Preemption raises complex issues of law, policy, and federalism. Under the Supremacy Clause of the U.S. Constitution, state law and policy (or law and policy of local subdivisions of states, acting under state law) that conflicts with federal law must yield to the exercise of Congress's powers. Whether a certain state action is preempted by federal law is a question of congressional intent. When it acts, Congress can preempt state action within a field entirely, allow for states to act freely, or permit state action to any intermediate degree. Federal statutes relating to protection of the environment and public health have taken a diverse range of approaches to preemption ranging between these two extremes of field preemption and no preemption. The Toxic Substances Control Act (TSCA), which defines the authority of the Environmental Protection Agency (EPA) to assess and regulate chemical substances in U.S. commerce, takes an intermediate approach. An overview of TSCA as a whole is necessary to place TSCA's preemption provisions in context. For more information, see CRS Report RL31905, The Toxic Substances Control Act (TSCA): A Summary of the Act and Its Major Requirements , by [author name scrubbed]. TSCA establishes a chemical regulatory program allowing EPA to review new chemical substances before they enter U.S. commerce, as well as significant new uses of chemical substances as designated by EPA (§5); to regulate chemicals in commerce that EPA determines to present unreasonable risks of injury to human health or the environment, using the least burdensome requirements that adequately protect against such risks (§6); and to require testing on chemicals that may present an unreasonable risk, or that merit testing by virtue of certain high production volume or high exposure potential criteria (§4). TSCA also authorizes and mandates that EPA require reporting of certain information on chemicals by manufacturers (including importers), processors, and sometimes distributors, and to maintain an inventory of chemicals and substances that have commenced non-exempt manufacture or import in the United States (§8). In addition, among other provisions, TSCA requires EPA to maintain confidentiality of trade secrets and privileged or confidential business information in a particular manner (§14), coordinate with other federal agencies and EPA offices (§9), and respond to citizens' petitions (§21). TSCA's core provisions (Title I) have not been significantly amended since the law's enactment in 1976. It is also important to note that TSCA applies to "chemical substances" (defined in §3). To avoid overlapping with other federal statutes, TSCA defines this term to exclude certain categories of materials and substances from its scope. Substances when used in pesticides, foods (including ingredients and additives), food contact substances (such as containers), personal care products, tobacco, nuclear materials, firearms, and shells and cartridges are not within the scope of TSCA—or consequently, within the scope of TSCA preemption. TSCA's preemption provisions (§18) base preemption on certain actions taken by EPA under a handful of specified statutory provisions. So long as EPA has not specifically addressed a chemical under TSCA, there is no preemption of state regulation of that same chemical. When EPA does take certain actions regarding a chemical, TSCA provides for preemption of duplicative testing requirements and of non-identical state regulations, other than total bans or disposal regulations, unless the state regulations are adopted under other federal laws or an exemption is granted. Specifically, TSCA Section 18(a) provides the following: If EPA promulgates a rule under TSCA Section 4 requiring manufacturers or processors of a chemical substance to conduct testing on that substance to develop data on its health and environmental effects, then a state or political subdivision cannot establish or continue its own testing requirement rule for that chemical substance "for purposes similar to those for which testing is required" by EPA. EPA has issued testing rules for several hundred chemicals under Section 4. These would preempt similar testing requirements by states, but research has uncovered no cases finding preemption of state testing requirements by EPA testing rules. If EPA prescribes a rule or order under TSCA Section 5 (new chemicals) or Section 6 (existing chemicals), which is designed to protect against a risk of injury to health or the environment associated with a chemical substance, then a state or political subdivision cannot establish or continue any non-identical requirement applicable to the same chemical substance (or article containing such chemical substance) that is designed to protect against the same risk. Since 1976, EPA has promulgated rules imposing restrictions to protect against risk of injury to health or the environment from six existing chemicals under Section 6 of TSCA and four new chemicals under Section 5(f). These few EPA rules have not preempted many state or local requirements. Because these are the only bases for preemption, EPA information-gathering rules under Section 8, or actions under any other section of TSCA, do not preempt state or local actions. Pursuant to exceptions, any preemption by EPA rules or orders under Sections 5 or 6 would not extend to the following: State regulations on the manner or method of disposal of chemicals; an EPA rule pertaining to disposal has no preemptive effect. State requirements that are identical to federal requirements, enabling states to provide additional enforcement (co-enforcement) of those requirements. State requirements that are adopted under the authority of any other federal law. Complete prohibitions on the use of particular chemicals within a state's jurisdiction. However, a state cannot prohibit the chemical's use in manufacturing or processing of other substances. TSCA Section 18(b) allows a state to apply to EPA for an exemption to allow a more stringent state chemical risk management regulation that would otherwise be preempted. EPA may, by rule, grant such an exemption so long as compliance with the state requirement would not cause a violation of the federal requirements for the chemical; would provide a significantly higher degree of protection from risk; and would not "unduly burden interstate commerce." It appears that no exemption rule has ever been issued. Section 18 of TSCA does not expressly preempt or affect lawsuits regarding chemical exposures under common law. Nonetheless, courts would generally interpret preempted "requirements" to include state common law. Thus, it is possible, for example, that a defendant's compliance with TSCA could provide a viable preemption defense to a lawsuit, if the common law or statutory requirement forming the basis for the lawsuit were designed to protect against the same risk as an EPA rule or order under TSCA Sections 5 or 6. The division of federal and state authority over chemicals is also affected by the lack of express authority, in Section 18 or elsewhere, for EPA to share with states any trade secret information that it collects that is confidential under TSCA Section 14. Legislative efforts to revise TSCA's chemical regulatory framework date back at least to 2005. The legislative proposals that have been discussed over this time generally would preserve the basic organization of TSCA Title I; for example, provisions related to testing would remain in Section 4, new chemicals and new uses in Section 5, regulation of existing chemicals in Section 6, and preemption in Section 18. They would also generally continue TSCA's exclusion of specified categories of substances regulated under other laws. Yet the proposals would make significant changes to the operation of TSCA. The proposals differ in various respects from one another, however, including with respect to preemption. In the 114 th Congress, S. 697 , the Frank R. Lautenberg Chemical Safety for the 21 st Century Act, was introduced on March 10, 2015. A hearing on the bill was held before the Senate Environment and Public Works Committee on March 18, 2015. Following a markup, an amended version of S. 697 was ordered to be reported out of the committee on April 28, 2015, on a 15-5 vote with bipartisan support. S. 697 was reported with an amendment in the nature of a substitute on June 17, 2015, and placed on the Senate legislative calendar. An alternate proposal, S. 725 , the Alan Reinstein and Trevor Schaefer Toxic Chemical Protection Act, has not been ordered to be reported out of committee. S. 725 would eliminate express preemption from TSCA entirely, and is not included in this report's comparison. In the House of Representatives, a discussion draft entitled the TSCA Modernization Act of 2015 was released in early April, and a hearing was held in the Subcommittee on Environment and the Economy of the Committee on Energy and Commerce on April 14, 2015. After some revisions, the House discussion draft was approved by the subcommittee on May 14, 2015, on a 21-0 voice vote, also on a bipartisan basis. The TSCA Modernization Act of 2015 was formally introduced, with some amendments, on May 26, 2015, as H.R. 2576 and referred to the full committee on Energy and Commerce. The committee ordered the bill to be reported on June 3, 2015, on a 47-0 vote, with one abstaining, and the bill was reported on June 23, 2015. The bill, as amended, was debated on the House floor and passed the House on June 23, 2015, on a 398-1 vote. S. 697 and H.R. 2576 address many of the same topics and provisions of TSCA, with important similarities and differences in their approaches. For additional discussion of select issues addressed by the bills, see CRS Report R44434, Proposed Amendments to the Toxic Substances Control Act (TSCA) in the 114th Congress: H.R. 2576 Compared with the Senate Substitute Amendment , by [author name scrubbed] and [author name scrubbed]. S. 697 generally takes a more comprehensive and detailed approach to amending TSCA.