Oscar Seidenberg and Harvey Hutchinson (defendants) leased a tract of land (the property) to the Columbia Organic Chemical Company (COCC) (defendant). COCC’s principals incorporated South Carolina Recycling and Disposal Inc. (SCRDI) (defendant) for the purpose of assuming COCC’s waste-handling business. SCRDI contracted with waste producers for the transport, recycling, and storing of hazardous waste. SCRDI haphazardly stored the waste on the property. The waste was kept in rusted drums, without regard to the chemical compatibility of the various waste products. As a result, in 1977, a toxic cloud formed when chemicals leaking from rusted drums reacted with rainwater, resulting in the hospitalization of 12 firemen. In 1979, an explosion and fire resulted when chemicals stored in glass jars leaked onto drums containing incompatible substances. The site manager for SCRDI could not identify the chemicals that caused the explosion, which made the fire difficult to extinguish. The United States (plaintiff) filed suit under § 107(a) of the Comprehensive Environmental Response, Compensation, and Liability Act of 1980 (CERCLA) against SCRDI, COCC, Seidenberg and Hutchinson. Additionally, the United States sued Allied Corporation, Monsanto Company, and EM Industries, Inc. (defendants), who had generated the waste. The United States argued the defendants were jointly and severally liable for the response costs of the toxic cloud, explosion, and fire under CERCLA. The defendants argued their conduct did not cause the damage, and they could not be liable under CERCLA. Alternatively, the defendants argued CERCLA’s liability scheme was retroactive and thus unconstitutional. The district court held that all defendants were strictly, and jointly and severally, liable under CERCLA. The district court further held CERCLA was prospective, directed at post-enactment consequences of past acts, and not retroactive. Alternatively, the district held that CERCLA, even if retroactive, was rationally related to a valid legislative purpose and therefore constitutional. Seidenberg, Hutchinson, Allied, Monsanto, and EM Industries appealed.