioned several aspects of OSHA's final rule, especially the benefits that the EtO rule would provide to society and its cost-effectiveness. These objections applied to both the PEL and the STEL. OMB objected to the STEL in particular as unsupported by any reasonable risk assessment or inference from the available scientific evidence. Letter from Christopher DeMuth, OMB, to Francis Lilly, Dep't of Labor (June 14, 1984), reprinted in J.A. at 415.

Notwithstanding OMB's objections, OSHA published as a final rule the long-term limits embodied in the PEL. The final rule requires employers to ensure that their employees are not exposed to an airborne concentration of EtO in excess of 1 ppm as an eight-hour TWA. Occupational Exposure to Ethylene Oxide, Final Standard, 49 Fed.Reg. 25,734, 25,796 (1984) (codified at 29 C.F.R. Sec. 1910.1047 (1985)). Employers must monitor exposure levels regularly, depending on the concentration of EtO in the air and changes in work practices. The rule sets an "action level" of 0.5 ppm, which if detected, requires employers to engage in regular employee monitoring. The rule also requires employers to inform employees of monitoring results, institute engineering controls to reduce the level of exposure, and, if necessary, provide respiratory protective equipment for employees. If the EtO exposure level is above the 1 ppm PEL, the employer is required to establish a written compliance program to achieve an exposure level below 1 ppm. Employers must institute a medical surveillance program for employees exposed to EtO at or above the action level, and the employer must provide medical examinations to certain employees. Finally, the employer must embark upon an employee information program that includes signs, labels, and training programs. All of these requirements are enforced through specific recordkeeping requirements. 49 Fed.Reg. at 25,796-800.

While OSHA did not heed OMB's objections with regard to the PEL, it did reserve judgment on the STEL. Rather than issue the STEL as part of the final EtO standard, OSHA reopened the record for comments on the desirability of a STEL. Commentary was once again divided, and on January 2, 1985, OSHA issued its final rule declining to impose a STEL. Occupational Exposure to Ethylene Oxide, Supplemental Statement of Reasons, 50 Fed.Reg. 64 (1985). This action contrasted with OSHA's earlier decision, prior to receiving OMB's comments, to issue a 10 ppm STEL.

Petitioner Public Citizen launches a two-pronged attack on OSHA's decision not to issue the STEL. First, Public Citizen claims that the decision not to issue a STEL is unsupported on the record. The Ethylene Oxide Industry Council (EOIC) appears as amicus to support OSHA's decision not to issue a STEL. Second, Public Citizen argues that OMB's role in these proceedings was unlawful. In the latter argument, Public Citizen is joined by several members of the House of Representatives as amici.

The Association of Ethylene Oxide Users (AEOU) also petitions this court for review, arguing that OSHA's decision to issue the PEL is unsupported on the record.

Our opinion treats four basic aspects of this case. First, we set out the appropriate substantive legal requirements applicable to an OSHA standard, as well as the standard of review courts apply to OSHA rulemaking. Second, we review the evidence supporting OSHA's decision to issue the 1 ppm PEL. Third, we consider the evidence supporting OSHA's decision not to issue a 10 ppm STEL. Finally, because we find OSHA's decision not to issue a STEL unlawful, we decline to pass on the legality of OMB's participation in the rulemaking process.

A. Legal Requirements Applicable to OSHA Standards

Although OSHA enjoys a broad congressional mandate to ensure safe working conditions, the agency is not unbridled in its ability to regulate industry. The Secretary may promulgate "occupational safety and health standards," defined as "standard[s] which require[ ] conditions, or the adoption or use of one or more practices, means, methods, operations, or processes, reasonably necessary or appropriate to provide safe or healthful employment and places of employment." 29 U.S.C. Sec. 652(8) (1982). If the Secretary chooses to regulate "toxic materials or harmful physical agents," he must:

set the standard which most adequately assures, to the extent feasible, on the basis of the best available evidence, that no employee will suffer material impairment of health or functional capacity even if such employee has regular exposure to the hazard dealt with by such standard for the period of his working life. Development of standards under this subsection shall be based upon research, demonstrations, experiments, and such other information as may be appropriate. In addition to the attainment of the highest degree of health and safety protection for the employee, other considerations shall be the latest available scientific information in the field, the feasibility of the standards, and experience gained under this and other health and safety laws.

29 U.S.C. Sec. 655(b)(5) (1982).

In Industrial Union Department, AFL-CIO v. American Petroleum Institute, 448 U.S. 607, 100 S.Ct. 2844, 65 L.Ed.2d 1010 (1980) (Benzene ), the Supreme Court endeavored to define more precisely the specific contours of the OSH Act. In that case, OSHA promulgated a standard regulating worker exposure to benzene, a proven carcinogen at high exposure levels. At that time, OSHA's regulations (also referred to as its "Cancer Policy") allowed the Secretary to assume that there is no safe level of exposure to a known carcinogen; thus OSHA set the PEL at the lowest economically and technically feasible level. Id. at 624, 100 S.Ct. at 2854. OSHA made no attempt whatsoever to estimate the significance of the risks to workers exposed at level equal to the lowest feasible PEL. Id. at 631-38, 100 S.Ct. at 2858-62.

The Supreme Court, in a plurality opinion, struck down the benzene standard, reasoning that "[i]n the end OSHA's rationale for lowering the permissible exposure limit to 1 ppm was based, not on any finding that leukemia has ever been caused by exposure to 10 ppm of benzene and that it will not be caused by exposure to 1 ppm, but rather on a series of assumptions indicating that some leukemias might result from exposure to 10 ppm and that the number of cases might be reduced by reducing the exposure level to 1 ppm." Id. at 634, 100 S.Ct. at 2860 (emphasis in original). Focusing on the "reasonably necessary and feasible" language of the OSH Act, a plurality of four Justices held that the Act does not allow the Secretary "to require employers to provide absolutely risk-free workplaces whenever it is technologically feasible to do so, so long as the cost is not great enough to destroy an entire industry." Id. at 641, 100 S.Ct. at 2863. Rather, the OSH Act requires the Secretary to make a threshold finding that the health risk posed by the substance is significant. Id. at 639-40, 641, 642, 100 S.Ct. at 2862-63, 2864.

Once the threshold finding of significant risk has been made, OSHA's standard must be reasonably necessary and appropriate to remedy that risk. Id. at 639, 642, 100 S.Ct. at 2862, 2864 (plurality opinion); id. at 665, 100 S.Ct. at 2875 (opinion of Powell, J.)

The statutory directive to courts reviewing standards adopted pursuant to the OSH Act is clear: the Secretary's determinations shall be sustained if they are "supported by substantial evidence in the record considered as a whole." 29 U.S.C. Sec. 655(f) (1982). The substantial evidence test is a familiar approach for courts reviewing agency action. The Supreme Court has defined substantial evidence as "such relevant evidence as a reasonable mind might accept as adequate to support a conclusion." Universal Camera Corp. v. NLRB, 340 U.S. 474, 477, 71 S.Ct. 456, 459, 95 L.Ed. 456 (1951) (quoting Consolidated Edison Co. v. NLRB, 305 U.S. 197, 229, 59 S.Ct. 206, 217, 83 L.Ed. 126 (1938)). The appellate court therefore does not review the evidence de novo. Even if the evidence is susceptible of two inconsistent inferences, the agency must be upheld if a reasonable person could come to either conclusion on that evidence. See Consolo v. Federal Maritime Commission, 383 U.S. 607, 620, 86 S.Ct. 1018, 1026, 16 L.Ed.2d 131 (1966).

Statutes like the OSH Act, however, create a peculiar problem for reviewing courts. The statute instructs the reviewing court to apply the substantial evidence test, which is normally reserved for formal adjudications of fact. OSHA rulemaking, however, is a hybrid, combining formal and informal aspects of decisionmaking and including essentially legislative tasks as well. See Industrial Union Department, AFL-CIO v. Hodgson, 499 F.2d 467, 472-74 (D.C.Cir.1974). As we noted in United Steelworkers v. Marshall, 647 F.2d 1189, 1206-07 (D.C.Cir.1980), cert. denied, 453 U.S. 913, 101 S.Ct. 3148, 69 L.Ed.2d 997 (1981):

The peculiar problem of reviewing the rules of agencies like OSHA lies in applying the substantial evidence test to regulations which are essentially legislative and rooted in inferences from complex scientific and factual data, and which often necessarily involve highly speculative projections of technological development in areas wholly lacking in scientific and economic certainty.

Because of this hybrid approach, courts view the substantial evidence test in this context as requiring OSHA "to identify relevant factual evidence, to explain the logic and the policies underlying any legislative choice, to state candidly any assumptions on which it relies, and to present its reasons for rejecting any significant contrary evidence and argument." Id. at 1207.

This deferential standard in no way conflicts with the substantive legal requirements established by the Benzene case. Benzene established that the burden of proving that the subject of the proposed regulation presents a significant health risk remains with the agency. 448 U.S. at 653, 100 S.Ct. at 2869. Three of the Justices in the plurality in the Benzene case, however, were careful to preserve courts' traditional deference to agency decisionmaking. Thus, "the requirement that a 'significant' risk be identified is not a mathematical straitjacket. It is the Agency's responsibility to determine, in the first instance, what it considers to be a 'significant' risk." Id. at 655, 100 S.Ct. at 2870. The plurality opinion further stated that "OSHA is not required to support its finding that a significant risk exists with anything approaching scientific certainty." Id. at 656, 100 S.Ct. at 2871. Noting that the statute allows the Secretary to regulate on the basis of the "best available evidence," 29 U.S.C. Sec. 655(b)(5) (1982), the opinion stated that "a reviewing court [must] give OSHA some leeway where its findings must be made on the frontiers of scientific knowledge." Id.

With these considerations in mind, we turn to the evidence adduced in the proceedings below.

III. EVIDENCE OF ETO'S ADVERSE HEALTH CONSEQUENCES

OSHA first determined whether exposure to EtO at the then-current PEL of 50 ppm presented adverse health effects, a necessary prerequisite to the finding of significant risk required by the Benzene case. AEOU challenges OSHA's findings, arguing that the Agency lacks substantial evidence for its conclusions. We have reviewed the evidence in the record and AEOU's challenges to that evidence. We find OSHA's findings on the adverse health consequences of EtO exposure supported by substantial evidence in the record as a whole. In this part, we treat only the substantial evidence question. In later parts of this opinion, we review OSHA's assessment of the magnitude of these health risks. See infra pp. 1493-99.

OSHA concluded that there is a relationship between exposure to EtO and a significant increase in the risk of death from cancer. 49 Fed.Reg. at 25,73 8. The agency based its finding on two types of evidence: epidemiological studies and experimental studies.

1. Epidemiological Evidence.

OSHA relied on a study by Morgan and two studies by Hogstedt for evidence of "a possible association between occupational exposure to ethylene oxide and death from leukemia." 49 Fed.Reg. at 25,740.

Morgan studied the mortality levels of workers employed at an EtO chemical plant over the course of twenty-two years. Morgan, Claxton, Divine, Kaplan & Harris, Mortality Among Ethylene Oxide Workers, 23 J. Occupational Med. 767 (1981), reprinted in J.A. at 1024. Measurements taken at the plant in 1977 showed no detectable EtO levels in most of the production areas. Measurements elsewhere at the plant showed levels well below 50 ppm. Id. at 768, J.A. at 1025. The mortality rate among the workers was less than expected from such a population sample and no incidence of leukemia was reported. The study did find, however, a significant increase in pancreatic cancer and Hodgkin's disease. Id. at 769, J.A. at 1026.

Hogstedt I revealed three cases of leukemia occurring between 1972 and 1977 among 230 workers at a Swedish factory where EtO was employed to sterilize hospital equipment. Hogstedt, Malmqvist & Wadman, Leukemia in Workers Exposed to Ethylene Oxide, 241 J.A.M.A. 1132 (1979), reprinted in J.A. at 577. Only 0.2 deaths were expected in such a population during that time span. In 1977, the EtO level in the area where two of the employees who died had worked was estimated at 10-30 ppm. Id. at 1133, J.A. at 578.

Hogstedt II examined 243 workers who had worked at a Swedish EtO production facility for at least one year. Hogstedt, Rohlen, Berndtsson, Axelson & Ehrenberg, A cohort study of mortality and cancer incidence in ethylene oxide production workers, 36 Brit.J.Indus.Med. 276 (1979), reprinted in J.A. at 642. The workers were tracked for 16 years, and the study revealed a significant excess mortality rate among employees who worked full-time in EtO production areas. The authors found no excess mortality among part-time or intermittently exposed workers. Id. at 278, J.A. at 644.

OSHA admitted that each of these studies was flawed in some way. 49 Fed.Reg. at 25,738-40. AEOU renews here its arguments against the probative value of these studies, urging us to fault the agency for relying on them at all. See Petitioner AEOU Brief at 24-27. OSHA, however, did not blindly rely on these studies; the agency recognized and accounted for the methodological weaknesses inherent in the studies.

The agency heard conflicting testimony on the Morgan study, including a statement from Morgan himself that the study failed to support a causal link between EtO and leukemia and thus should be considered as negative evidence on EtO's carcinogenic properties. The study clearly demonstrates, however, a link between EtO and pancreatic cancer and Hodgkin's disease. As some commenters suggested, the study underestimated the cancer risk, see Hearing Transcript (Tr.) at 792, J.A. at 1796, the study was inconclusive rather than negative on leukemia risk, Tr. at 341, J.A. at 1789, and the study was "a strong piece of evidence indicating that even in very small cohorts, with exposures well below the current OSHA standard, excess cancer risk ... was detected." 49 Fed.Reg. at 25,739 (quoting Submission of Dr. Jeanne Stellman at 8, J.A. at 1001).

Commenters criticized Hogstedt I because of the small sample size, see, e.g., Tr. 634, J.A. at 1794, and the fact that the subject workers were exposed to methyl formate along with EtO, see, e.g., Health Industry Manufacturers Association (HIMA) Comments at 25, reprinted in J.A. at 1151, 1182; EOIC Hazard Assessment of Ethylene Oxide at 112, reprinted in J.A. at 1443, 1464. OSHA, however, noted that it had no evidence that methyl formate was carcinogenic. Hogstedt II faced similar criticism: workers were exposed to multiple chemical combinations along with EtO. See, e.g., HIMA Comments at 25, J.A. at 1182. Despite these criticisms, only two commenters concluded that the evidence did not establish an increased risk of cancer. Even one of those commenters stated that the studies "may not provide as much reassurance [that EtO is not a carcinogen] as some would like." 49 Fed.Reg. at 25,740 (quoting Submission of Robert W. Morgan). In contrast, NIOSH concluded that the Hogstedt studies provide evidence of a possible relationship between EtO exposure and leukemia. See Tr. at 341, J.A. at 1789; NIOSH Current Intelligence Bulletin 5 (May 22, 1981), reprinted in J.A. at 619, 625.

We cannot accept AEOU's proposition that these studies are rendered totally valueless by their methodological flaws. While each study does suffer from defects, OSHA did not rely on the epidemiological studies as conclusive evidence of the carcinogenic nature of EtO. Rather, OSHA found "that the epidemiological evidence, although not by itself conclusive, is supportive of EtO's potential carcinogenic ... effects." 49 Fed.Reg. at 25,740. Moreover, OSHA did not reach this position in the face of unified, contrary scientific thought. Indeed, the record demonstrates that OSHA's acceptance of these studies for some, but not conclusive, evidence of EtO's potential carcinogenic effects was supported by a substantial portion of the scientists who attended the rulemaking hearings.

These epidemiological studies, while of some value by themselves, take on added significance when viewed in light of the experimental evidence in the record.

2. Experimental Evidence.

OSHA relied on two studies examining the effects of EtO exposure on animals for further evidence of the chemical's carcinogenicity: the Bushy Run rat study and the NIOSH rat and monkey study.

In the Bushy Run study, researchers at the Bushy Run Research Center in Pittsburgh exposed rats to EtO at concentrations of 100, 33, and 10 ppm for 6 hours per day, 5 days per week. Snellings, Weil & Maronpot, Final Report, Ethylene Oxide Two-Year Inhalation Study on Rats at v (unpublished manuscript Jan. 28, 1981), reprinted in J.A. at 579, 583. This study produced a number of significant results, indicating that EtO exposure was related to development of various types of cancers. See 49 Fed.Reg. at 25,740-41. Many commenters objected to four particular aspects of the Bushy Run study.

First, the rats suffered a viral infection during the course of the study. Some commenters suggested that the infection could have adversely affected the immune systems of the rats, thus invalidating the results. See id. at 25,74 1 (noting criticism). OSHA concluded, however, based on testimony of the director of the study and an expert pathologist from the National Cancer Institute, that the outbreak had no substantial effect on the outcome of the study. Id.

Second, other commenters suggested that the type of leukemia produced in the Bushy Run rats had no counterpart in other strains of rats, in mice, or in humans. Id. OSHA specifically rejected this assertion, relying on expert testimony that recent studies had described similar leukemia in humans. Id. (citing testimony of Jerrold M. Ward).

Third, EOIC suggested that the only types of tumors appearing in the test rats were types that occur in rats spontaneously. This fact indicated that ETO might promote tumor growth, rather than initiate it. Id. OSHA heard expert testimony that tumor-promoters can simultaneously be tumor-initiators, and thus carcinogenic, as well. Id. (citing testimony of Jerrold M. Ward). The same expert stated that tumor-promoters might be as dangerous in ultimately causing cancer as chemicals that are not categorized as promoters. Id. at 25,742.2

Finally, OSHA rejected an argument that the statistical analyses performed on the Bushy Run data were misleading. See, e.g., HIMA Comments App. H at 5-7, J.A. at 1293-95. OSHA confirmed the Bushy Run researchers' statistical methodology and specifically reanalyzed the evidence under a "worst case" theory, concluding that the results were essentially accurate. Thus, OSHA found that the Bushy Run study demonstrates that exposure to EtO significantly increases mortality and the incidence of tumors among rats. 49 Fed.Reg. at 25,742.

The second experimental study was a two-year study of rats and monkeys conducted by NIOSH. Chronic Inhalation Toxicity of Ethylene Oxide and Propylene Oxide in Rats and Monkeys--A Preliminary Report (Feb. 17, 1982), reprinted in J.A. at 1131. These animals were exposed to EtO at concentrations of 50 and 100 ppm. During the study, however, the rats contracted an infection that killed a large portion of the group. Once the infection had dispersed, researchers resumed the study. 49 Fed.Reg. at 25,742.

The NIOSH study revealed an increased incidence of leukemia in rats exposed to 50 ppm EtO, but not in rats exposed to 100 ppm EtO. See J.A. at 1144. The study did reveal a relationship in rats between tumor production and EtO exposure. The results paralleled those of the Bushy Run study. See 49 Fed.Reg. at 25,743. None of the monkeys demonstrated any evidence of leukemia, but there was some evidence of central nervous system damage. Id.

Significantly, "[t]he overwhelming majority of comments on the NIOSH study agreed with OSHA's conclusions that these preliminary results provide additional evidence of EtO's carcinogenicity in experimental animals." Id. Two commenters stated that taken together, the Bushy Run and NIOSH studies produced consistent, disturbing results with regard to tumor production. Id. (statements of Leon Goldberg and Jerrold Ward).3

3. OSHA's Conclusions.

Taking the epidemiological and experimental studies together, OSHA found that EtO causes cancer in laboratory animals and poses a significant cancer risk for humans. While each study individually may not be a model of textbook scientific inquiry, the cumulative evidence is compelling. OSHA takes precisely this view: "[A]lthough these studies do not provide definitive evidence of carcinogenicity, they are suggestive of an association between occupational exposure to EtO and cancer (leukemia) mortality." 49 Fed.Reg. at 25,744.

This court's role is not to review the evidence de novo to arrive at our own estimate of the risks; rather, we look for substantial evidence supporting OSHA's finding of EtO carcinogenicity. 29 U.S.C. Sec. 655(f) (1982). A reasonable person could draw from this evidence the conclusion that exposure to EtO presents a risk of cancer. Thus, the substantial evidence test is met. Even if a reasonable person could also draw the opposite conclusion, we must uphold the agency's findings. See Consolo v. Federal Maritime Commission, 383 U.S. 607, 620, 86 S.Ct. 1018, 1026, 16 L.Ed.2d 131 (1966).

OSHA also found that EtO exposure is related to cell genetic damage and to reproductive problems. We review these conclusions in turn.