The United States has made progress in cleaning up air pollution, but 154.5 million people, about half the population, live where the air is so polluted with smog and particles that it is often dangerous to breathe, the American Lung Association said Wednesday.

In its annual report on air quality, State of the Air 2011, the?organization says that the Clean Air Act is working and warns against legislators who are trying to weaken the law.

"State of the Air tells us that the progress the nation has made cleaning up coal-fired power plants, diesel emissions and other pollution sources has drastically cut dangerous pollution from the air we breathe," said Charles Connor, American Lung Association president and chief executive.

"We owe our cleaner air to the Clean Air Act," Connor said. "We have proof that cleaning up pollution results in healthier air to breathe. That's why we cannot stop now. Half of our nation is still breathing dangerously polluted air. Everyone must be protected from air pollution."

The threats to the Clean Air Act are coming from the Republican, not the Democratic, side of Congress, in bills to strip the U.S. EPA of funding and curtail its powers to regulate air emissions.

The air is so polluted in some areas that it is often dangerous to breathe, the ALA said in its annual report on air quality across the United States.

About 48 percent of U.S. residents live in counties where smog (ozone) is too high, 20 percent live in areas where there are too many short-term spikes in pollution and six percent live in areas with harmful year-round soot (particle pollution), said the report.

About 17 million Americans live in areas afflicted by all three air pollution hazards, the report noted.

The report listed California as the most polluted state, where people are breathing some of the worst air.

Compared with other states, California has more polluted places, including Los Angeles, Long Beach, Riverside, Bakersfield and Fresno, the report said.

Honolulu in Hawaii and Santa Fe-Espanola in New Mexico are the only two cities in the nation that had no days in which smog and soot levels reached unhealthy ranges, making them the cleanest cities in the nation, said the report.

Research suggests air pollution threatens human health -- and not just the lungs.

Small particles of pollution can lodge deep in the lungs, triggering an inflammatory process that, over time, can spread elsewhere in the body and damage blood vessels and the heart, according to Dr. Norman Edelman, the ALA's chief medical officer.

On days in which smog levels spike, there's an increase in hospital admissions for respiratory illnesses, heart attacks and stroke in the two or three days following it, said Michael Jerrett, a professor of environmental health sciences at University of California, Berkeley's School of Public Health.

In addition to posing both long-term and short-term risks, pollution can also contribute to low birth weights, diabetes, cardiovascular disease, heart attack, stroke and, ultimately, shorter life spans, he warned.