Despite continuing air-quality improvements, Lancaster was the 10th-smoggiest midsize metropolitan area in the United States during 2010, according to a report.

Using data from government air-quality monitors, the Philadelphia-based group PennEnvironment also found the Lancaster area tied with Pittsburgh for the second-most number of unhealthy smog days among all metropolitan areas in Pennsylvania.

Philadelphia had the worst smog in the state, according to the report, "Danger in the Air: Unhealthy Air Days in 2010 and 2011."

Despite the poor air-quality marks, Lancaster is in compliance with the federal Environmental Protection Agency's attainment requirements for ground-level ozone, the main component of smog. For many years it was out of attainment, including as recently as 2009.

There were 13 days in the summer of 2010 in which the air violated smog limits. Ozone monitors are maintained atop Lincoln Middle School in Lancaster city, and there is a downwind monitor in Leacock Township.

Smog is formed usually in the summer months when ozone pollution from vehicles, coal-fired power plants and industrial processes bakes in the heat.

Ozone is one of the most harmful and pervasive air pollutants in the country, according to the PennEnvironment Research & Policy Center.

High ozone levels particularly affect children, the elderly and people with respiratory ailments, such as asthma. But even healthy adults can experience reduced lung function if exercising amid ozone pollution.

"It's important to keep this issue in front of the public so they understand that air quality is not all it should be," said Kevin M. Stewart, director of environmental health for the American Lung Association's Mid-Atlantic region.

"It's not like the old days where you could cut smog with a knife. Those days are thankfully rare. But there also have been discoveries from scientists that air pollution is hazardous at levels that are lower than earlier thought.

"We want everyone to understand that even with invisible ozone you can still have health effects, especially among certain at-risk individuals."

Lancaster has had poor ozone levels for years. It's not all the area's fault. Levels are exacerbated because the area sits downwind of plumes of high smog pollution blowing in from the Washington-Baltimore area and from power plants in western Pennsylvania and the Midwest.

Another poor showing in Lancaster's air comes amid controversy over federal efforts to toughen smog levels.

In 2008, the Bush administration set tougher smog limits, but far below what the EPA's scientific panel had urged. Some environmental groups, including the American Lung Association, sued.

President Barack Obama seemed poised to implement those tougher ozone limits but announced earlier this month that he would hold off until at least 2013.

The president cited the need to ease regulatory burdens as the country tries to recover from a sick economy.

Business groups hailed the decision, but environmental groups were critical. The ALA said it might revive its litigation.

EPA's scientific panel had claimed the more stringent ozone standards could save up to 12,000 lives annually from heart attacks, lung disease and asthma attacks.

On Tuesday, the National Latino Coalition on Climate Change criticized the non-action, saying one of every two Hispanic Americans lives in a county that frequently violate air pollution standards.

There is one new air-pollution move by the federal government that could bode well for Lancaster's air.

In July, the EPA finalized rules that would require cuts in smokestack emissions at power plants that can travel hundreds of miles by wind.

The Cross-State Air Pollution Rule, proposed to take effect in 2014, would require power plants to cut nitrogen oxides — a main component of smog — by 54 percent from 2005 levels.

The lives of an estimated 2,900 Pennsylvanians could be saved annually by the pollution cuts, according to EPA.

But already there is a battle in Congress to defeat the proposed regulation.

The five cities with the most smog in 2010 were all in California. They were followed by Baltimore, Washington, D.,C. and Philadelphia.

acrable@lnpnews.com