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Amendments to the Clean Air Act, adopted in 1990, ordered the Environmental Protection Agency to tighten controls on ozone, which leads to smog, which leads to asthma and many other respiratory difficulties. In 1997 (these things take time) the EPA imposed stiff new regulations to protect public health.
In 2004, EPA issued revised regulations that were weaker than the 1997 regs, weaker even than the regs that the 1997 regs replaced. Environmental and public health organizations filed suit, and the court found the 2004 regs illegal.
The EPA and industry groups appealed the decision and in late 2006, a three-judge panel of appeals-court judges upheld the ruling, affirming that the 2004 regs are illegal. Various parties asked the judges to rehear the case. In June 2007, they refused and actually strengthened their earlier ruling.