448 U.S. 607 (1980)

INDUSTRIAL UNION DEPARTMENT, AFL-CIO
v.
AMERICAN PETROLEUM INSTITUTE ET AL.

No. 78-911.

Supreme Court of United States.

Argued October 10, 1979. Decided July 2, 1980.^[*] CERTIORARI TO THE UNITED STATES COURT OF APPEALS FOR THE FIFTH CIRCUIT.

*610 George H. Cohen argued the cause for petitioner in No. 78-911. With him on the briefs were Robert M. Weinberg, J. Albert Woll, Laurence Gold, Elliot Bredhoff, and George Kaufmann. William Alsup argued the cause for petitioner in No. 78-1036. With him on the briefs were Solicitor General McCree, Deputy Solicitor General Easterbrook, Benjamin W. Mintz, and Dennis K. Kade.

Edward W. Warren argued the cause for respondents American Petroleum Institute et al. in both cases. With him on the brief were Stark Ritchie, Martha Beauchamp, Neil J. King, *611 John H. Pickering, Robert R. Bonczek, John F. Dickey, Robert L. Ackerly, and Harold B. Scoggins, Jr. Charles F. Lettow argued the cause for respondents Rubber Manufacturers Association, Inc., et al. in both cases. With him on the brief was John C. Murphy, Jr.^[†]

MR. JUSTICE STEVENS announced the judgment of the Court and delivered an opinion, in which THE CHIEF JUSTICE and MR. JUSTICE STEWART joined and in Parts, I, II, III-A, III-B, III-C, and III-E of which MR. JUSTICE POWELL joined.

The Occupational Safety and Health Act of 1970 (Act), 84 Stat. 1590, 28 U. S. C. § 651 et seq., was enacted for the purpose of ensuring safe and healthful working conditions for every working man and woman in the Nation. This litigation concerns a standard promulgated by the Secretary of Labor to regulate occupational exposure to benzene, a substance which has been shown to cause cancer at high exposure levels. The principal question is whether such a showing is a sufficient basis for a standard that places the most stringent limitation on exposure to benzene that is technologically and economically possible.

The Act delegates broad authority to the Secretary to promulgate different kinds of standards. The basic definition *612 of an "occupational safety and health standard" is found in § 3 (8), which provides:

"The term `occupational safety and health standard' means a standard which requires 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." 84 Stat. 1591, 28 U. S. C. § 652 (8).

Where toxic materials or harmful physical agents are concerned, a standard must also comply with § 6 (b) (5), which provides:

"The Secretary, in promulgating standards dealing with toxic materials or harmful physical agents under this subsection, shall 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 data in the field, the feasibility of the standards, and experience gained under this and other health and safety laws." 84 Stat. 1594, 28 U. S. C. § 655 (b) (5).^[1]

*613 Wherever the toxic material to be regulated is a carcinogen, the Secretary has taken the position that no safe exposure level can be determined and that § 6 (b) (5) requires him to set an exposure limit at the lowest technologically feasible level that will not impair the viability of the industries regulated. In this case, after having determined that there is a casual connection between benzene and leukemia (a cancer of the white blood cells), the Secretary set an exposure limit on airborne concentrations of benzene of one part benzene per million parts of air (1 ppm), regulated dermal and eye contact with solutions containing benzene, and imposed complex monitoring and medical testing requirements on employers whose workplaces contain 0.5 ppm or more of benzene. 29 CFR §§ 1910.1028 (c), (e) (1979).

On pre-enforcement review pursuant to 29 U. S. C. § 655 (f), the United States Court of Appeals for the Fifth Circuit held the regulation invalid. American Petroleum Institute v. OSHA, 581 F. 2d 493 (1978). The court concluded that the Occupational Safety and Health Administration (OSHA)^[2] had exceeded its standard-setting authority because it had not shown that the new benzene exposure limit was "reasonably necessary or appropriate to provide safe or healthful employment" as required by § 3 (8),^[3] and because § 6 (b) (5) *614 does "not give OSHA the unbridled discretion to adopt standards designed to create absolutely risk-free workplaces regardless of costs."^[4] Reading the two provisions together, the Fifth Circuit held that the Secretary was under a duty to determine whether the benefits expected from the new standard bore a reasonable relationship to the costs that it imposed. Id., at 503. The court noted that OSHA had made an estimate of the costs of compliance, but the record lacked substantial evidence of any discernible benefits.^[5]

We agree with the Fifth Circuit's holding that § 3 (8) requires the Secretary to find, as a threshold matter, that the *615 toxic substance in question poses a significant health risk in the workplace and that a new, lower standard is therefore "reasonably necessary or appropriate to provide safe or healthful employment and places of employment." Unless and until such a finding is made, it is not necessary to address the further question whether the Court of Appeals correctly held that there must be a reasonable correlation between costs and benefits, or whether, as the federal parties argue, the Secretary is then required by § 6 (b) (5) to promulgate a standard that goes as far as technologically and economically possible to eliminate the risk.

Because these are unusually important cases of first impression, we have reviewed the record with special care. In this opinion, we (1) describe the benzene standard, (2) analyze the Agency's rationale for imposing a 1 ppm exposure limit, (3) discuss the controlling legal issues, and (4) comment briefly on the dermal contact limitation.

I

Benzene is a familiar and important commodity. It is a colorless, aromatic liquid that evaporates rapidly under ordinary atmospheric conditions. Approximately 11 billion pounds of benzene were produced in the United States in 1976. Ninety-four percent of that total was produced by the petroleum and petrochemical industries, with the remainder produced by the steel industry as a byproduct of coking operations. Benzene is used in manufacturing a variety of products including motor fuels (which may contain as much as 2% benzene), solvents, detergents, pesticides, and other organic chemicals. 43 Fed. Reg. 5918 (1978).

The entire population of the United States is exposed to small quantities of benzene, ranging from a few parts per billion to 0.5 ppm, in the ambient air. Tr. 1029-1032. Over one million workers are subject to additional low-level exposures as a consequence of their employment. The majority of these employees-work in gasoline service stations, benzene *616 production (petroleum refineries-and coking operations), chemical processing, benzene transportation, rubber manufacturing, and laboratory operations.^[6]

Benzene is a toxic substance. Although it could conceivably cause harm to a person who swallowed or touched it, the principal risk harm comes from inhalation of benzene vapors. When these vapors are inhaled, the benzene diffuses through the lungs and is quickly absorbed into the blood. *617 Exposure to high concentrations produces an almost immediate effect on the central nervous system. Inhalation of concentrations of 20,000 ppm can be fatal within minutes; exposures in the range of 250 to 500 ppm can cause vertigo, nausea, and other symptoms of mild poisoning. 43 Fed. Reg. 5921 (1978). Persistent exposures at levels above 25-40 ppm may lead to blood deficiencies and diseases of the blood-forming organs, including aplastic anemia, which is generally fatal.

Industrial health experts have long been aware that exposure to benzene may lead to various types of nonmalignant diseases. By 1948 the evidence connecting high levels of benzene to serious blood disorders had become so strong that the Commonwealth of Massachusetts imposed a 35 ppm limitation on workplaces within its jurisdiction. In 1969 the American national Standards Institute (ANSI) adopted a national consensus standard of 10 ppm averaged over an 8-hour period with a ceiling concentration of 25 ppm for 10-minute periods or a maximum peak concentration of 50 ppm. Id., at 5919. In 1971, after the Occupational Safety and Health Act was passed, the Secretary adopted this consensus standard as the federal standard, pursuant to 29 U. S. C. § 655 (a).^[7]

*618 As early as 1928, some health experts theorized that there might also be a connection between benzene in the workplace and leukemia.^[8] In the late 1960's and early 1970's a number of epidemiological studies were published indicating that workers exposed to high concentrations of benzene were subject to a significantly increased risk of leukemia.^[9] In a 1974 report recommending a permanent standard for benzene, the National Institute for Occupational Safety and Health *619 (NIOSH), OSHA's research arm,^[10] noted that these studies raised the "distinct possibility" that benzene caused leukemia. But, in light of the fact that all known cases had occurred at very high exposure levels, NIOSH declined to recommend a change in the 10 ppm standard, which it considered sufficient to protect against nonmalignant diseases. NIOSH suggested that further studies were necessary to determine conclusively whether there was a link between benzene and leukemia and, if so, what exposure levels were dangerous.^[11]

Between 1974 and 1976 additional studies were published which tended to confirm the view that benzene can cause leukemia, at least when exposure levels are high.^[12] In an *620 August 1976 revision of its earlier recommendation, NIOSH stated that these studies provided "conclusive" proof of a causal connection between benzene and leukemia. 1 Record. Ex. 2-5 p. 100. Although it acknowledged that none of the intervening studies had provided the dose-response data it had found lacking two years earlier, id., at 9, NIOSH nevertheless recommended that the exposure limit be set as low as possible. As a result of this recommendation, OSHA contracted with a consulting firm to do a study on the costs to industry of complying with the 10 ppm standard then in effect or, alternatively, with whatever standard would be the lowest feasible. Tr. 505-506.

In October 1976, NIOSH sent another memorandum to OSHA, seeking acceleration of the rulemaking process and "strongly" recommending the issuance of an emergency temporary standard pursuant to § 6 (c) of the Act, 29 U. S. C. § 655 (c),^[13] for benzene and two other chemicals believed to *621 be carcinogens. NIOSH recommended that a 1 ppm exposure limit be imposed for benzene.^[14] 1 Record, Ex. 2-6. Apparently because of the NIOSH recommendation, OSHA asked its consultant to determine the cost of complying with a 1 ppm standard instead of with the "minimum feasible" standard. Tr. 506-507. It also issued voluntary guidelines for benzene, recommending that exposure levels be limited to 1 ppm on an 8-hour time-weighted average basis wherever possible. 2 Record, Ex. 2-44.

In the spring of 1976, NIOSH had selected two Pliofilm plants in St. Marys and Akron, Ohio, for an epidemiological study of the link between leukemia and benzene exposure. In April 1977, NIOSH forwarded an interim report to OSHA indicating at least a fivefold increase in the expected incidence of leukemia for workers who had been exposed to benzene *622 at the two plants from 1940 to 1949.^[15] The report submitted to OSHA erroneously suggested that exposures in the two plants had generally been between zero and 15 ppm during the period in question.^[16] As a result of this new evidence *623 and the continued prodding of NIOSH, 1 Record, Ex. 2-7, OSHA did issue an emergency standard, effective May 21, 1977, reducing the benzene exposure limit from 10 ppm to 1 ppm, the ceiling for exposures of up to 10 minutes from 25 ppm, and eliminating the authority for peak concentrations of 50 ppm. 42 Fed. Reg. 22516 (1977). In its explanation accompanying the emergency standard, OSHA stated that benzene had been shown to cause leukemia at exposures below 25 ppm and that, in light of its consultant's report, it was feasible to reduce the exposure limit to 1 ppm. Id., at 22517, 22521.

On May 19, 1977, the Court of Appeals for the Fifth Circuit entered a temporary restraining order preventing the emergency standard from taking effect. Thereafter, OSHA abandoned its efforts to make the emergency standard effective and instead issued a proposal for a permanent standard patterned almost entirely after the aborted emergency standard Id., at 27452.

In its published statement giving notice of the proposed permanent standard, OSHA did not ask for comments as to whether or not benzene presented a significant health risk at exposures of 10 ppm or less. Rather, it asked for comments as to whether 1 ppm was the minimum feasible exposure limit.^[17]Ibid. As OSHA's Deputy Director of Health Standards, Grover Wrenn, testified at the hearing, this formulation *624 of the issue to be considered by the Agency was consistent with OSHA's general policy with respect to carcinogens.^[18] Whenever a carcinogen is involved, OSHA will presume that no safe level of exposure exists in the absence of clear proof establishing such a level and will accordingly set the exposure limit at the lowest level feasible.^[19] The proposed 1 ppm exposure *625 limit in this case thus was established not on the basis of a proven hazard at 10 ppm. but rather on the basis of "OSHA'S best judgment at the time of the proposal of the feasibility of compliance with the proposed standard by the [a]ffected industries." Tr. 30. Given OSHA's cancer policy, it was in face irrelevant whether there was any evidence at all of a leukemia risk at 10 ppm. The important point was that there was no evidence that there was not some risk, however small, at the level. The fact that OSHA did not ask for comments on whether there was a safe level of exposure for benzene was indicative of its further view that a demonstration of such absolute safety simply could not be made.^[20]

Public hearings were held on the proposed standard, commencing on July 19, 1977. The final standard was issued on February 10, 1978. 29 CFR § 1910.1028 (1979).^[21] In its final form, the benzene standard is designed to protect workers from whatever hazards are associated with low-level benzene *626 exposures by requiring to monitor workplaces to determine the level of exposure, to provide medical examinations when the level rises above 0.5 ppm. and to institute whatever engineering or other controls are necessary to keep exposures at or below 1 ppm.

In the standard as originally proposed by OSHA, the employer's duty to monitor, keep records, and provide medical examinations arose whenever any benzene was present in a workplace covered by the rule.^[22] Because benzene is omni-present in small quantities, NIOSH and the President's Council on Wage and Price Stability recommended the use of an "action level" to trigger monitoring and medical examination requirements. Tr. 1030-1032; App. 121-133. OSHA accepted this recommendation, providing under the final standard that, if initial monitoring discloses benzene concentrations below 0.5 ppm averaged over an 8-hour work day, no further action is required unless there is a change in the company's practices.^[23] If exposures are above the action *627 level, but below the 1 ppm exposure limit, employers are required to monitor exposure levels on a quarterly basis and to provide semiannual medical examinations for their exposed employees. Neither the concept of an action level, nor the specific level selected by OSHA, is challenged in this proceeding.

Whenever initial monitoring indicates that employees are subject to airborne concentrations of benzene above 1 ppm averaged over an 8-hour workday, with a ceiling of 5 ppm for any 15-minute period, employers are required to modify their plants or institute work practice controls to reduce exposures within permissible limits. Consistent with OSHA's general policy, the regulation does not allow respirators to be used if engineering modifications are technologically feasible.^[24] Employers in this category are also required to perform monthly monitoring so long as their workplaces remain above 1 ppm, provide semiannual medical examinations to exposed workers, post signs in and restrict access to "regulated areas" where the permissible exposure limit is exceeded, and conduct employee training programs where necessary.

The standard also places strict limits on exposure to liquid *628 benzene. As originally framed, the standard totally prohibited any skin or eye contact with any liquid containing any benzene. Ultimately, after the standard was challenged, OSHA modified this prohibition by excluding liquids containing less than 0.5% benzene. After three years, that exclusion will be narrowed to liquids containing less than 0.1% benzene.

The permanent standard is expressly inapplicable to the storage, transportation, distribution, sale, or use of gasoline or other fuels subsequent to discharge from bulk terminals.^[25] This exception is particularly significant in light of the fact that over 795,000 gas station employees, who are exposed to an average of 102,700 gallons of gasoline (containing up to 2% benzene) annually, are thus excluded from the protection of the standard.^[26]

As presently formulated, the benzene standard is an expensive way of providing some additional protection for a relatively small number of employees. According to OSHA's figures, the standard will require capital investments in engineering controls of approximately $266 million, first-year operating costs (for monitoring, medical testing, employee training, and respirators) of $187 million to $205 million and *629 recurring annual costs of approximately $34 million.^[27] 43 Fed. Reg. 5934 (1978). The figures outlined in OSHA's explanation of the costs of compliance to various industries indicate that only 35,000 employees would gain any benefit from the regulation in terms of a reduction in their exposure to benzene.^[28] Over two-thirds of these workers (24,450) are employed in the rubber-manufacturing industry. Compliance costs in that industry are estimated to be rather low with no capital costs and initial operating expenses estimated at only $34 million ($1,390 per employee); recurring annual costs would also be rather low, totaling less than $1 million. By contrast, the segment of the petroleum refining industry that produces benzene would be required to incur $24 million in capital costs and $600,000 in first-year operating expenses to provide additional protection for 300 workers ($82,000 per employee), while the petrochemical industry would be required to incur $20.9 million in capital costs and $1 million in initial operating expenses for the benefit of 552 employees ($39,675 per employee).^[29]Id., at 5936-5938.

*630 Although OSHA did not quantify the benefits to each category of worker in terms of decreased exposure to benzene, it appears from the economic impact study done at OSHA's direction that those benefits may be relatively small. Thus, although the current exposure limit is 10 ppm, the actual exposures outlined in that study are often considerably lower. For example, for the period 1970-1975 the petrochemical industry reported that, out of a total of 496 employees exposed to benzene, only 53 were exposed to levels between 1 and 5 ppm and only 7 (all at the same plant) were exposed to between 5 and 10 ppm. 1 Economic Impact Statement, p. 4-6, Table 4-2, 11 Record, Ex. 5A, p. 4-6, Table 4-2. See also id., Tables 4.3-4.8 (indicating sample exposure levels in various industries).

II

The critical issue at this point in the litigation is whether the Court of Appeals was correct in refusing to enforce the 1 ppm exposure limit on the ground that it was not supported by appropriate findings.^[30]

*631 Any discussion of the 1 ppm exposure limit must, of course, being with the Agency's rationale for imposing that limit.^[31] The written explanation of the standard fills 184 pages of the printed appendix. Much of it is devoted to a discussion of the voluminous evidence of the adverse effects of exposure to benzene at levels of concentration well above 10 ppm. This discussion demonstrates that there is ample justification for regulating occupational exposure to benzene and that the prior limit of 10 ppm, with a ceiling of 25 ppm (or a peak of 50 ppm) was reasonable. It does not, however, provide direct support for the Agency's conclusion that the limit should be reduced from 10 ppm to 1 ppm.

The evidence in the administrative record of adverse effects of benzene exposure at 10 ppm is sketchy at best. OSHA noted that there was "no dispute" that certain nonmalignant blood disorders, evidenced by a reduction in the level of red or white cells or platelets in the blood, could result from exposures of 25-40 ppm. It then stated that several studies had indicted that relatively slight changes in normal blood values could result from exposures below 25 ppm and perhaps below 10 ppm. OSHA did not attempt to make any estimate based on these studies of how significant the risk of nonmalignant disease would be at exposures of 10 ppm of less.^[32] Rather, it stated that because of the lack of data concerning the linkage between low-level exposures and blood abnormalities, it was impossible to construct a dose-response *632 curve at this time.^[33] OSHA did conclude, however, that the studies demonstrated that the current 10 ppm exposure limit was inadequate to ensure that no single worker would suffer a nonmalignant blood disorder as a result of benzene exposure. Nothing that it is "customary" to set a permissible exposure limit by applying a safety factor 10-100 to the lowest level at which adverse effects had been observed, the Agency stated that the evidence supported the conclusion that the limit should be set at a point "substantially less than 10 ppm" even if benzene's leukemic effects were not considered. 43 Fed. Reg. 5924-5925 (1978). OSHA did not state, however, that the nonmalignant effects of benzene exposure justified a reduction in the permissible exposure limit to 1 ppm.^[34]

OSHA also noted some studies indicating an increase in chromosomal aberrations in workers chronically exposed to *633 concentrations of benzene probably less than 25 ppm."^[35] However, the Agency took no definitive position as to what these aberrations meant in terms of demonstrable health effects and stated that no quantitative dose-response relationship had yet been established. Under these circumstances, chromosomal effects were categorized by OSHA as an "adverse biological event of serious concern which may pose or reflect a potential health risk and as such, must be considered in the larger purview of adverse health effects associated with benzene. Id., at 5932-5934.

With respect to leukemia, evidence of an increased risk (i. e., a risk greater than that borne by the general population) due to benzene exposures at or below 10 ppm was even sketchier. Once OSHA acknowledged that the NIOSH study it had relied upon in promulgating the emergency standard did not support its earlier view that benzene had been shown to cause leukemia at concentrations below 25 ppm, see n. 12, supra, there was only one study that provided any evidence of such an increased risk. That study, conducted by the Dow Chemical Co., uncovered three leukemia deaths, versus 0.2 expected deaths, out of a population of 594 workers; it appeared that the three workers had never been exposed to more than 2 to 9 ppm of benzene. The authors of the study, however, concluded that it could not be viewed as proof of a relationship between low-level benzene exposure and leukemia because all three workers had probably been occupationally exposed to a number of other potentially carcinogenic chemicals at other points in their careers and because no leukemia deaths had been uncovered among workers who had been exposed to much higher levels of benzene. In its explanation of the permanent standard, OSHA stated that the possibility that these three leukemias had been caused by benzene exposure could not be *634 ruled out and that the study, although not evidence of an increased risk of leukemia at 10 ppm, was therefore "consistent with the findings of many studies that there is an excess leukemia risk among benzene exposed employees." 43 Fed. Reg. 5928 (1978). The Agency made to finding that the Dow study, any other empirical evidence, or any opinion testimony demonstrated that exposure to benzene at or below the 10 ppm level had ever in fact caused leukemia. See 581 F. 2d, at 503, where the Court of Appeals noted that OSHA was "unable to point to any empirical evidence documenting a leukemia risk at 10 ppm. . . ."

In 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. In reaching that result, the Agency first unequivocally concluded that benzene is a human carcinogen.^[36] Second, it concluded that industry had failed to prove that there is a safe threshold level of exposure to benzene below which no excess leukemia cases would occur. In reaching this conclusion OSHA rejected industry contentions that certain epidemiological studies indicating no excess risk of leukemia among workers exposed at levels below 10 ppm were sufficient to establish that the threshold level of safe exposure was at or above *635 10 ppm.^[37] It also rejected an industry witness' testimony that a dose-response curve could be constructed on the basis of the reported epidemiological studies and that this curve indicated that reducing the permissible exposure limit from 10 to 1 ppm would prevent at most one leukemia and one other cancer death every six years.^[38]

Third, the Agency applied its standard policy with respect to carcinogens.^[39] concluding that, in the absence of definitive *636 proof of a safe level, it must be assumed that any level above zero presents some increased risk of cancer.^[40] As the federal parties point out in their brief, there are a number of scientists and public health specialists who subscribe to this view, theorizing that a susceptible person may contract cancer from the absorption of even one molecule of a carcinogen like benzene. Brief for Federal Parties 18-19.^[41]

*637 Fourth, the Agency reiterated its view of the Act, stating that it was required by § 6 (b) (5) to set the standard either at the level that has been demonstrated to be safe or at the lowest level feasible, whichever is higher. If no safe level is established, as in this case, the Secretary's interpretation of the statute automatically leads to the selection of an exposure limit that is the lowest feasible.^[42] Because of benzene's importance to the economy, no one has ever suggested that it would be feasible to eliminate its use entirely, or to try to limit exposures to the small amounts that are omni-present. Rather, the Agency selected 1 ppm as a workable exposure level, see n. 14, supra, and then determined that compliance with that level was technologically feasible and that "the economic impact of . . . [compliance] will not be such as to threaten the financial welfare of the affected firms or the general economy." 43 Fed. Reg. 5939 (1978). It therefore held that 1 ppm was the minimum feasible exposure level within the meaning of § 6 (b) (5) of the Act.

Finally, although the Agency did not refer in its discussion of the pertinent legal authority to any duty to identify the anticipated benefits of the new standard, it did conclude that some benefits were likely to result from reducing the exposure limit from 10 ppm to 1 ppm. This conclusion was based, again, not on evidence, but rather on the assumption that the risk of leukemia will decrease as exposure levels decrease. Although the Agency had found it impossible to construct a dose-response curve that would predict with any accuracy the *638 number of leukemias that could be expected to result from exposures at 10 ppm, at 1 ppm, or at any intermediate level, it nevertheless "determined that the benefits of the proposed standard are likely to be appreciable."^[43] 43 Fed. Reg. 5941 (1978). In light of the Agency's disavowal of any ability to determine the numbers of employees likely to be adversely affected by exposures of 10 ppm, the Court of Appeals held this finding to be unsupported by the record. 581 F. 2d, at 503.^[44]

It is noteworthy that a no point in its lengthy explanation did the Agency quote or even cite § 3 (8) of the Act. It made no finding that any of the provisions of the new standard were "reasonably necessary or appropriate to provide safe or healthful employment and places of employment." Nor did it allude to the possibility that any such finding might have been appropriate.

*639 III

Our resolution of the issues in these cases turns, to a large extent, on the meaning of and the relationship between § 3 (8), which defines a health and safety standard as a standard that is "reasonably necessary and appropriate to provide safe or healthful employment," and § 6 (b) (5), which directs the Secretary in promulgating a health and safety standard for toxic materials to "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. . . ."

In the Government's view, § 3 (8)'s definition of the term "standard" has no legal significance or at best merely requires that a standard not be totally irrational. It takes the position that § 6 (b) (5) is controlling and that it requires OSHA to promulgate a standard that either gives an absolute assurance of safety for each and every worker or reduces exposures to the lowest level feasible. The Government interprets "feasible" as meaning technologically achievable at a cost that would not impair the viability of the industries subject to the regulation. The respondent industry representatives, on the other hand, argue that the Court of Appeals was correct in holding that the "reasonably necessary and appropriate" language of § 3 (8), along with the feasibility requirement of § 6 (b) (5), requires the Agency to quantify both the costs and the benefits of a proposed rule and to conclude that they are roughly commensurate.

In our view, it is not necessary to decide whether either the Government or industry is entirely correct. For we think it is clear that § 3 (8) does apply to all permanent standards promulgated under the Act and that it requires the Secretary, before issuing any standard, to determine that it is reasonably necessary and appropriate to remedy a significant risk of material health impairment. Only after the Secretary has made the threshold determination that such a risk exists *640 with respect to a toxic substance, would it be necessary to decide whether § 6 (b) (5) requires him to select the most protective standard he can consistent with economic and technological feasibility, or whether, as respondents argue, the benefits of the regulation must be commensurate with the costs of its implementation. Because the Secretary did not make the required threshold finding in these cases, we have no occasion to determine whether costs must be weighed against benefits in an appropriate case.

A

Under the Government's view, § 3 (8), if it has any substantive content at all,^[45] merely requires OSHA to issue standards *641 that are reasonably calculated to produce a safer or more healthy work environment. Tr. of Oral Arg. 18, 20. Apart from this minimal requirement of rationality, the Government argues that § 3 (8) imposes no limits on the Agency's power, and thus would not prevent it from requiring employers to do whatever would be "reasonably necessary" to eliminate all risks of any harm from their workplaces.^[46] With respect to toxic substances and harmful physical agents, the Government takes an even more extreme position. Relying on § 6 (b) (5)'s direction to set a standard "which most adequately assures . . . that no employee will suffer material impairment of health or functional capacity," the Government contends that the Secretary is required to impose standards that either guarantee workplaces that are free from any risk of material health impairment, however small, or that come as close as possible to doing so without ruining entire industries.

If the purpose of the statute were to eliminate completely and with absolute certainty any risk or serious harm, we would agree that it would be proper for the Secretary to interpret §§ 3 (8) and 6 (b) (5) in this fashion. But we think it is clear that the statute was not designed 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. Rather, both the language and structure of the Act, as well as its legislative history, indicate that it was intended to require the elimination, as far as feasible, of significant risks of harm.

*642 B

By empowering the Secretary to promulgate standards that are "reasonably necessary or appropriate to provide safe or healthful employment and places of employment." the Act implies that, before promulgating any standard, the Secretary must make a finding that the workplaces in question are not safe. But "safe" is not equivalent of "risk-free." There are many activities that we engage in every day—such as driving a car or even breathing city air—that entail some risk of accident or material health impairment; nevertheless, few people would consider these activities "unsafe." Similarly, a workplace can hardly be considered "unsafe" unless it threatens the workers with a significant risk of harm.

Therefore, before he can promulgate any permanent health or safety standard, the Secretary is required to make threshold finding that a place of employment is unsafe—in the sense that significant risks are present and can be eliminated or lessened by a change in practices. This requirement applies to permanent standards promulgated pursuant to § 6 (b) (5), as well as to other types of permanent standards. For there is no reason why § 3 (8)'s definition of a standard should not be deemed incorporated by reference into § 6 (b) (5). The standards promulgated pursuant to § 6 (b) (5) are just one species of the genus of standards governed by the basic requirement. That section repeatedly uses the term "standard" without suggesting any exception from, or qualification of, the general definition; on the contrary, it directs the Secretary to select "the standard"—that is to say, one of various possible alternatives that satisfy the basic definition in § 3 (8)—that is most protective.^[47] Moreover, requiring the *643 Secretary to make a threshold finding of significant risk is consistent with the scope of the regulatory power granted to him by § 6 (b) (5), which empowers the Secretary to promulgate standards, not for chemicals and physical agents generally, but for "toxic materials" and "harmful physical agents."^[48]

This interpretation of §§ 3 (8) and 6 (b) (5) is supported by the other provisions of the Act. Thus, for example § 6 (g) provides in part that

"[i]n determining the priority for establishing standards under this section, the Secretary shall give due regard to the urgency of the need for mandatory safety and health standards for particular industries trades, *644 crafts, occupations, business, workplaces or work environments."

The Government has expressly acknowledged that this section requires the Secretary to undertake some cost-benefit analysis before he promulgates any standard, requiring the elimination of the most serious hazards first.^[49] If such an analysis must precede the promulgation of any standard, it seems manifest that Congress intended, at a bare minimum, that the Secretary find a significant risk of harm and therefore a probability of significant benefits before establishing a new standard.

Section 6 (b) (8) lends additional support to this analysis. That subsection requires that, when the Secretary substantially alters an existing consensus standard, he must explain how the new rule will "better effectuate" the purposes of the Act.^[50] If this requirement was intended to be more than a meaningless formality, it must be read to impose upon the Secretary the duty to find that an existing national consensus standard is not adequate to protect workers from a continuing and significant risk of harm. Thus, in this case, the Secretary was required to find that exposures at the current permissible *645 exposure level of 10 ppm present a significant risk of harm in the workplace.

In the absence of a clear mandate in the Act, it is unreasonable to assume that Congress intended to give the Secretary the unprecedented power over American industry that would result from the Government's view of §§ 3 (8) and 6 (b) (5), coupled with OSHA's cancer policy. Expert testimony that a substance is probably a human carcinogen—either because it has caused cancer in animals or because individuals have contracted cancer following extremely high exposures—would justify the conclusion that the substance poses some risk of serious harm no matter how minute the exposure and no matter how many experts testified that they regarded the risk as insignificant. That conclusion would in turn justify pervasive regulation limited only by the constrain of feasibility. In light of the fact that there are literally thousands of substances used in the workplace that have been identified as carcinogens or suspect carcinogens, the Government's theory would give OSHA power to impose enormous costs that might produce little, if any, discernible benefit.^[51]

*646 If the Government were correct in arguing that neither § 3 (8) nor § 6 (b) (5) requires that the risk from a toxic substance be quantified sufficiently to enable the Secretary to characterize it as significant in an understandable way, the statute would make such a "sweeping delegation of legislative power" that it might be unconstitutional under the Court's reasoning in A. L. A. Schechter Poultry Crop. v. United States, 295 U. S. 495, 539, and Panama Refining Co. v. Ryan, 293 U. S. 388. A construction of the statute that avoids this kind of open-ended grant should certainly be favored.

C

The legislative history also supports the conclusion that Congress was concerned, not with absolute safety, but with the elimination of significant harm. The examples of industrial hazards referred to in the Committee hearings and debates all involved situations in which the risk was unquestionably significant. For example. the Senate Committee on Labor and Public Welfare noted that byssinosis, a disabling lung disease caused by breathing cotton dust, affected as many as 30% of the workers in carding or spinning rooms in some American cotton mils and that as many as 100,000 active or retired workers were then suffering from the disease. It also noted that statistics indicated that 20,000 out of 50,000 workers who had performed insulation work were likely to die of asbestosis, lung, cancer, or mesothelyioma as a result of breathing asbestos fibers. Another example given of an occupational health hazard that would be controlled by the Act was betanaphthylamine, a "chemical so toxic that any exposure at all is likely to cause the development of bladder cancer over a period of years." S. Rep. No. 91-1282, pp. 3-4 (1970); Legislative History of the Occupational Safety and health Act of 1970 (Committee Print compiled for the Senate Committee on Labor and Public Welfare), pp. 143-144 (1971) (hereafter Leg. Hist.).

Moreover, Congress specifically amended § 6 (b) (5) to make *647 it perfectly clear that it does not require the Secretary to promulgate standards that would assure an absolutely risk-free workplace. Section 6 (b) (5) of the initial Committee bill provided that

"[t]he Secretary, in promulgating standards under this subsection, shall set the standard which most adequately and feasibly assures, on the basis of the best available evidence, that no employee will suffer any impairment of health or functional capacity, or diminished life expectancy even if such employee has regular exposure to the hazard dealt with by such standard for the period of his working life." (Emphasis supplied.) S. 2193, 91st Cong., 2d Sess., p. 39 (1970), Leg. Hist. 242.

On the floor of the Senate, Senator Dominick questioned the wisdom of this provision, stating:

"How in the world are we ever going to live up to that? What are we going to do about a place in Florida where mosquitoes are getting at the employee—perish the thought that there may be mosquitoes in Florida? But there are black flies in Minnesota and Wisconsin. Are we going to say that if employees get bitten by those for the rest of their lives they will not have been done any harm at all? Probably they will not be, but do we know?" 116 Cong. Rec. 36522 (1970), Leg. Hist. 345.

He then offered an amendment deleting the entire subsection.^[52]*648 After discussions with the sponsors of the Committee bill, Senator Dominick revised his amendment. Instead of deleting the first sentence of § 6 (b) (5) entirely, his new amendment limited the application of that subsection to toxic materials and harmful physical agents and changed "any" impairment of health to "material" impairment.^[53] In discussing this change, Senator Dominick noted that the Committee's bill read as if a standard had to "assure that, no matter what anybody was doing, the standard would protect him for the rest of his life against any foreseeable hazard." Such an "unrealistic standard," he stated, had not been intended by the sponsors of the bill. Rather, he explained that the intention of the bill, as implemented by the amendment, was to require the Secretary

"to use his best efforts to promulgate the best available standards, and in so doing, . . . he should take into account that anyone working in toxic agents and physical *649 agents which might be harmful may be subjected to such conditions for the rest of his working life, so that we can get at something which might not be toxic now, if he works in it a short time, but if he works in it the rest of his life might be very dangerous; and we want to make sure that such things are taken into consideration in establishing standards." 116 Cong. Rec., at 37622-37623, Leg. Hist. 502-503.^[54]

Senator Williams, one of the sponsors of the Committee bill, agreed with the interpretation, and the amendment was adopted.

In their reply brief the federal parties argue that the Dominick amendment simply means that the Secretary is not required to eliminate threats of insignificant harm; they argue that § 6 (b) (5) still requires the Secretary to set standards that ensure that not even one employee will be subject to any risk of serious harm—no matter how small that risk may be.^[55]*650 This interpretation is at odds with Congress' express recognition of the futility of trying to make all workplaces totally risk-free. Moreover, not even OSHA follows this interpretation of § 6 (b) (5) to its logical conclusion. Thus, if OSHA is correct that the only no risk level for leukemia due to benzene exposure is zero and if its interpretation of § 6 (b) (5) is correct, OSHA should have set the exposure limit as close to zero as feasible. But OSHA did not go about its task in that way. Rather, it began with a 1 ppm level, selected at least in part to ensure that employers would not be required to eliminate benzene concentrations that were little greater than the so-called "background" exposures experienced by the population at large. See n. 14 supra. Then, despite suggestions by some labor unions that it was feasible for at least some industries to reduce exposures to well below 1 ppm,^[56] OSHA decided to apply the same limit to all, largely as a matter of administrative convenience. 43 Fed. Reg. 5947 (1978).

OSHA also deviated from its own interpretation of § 6 (b) (5) in adopting a