Half of Maine’s counties are violating federal air pollution standards for ozone, according to an Environmental Protection Agency report released Thursday.

Ground-level ozone, also known as smog, is an odorless gas created from the combination of sunlight and a soup of airborne chemicals that result from burning fossil fuels in vehicles and factories.

Maine’s location, frequently described by state environmental officials as “the tailpipe of America,” guarantees that pollutants will be available along the coast although Maine has few “dirty” industries. Maine likely produces a bit of domestic smog from cars clogging the roads during the summer tourist season, but the majority of the pollution travels on prevailing winds from the Midwest and southern New England, according to state meteorologists.

State officials have long known of air quality problems. Past efforts to comply with federal standards included Gov. Angus King’s short-lived tailpipe emissions testing as well as the state’s shift to gasoline with MTBE. But the problem continues to grow.

Public health advocates worry that the rise in ozone levels has been a factor in the state’s unusually high population of asthma sufferers, and that smog harms those who are already sensitive to air pollution. Dangerous ozone alerts, suggesting that asthmatics, the elderly and small children avoid strenuous activities outside, have become common during the Maine summer.

Nationwide 31 states and 474 counties were cited for nonattainment. The Los Angeles basin had the dubious honor of having the nation’s worst smog. The EPA report states that the entire states of Rhode Island, Connecticut and Massachusetts, as well as all of New England’s coastal counties failed to meet standards.

In Maine, at least a portion of Androscoggin, Cumberland, Hancock, Knox, Lincoln, Sagadahoc, Waldo and York counties failed to meet the standard. In total, 108 Maine towns were cited for unhealthful ozone levels, and Sagadahoc was the sole county to have every community in violation.

The EPA report mirrored DEP’s own recommendations based on the past three years’ data from air quality monitors scattered throughout the state that measure ozone levels over an eight-hour period. Previously, the federal government has used one-hour measurements, which critics said were not sufficient, as ozone levels rise and fall during the day.

The EPA proposes to address the growing air quality programs with a suite of changes known as the Clean Air Rules of 2004. EPA Administrator Mike Leavitt Thursday called the rules “strong medicine.”

The ozone rules require that Maine prepare a plan for reducing ozone levels in its nonattainment areas – by 2007 in the four most southerly counties and by 2009 in Hancock, Knox Lincoln and Waldo – or risk sanctions, which could include the loss of federal transportation funds. Other rules seek to address the transport of pollution across state borders, though the implementation of new restrictions could take years.

New England is increasingly taking a regional approach to its air quality problem, proposing joint legislation across the region to reduce sources of pollution, as well as testifying against the construction of new projects elsewhere that are expected to cause new pollution drift.

Jim Brooks, who heads DEP’s air bureau, said Thursday that his department is developing new rules for all-terrain vehicles, paints, marine engines and gasoline cars that will help the state meet the EPA deadlines.

“We’ll look over the rules and we’ll develop a strategy,” he said. “I feel confident that over the next three to six years a large part of the eastern United States can come into attainment, but I can’t guarantee it.”

Environmentalists Thursday criticized the Bush administration’s approach, calling the Clean Air Rules of 2004 “smoke and mirrors.”

“The air is dirty,” said Matthew Davis of Portland-based Environment Maine. “People in these towns need to tell the EPA they want strong federal action to clean up the air.”

The Associated Press contributed to this story.