This section contains the nature of the case and procedural background.

Lead 'antiknock' compounds, when added to gasoline, dramatically increase the fuel's octane rating. Increased octane allows for higher compression engines, which operate with greater efficiency. Since 1923 antiknocks have been regularly added to gasoline. Scientists have always questioned whether the addition of lead to gasoline, and its consequent diffusion into the atmosphere from the automobile emission, poses a danger to the public health. Despite its universal presence, lead serves no known purpose in the human body, and at higher concentrations is toxic, causing anemia, severe intestinal cramps, paralysis of nerves, fatigue, and even death. A major source of lead exposure for humans is the ambient air. This source is easily the most controllable, since approximately 90 percent of lead in the air comes from automobile emissions, and can be simply eliminated by removing lead from gasoline. The Clean Air Act authorizes D to regulate gasoline additives whose emission products 'will endanger the public health or welfare * * *.' D, after notice and comment, determined that the automotive emissions caused by leaded gasoline present 'a significant risk of harm' to the public health. D promulgated regulations that reduce, in step-wise fashion, the lead content of leaded gasoline. Ps appealed the promulgation of low-lead regulations. Ps' main claim is that D misinterpreted the statutory standard of 'will endanger' and that his application of that standard is without support in the evidence and arbitrary and capricious. Ps argue that the 'will endanger' standard requires a high quantum of factual proof, proof of actual harm rather than of a 'significant risk of harm.' The appeal was heard, and the judge ordered the regulations set aside. D requested a rehearing en banc. D claims that the statutory language * * * does not require a determination that automobile emissions alone create the endangerment on which controls may be based.

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