Under the supremacy clause of the United States Constitution (U.S. Const., art. VI, cl. 2), federal law "shall be the supreme Law of the Land." (See Brown v. Mortensen (2011) 51 Cal.4th 1052, 1059 (Brown).) Therefore Congress may preempt state laws to the extent it believes such action is necessary to achieve its purposes. (City of New York v. FCC (1988) 486 U.S. 57, 63 (City of New York).) Congress may exercise that power expressly, or the courts may infer preemption under one of three implied preemption doctrines: conflict, obstacle, or field preemption. (Brown, supra, 51 Cal.4th at p. 1059.) Express preemption occurs when Congress defines the extent to which a statute preempts state law. (Viva! Internat. Voice for Animals v. Adidas Promotional Retail Operations, Inc. (2007) 41 Cal.4th 929, 935.) Conflict preemption exists when it is impossible to simultaneously comply with both state and federal law. (Ibid.) Obstacle preemption occurs when state law stands in the way of full accomplishment and execution of federal law. (Ibid.) Field preemption applies when comprehensive federal regulations leave no room for state regulation. (Ibid.) Preemption may be based either on federal statutes or on federal regulations that are properly adopted in accordance with statutory authorization. As a result, a federal agency acting within the scope of its congressionally delegated authority may preempt state regulation and "render unenforceable state or local laws that are otherwise not inconsistent with federal law." (City of New York, supra, 486 U.S. at p. 64.) The current appeal presents primarily the question of regulatory, not statutory, preemption. When regulatory preemption is at issue, the regulation's force does not depend on express congressional authorization to displace state law. As a result, a narrow focus on Congress' intent to supersede state law is not appropriate. (City of New York, supra, 486 U.S. at p. 64.) Therefore the focus is on the agency and whether it acted within the bounds of its lawful authority. (Ibid.) A federal agency's regulations will preempt any state or local laws that conflict with or frustrate the regulations' purpose. Beyond that, in proper circumstances an agency may determine that its authority is exclusive and preempts any state efforts to regulate at all in the forbidden area. (City of New York, supra, 486 U.S. at p. 64.) Many of the responsibilities conferred on federal agencies include a broad grant of authority to reconcile conflicting policies. "Where this is true, the Supreme Court has cautioned that even in the area of pre-emption, if the agency's choice to pre-empt 'represents a reasonable accommodation of conflicting policies that were committed to the agency's care by the statute, we should not disturb it unless it appears from the statute or its legislative history that the accommodation is not one that Congress would have sanctioned.'" (Ibid.)