A big meeting in Copenhagen. A cap-and-trade bill in Congress. And now, a determination by the Environmental Protection Agency that global warming pollution is a threat to public health _ a move that clears the way for the first-ever federal regulations targeting climate-changing emissions.

Confused about the EPA's action on greenhouse gases?

Here are some questions and answers about what was announced by the EPA on Monday, and what it means for international climate negotiations, action in Congress and global warming in general:

Q: What did the EPA do exactly?

A: The EPA classified six greenhouse gases _ carbon dioxide, methane, nitrous oxide, hydrofluorocarbons, perfluorocarbons and sulfur hexafluoride _ as pollutants that threaten the health and welfare of the American people. This will compel the federal government _ unless it is blocked by Congress _ to regulate emissions of greenhouse gases for the first time under the 1970 Clean Air Act. The government already controls releases of sulfur dioxide, ground-level ozone, nitrogen oxides and carbon monoxide under the law.

Q: How do these six newly classified gases threaten us?

A: Unlike other pollutants, greenhouse gases threaten public health and welfare indirectly, by warming the planet rather than directly by having people breathe them in. In terms of health, the EPA expects global warming to expand the range of certain diseases such as Lyme disease, boost many allergens and worsen smog, which is known to trigger asthma attacks and cause lung damage. On the welfare front, a rise in sea level brought about by climate change could worsen flooding.

Q: Why did the EPA decide to do this?

A: A 2007 Supreme Court ruling declared that carbon dioxide and other greenhouse gases are air pollutants as defined by the Clean Air Act and that the EPA had the authority to regulate them if they are found to endanger public health and welfare. It directed the EPA to make that determination. The Bush administration never acted on the court order. By contrast, the Obama administration found the science overwhelming and announced it would set the first-ever greenhouse gas standards for automobiles and would move to control heat-trapping emissions from large industrial sources such as factories and power plants. The endangerment finding announced Monday was needed to make those proposed rules a reality.

Q: How does the EPA finding related to the climate negotiations now under way in Copenhagen, Denmark?