Maryland has cleaned up its air enough since the late 1980s that the Environmental Protection Agency reclassified yesterday much of the state's ozone pollution as moderate from severe - even under a new, stricter definition of clean air.
But pollution here remains among the country's worst, and state officials say they will have a tough time cutting ground-level ozone further.
The better grade the state got yesterday reflects a decade of tightening controls on everything from smokestacks and auto exhaust to gas canisters, hairspray and perfume bottles.
Dirty air blowing in from power plants in the Ohio River Valley accounts for about half of Maryland's ozone problem, and state officials say little more can be done locally.
"The low-hanging fruit is pretty much all gone in Maryland," said George Aburn, manager of air quality planning at the Maryland Department of the Environment. "Without significant reductions in pollution that floats in from other states, we'll never meet the [new] ozone standard."
The EPA released the new "moderate" classification for Baltimore and 13 Maryland counties yesterday as part of a list of 474 counties, in 31 states, failing to meet stricter ozone standards. The Maryland localities also had fallen short of the old standards.
The EPA for the first time raised a flag about ozone in Washington County, putting it on a kind of probation with a 2007 deadline to improve.
Ozone, the primary ingredient in smog, is formed when paint and auto fumes chemically react in sunlight with nitrogen oxides, a product of burning fuels in car engines and power plants. Ozone damages lung cells, making breathing difficult and aggravating asthma.
The EPA revised its standard for ozone in 1997, after scientific studies found harm to human health at lower exposure levels over longer periods. Three states and dozens of companies sued to stop the new rule, but they lost before the Supreme Court in 2001.
Under the new standard, air is unhealthy if its ozone concentrations are 85 parts per billion or higher, averaged over eight hours. The old standard was 120 parts per billion, measured hourly.
Ozone levels in Baltimore and its suburbs average 103 parts per billion under the eight-hour standard, said Judith M. Katz, the top air protection official at the EPA's regional office in Philadelphia.
