To underscore the intuition behind precautionary risk regulation and the harm that can result from the absence of a requisite standard of due care, this final substantive chapter examines a landmark case of an uncertain threat of environmental harm: namely, the public health hazard that the manufacture and emission of tetraethyl lead (TEL) and leaded gasoline involved for more than five decades. Tetraethyl lead was commonly used as a gasoline additive to prevent engine knocking and improve automotive efficiency. With its mass production in the early 1920s, various high-profile instances of acute lead poisoning surfaced, which affirmed for many the danger to the broader public. Yet corporations like General Motors, DuPont, and Standard Oil, which had strong vested interests in the continued and expanded production of TEL and leaded gasoline, capitalized on the prevailing uncertainty to block initiatives to implement mandatory emission standards until the 1970s. These companies enjoyed proprietary rights to the scientific data that could corroborate the health effects of exposure to leaded gasoline exhaust, which was purposefully withheld from regulators, consumers, and the general public. Moreover, these corporations commonly misrepresented the dangers of exposure, funded industry-led studies to cast doubt on the empirical findings of public health scientists who confirmed that exposure to TEL had adverse health effects, and ignored safer alternative additives that could have served the same purpose—though, perhaps, with a less profitable return—without endangering the public.