SEATTLE (AP) - Gov. Chris Gregoire is among those who plan to urge the U.S. Environmental Protection Agency to finalize a determination that greenhouse gases threaten public health.

Nearly 200 environmentalists, businesses, government officials, students and others are scheduled to testify Thursday at the EPA hearing in Seattle, the second of two public forums nationwide.

In April, the EPA found that concentrations of six greenhouse gases, including carbon dioxide and methane, pose dangers to human health and welfare. Its finding could lead to regulating the gases under the Clean Air Act.

The agency won't make a final decision until after listening to the public comments.

Environmentalists and others say quick action is needed because climate change already is affecting human health and the environment.

"We've just really run out of time for anything but the most ambitious, focused, determined clean energy transition," said K.C. Golden, policy director for the environmental group Climate Solutions. Golden is scheduled to testify.

Hundreds are expected to rally at noon Thursday outside the hearing location to support the EPA finding.

Opponents say new emission limits would be costly for companies and hurt the economy.

"The bottom line is climate change policy is going to be extremely expensive for citizens and businesses in the country," said Grant Nelson, governmental affairs director of the Association of Washington Business, who also planned to testify.

Last month, the EPA also found that tailpipe emissions from vehicles were contributing to climate change. The EPA said high levels of greenhouse gases are the result of human activities and will result in climate change effects such as more wildfires, flooding and harm to agriculture, wildlife and ecosystems.

"There's no question that at some level greenhouse gases will cause considerable harm to the planet and the things on it," said Philip Mote, Washington state's climatologist and a researcher at the Climate Impacts Group. "Finding a way to avoid harmful levels strikes many scientists as prudent given what we know."

Dr. Evan Kanter, national president of Physicians for Social Responsibility, said global warming will increase health risks as a result of more frequent and intense heat waves, extreme weather events such as heavy rainfall and storms, and the spread of infectious diseases from weather pattern changes.

"Global warming is one of those threats to human health," said Kanter, a Seattle physician.

Environmentalists praised the EPA's finding and said they hope it leads to legislative or regulatory action.

"It looks at this problem of climate change from a science-base approach. It acknowledges the threat that climate change poses to people," said Becky Kelley, campaign director for the Washington Environmental Council. "It paves the way for the executive branch to take action."

Earlier this week, President Barack Obama announced a plan to require the auto industry to build vehicles that average 35.5 miles per gallon by 2016 and reduce vehicle carbon dioxide emissions by about one-third.

Bryan Brendle with the National Association of Manufacturers, representing 11,000 companies, urged EPA regulators at Monday's hearing in Arlington, Va., not to finalize its determination. He said the Clean Air Act isn't the proper tool to deal with greenhouse gas emissions and that climate policy should be left to Congress.

A bill that would cut greenhouse gas emissions by 17 percent over the next 11 years is before the House Energy and Commerce Committee.

The Bush administration had stalled on making a determination on whether greenhouse gases contribute to air pollution that endangers public health. The Supreme Court ordered that determination in 2007 when it declared greenhouse gases were air pollutants covered by the Clean Air Act.

"This is an important reversal from the previous administration," said Janette Brimmer, staff attorney of Earthjustice, a public interest law firm in Seattle. "The EPA is right on. I'm going to urge them to take it to completion and do so with all haste."