<human>:  How did the Clean Air Act of 1990 affect the regulation of greenhouse gases?
<bot>: The Clean Air Act of 1990 was a landmark piece of legislation that regulated greenhouse gases for the first time. It was passed in response to the growing concern about global warming. The act established the Environmental Protection Agency (EPA) to regulate greenhouse gases, and it required the EPA to set standards for emissions of these gases. The EPA has since set standards for emissions of greenhouse gases, including carbon dioxide, methane, and nitrous oxide.