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When triple-digit temperatures and a high ozone alert combined last week to make breathing oppressive for vulnerable groups such as asthma sufferers, at least one good thing happened.

People who would have been most adversely affected stayed indoors and avoided strenuous activity, said Denise Cuneo, director of respiratory therapy at Sturdy Memorial Hospital in Attleboro.

"Our emergency room didn't have a lot of individuals come in," Cuneo said. "I think a lot of people used good sense and used their air conditioners."

Cuneo also said advances in treatment, new drugs and exercise regimens have helped an increasing number of people suffering from emphysema and other disorders slow the progressive effects of lung disease.

But if there were few casualties as a result of air pollution in the Attleboro area last week, the air itself gets little of the credit. That's mostly because of high ozone contamination.

The American Lung Association gave the air in Bristol County a "D" grade for ozone content in its 2008 "State of the Air" report released in March. That's up slightly from the prior year, when the county rated an "F."

Although EPA measurements have shown reductions in key pollutants, such as carbon monoxide and airborne particles over the years, Bristol County is one of 11 Bay State counties currently not in compliance with federal standards for ozone, according to a recent report.

Last year, the county experienced days of elevated ozone levels that could affect public health as well as two for particles in the air, according to the Lung Association. That number is expected to climb this year because of more stringent EPA ozone standards.

Ozone is the primary ingredient in smog, and harms breathing by chemically attacking lung tissue, the Lung Association says. The gas is formed by chemical reactions in the atmosphere involving nitrogen oxides and volatile organic compounds, produced primarily by tailpipe emissions, coal-fired power plants and other combustion. When the two contaminants come in contact with heat and sunlight, they form ozone smog.

Children and teens, senior citizens and people with pre-existing lung diseases are particularly vulnerable to ozone pollution, respiratory experts say.

Studies have shown that the risk of premature death increases with higher levels of ozone.

Ominously, recent research has shown a correlation between health effects and even short-term exposure, officials with the Lung Association say. Working independently, teams at Harvard, Johns Hopkins and New York University found a small but clear connection between daily ozone levels and increased deaths.

Breathing ozone for longer periods can alter the lungs' ability to function. And repeated inflammation due to exposure over a period of years can lead to a chronic "stiffening" of the lungs, and can worsen symptoms for those with disorders such as chronic obstructive pulmonary disease (COPD).

Curiously, the county's bad grades for ozone come at a time when ozone and other forms of air pollution have been declining locally and across the country. Nationally, particle pollution in the atmosphere declined by 11 percent from 2000 to 2006, and ozone dropped by 7 percent according to EPA figures.

In Bristol County, particle contamination declined from 12 micrograms per cubic meter in 2000 to 9.2 micrograms in 2007. Measurements for ozone also dropped.

"Over the past several years, levels of many pollutants, such as carbon monoxide have been improving," said Richard Burkhart of the Boston office of the EPA. The removal from lead from gasoline, increased efficiency of auto and truck engines and emphasis on cleaner-burning power generating plants have all helped clear the air.

But New England is not the master of its own destiny when it comes to clean air, Burkhart said.

Many of the contaminants that influence the quality of air in Massachusetts come from cars and power plants in the New York and Philadelphia areas, adding their impurities to those from traffic in Boston and power plants in Fall River and Salem.

Of course, the Attleboro area pollutes plenty on its own.

According to a 2001 report by the EPA, vehicles, industries and homes in Bristol County belched a total of 154,000 tons of carbon monoxide, alone, into the air in a 12-month period. The vast majority -114,000 tons - came from highway vehicles. A significant amount, however, was produced by power plants and factories.

According to EPA reports, about 1,100 tons came from electric utilities, mostly from the coal-fired Brayton Point generating plant.

While that that might seem incredibly dirty, Bristol County's air still would get relatively high marks if it wasn't for ozone.

Last year, the air was clean enough by comparison to earn a "B" from the American Lung Association for particle pollution.

What fails to impress Lung Association officials and others is the federal government's response to the ozone threat. While the federal government in March tightened the allowable amount of ozone in the air at 75 parts per billion, that was higher than the EPA's scientific advisers had recommended.

The Lung Association and others back the more stringent standard.

Margaret LaCroix, vice president for marketing and communication for the American Lung Association of Massachusetts, said greater reductions in ozone are supported by research showing that even relatively low levels can be damaging to human lungs.

"It can be likened to second-hand cigarette smoke," LaCroix said. "At one time, it had been regarded as more of a nuisance. But as research has progressed, we understand just how serious the effects can be."

If there's no immediate protection from ozone, area residents can take their own protective measures by watching pollution levels reported in the daily Air Quality Index, limiting strenuous outdoor activity at peak times and avoiding areas such as crowded highways and industrial areas when pollution is at its peak, the Lung Association says.

The Air Quality Index, or AQI, is the standard system used to notify the public about levels of air pollution. The index, which is widely reported in the media, tracks levels of both ozone and particle pollution.

The AQI reports air pollution levels on a daily basis ranked on a scale of 0 for pristine air all the way to 500 for air pollution levels that pose immediate danger to the public.

The scale is also broken down by color with green representing good air quality conditions, yellow for slightly dirtier air, orange, red, and so on.

If the day's ozone level is unhealthy, the Lung Association says, adjust your plans for the day, and that means avoiding prolonged vigorous activity outdoors.

The health effects of ozone are worsened over extended periods of exposure, and by the deep, rapid breathing that accompanies exercise. Plan the most strenuous activities for the early morning hours, which in most areas occurs before ozone levels climb.

Also adjust plans if the day's particle pollution level is unhealthy. That means staying away from high-traffic areas.

AQI levels can be found online through the U.S. Environmental Protection Agency's web site, www.epa.gov/airnow.