The OSH Act authorizes the Secretary of Labor to promulgate federal occupational safety and health standards. 29 U.S.C. § 655. In the Superfund Amendments and Reauthorization Act of 1986 (SARA), Congress directed the Secretary of Labor to “promulgate standards for the health and safety protection of employees engaged in hazardous waste operations” pursuant to her authority under the OSH Act. OSHA promulgated regulations on “Hazardous Waste Operations and Emergency Response,” including detailed regulations on worker training requirements. The State of Illinois enacted the licensing acts at issue here. The laws are designated as acts “in relation to environmental protection,” and their stated aim is to protect both employees and the general public by licensing hazardous waste equipment operators and laborers working at certain facilities. Both licensing acts require a license applicant to provide a certified record of at least 40 hours of training under an approved program conducted within Illinois, to pass a written examination, and to complete an annual refresher course of at least eight hours of instruction. P is a national trade association whose members are subject to the OSH Act and OSHA regulations, and are therefore required to train, qualify, and certify their hazardous waste remediation workers. The workers employed by P's members are also required to obtain licenses pursuant to the Illinois licensing acts. P brought a declaratory judgment action in United States District Court against D seeking to enjoin D from enforcing the Illinois licensing acts claiming that the acts were pre-empted by the OSH Act and OSHA regulations and that they violated the Commerce Clause of the United States Constitution. The District Court held that state laws that attempt to regulate workplace safety and health are not pre-empted by the OSH Act when the laws have a “legitimate and substantial purpose apart from promoting job safety.” The District Court invalidated the requirement that applicants for a hazardous waste license be trained “within Illinois” on the ground that the provision did not contribute to D's stated purpose of protecting public safety. The Seventh Circuit held that the OSH Act pre-empts all state law that “constitutes, in a direct, clear and substantial way, regulation of worker health and safety,” unless the Secretary has explicitly approved the state law. The Court of Appeals remanded the case to the District Court without considering which, if any, of the Illinois provisions, would be pre-empted. The court made clear that D “cannot regulate worker health and safety under the guise of environmental regulation,” and it rejected the District Court's conclusion that the State's 4,000-hour experience requirement could survive pre-emption simply because the rule might also enhance public health and safety. The Supreme Court granted certiorari.