The sources of particle pollution can range from woodstoves and diesel trucks to coal-fired power plants and heavy highway traffic -- to name a few. And while particle pollutants come in different sizes, the ones that do the most damage are the extremely tiny ultrafine particles.
These teeny particles can creep deep into the lungs where they cause irritation and inflammation, and can be troublesome to people with existing respiratory problems such as asthma and chronic obstructive pulmonary disease.
But it's also suspected that these same tiny particles might escape the lungs and make their way into the bloodstream, where they can they have body-wide effects, especially on the heart.
Cheyenne, Wyo., was the cleanest place in the United States for long-term particle pollution. Santa Fe, N.M., was second in this category, Honolulu was third, with Great Falls, Mont., and Flagstaff, Ariz., tied for fourth.
Fargo had the distinction of being the cleanest city in the land in this year's report. It was the only location to earn a place on all three clean city lists.
One big contrast between the 2009 report and the data from the previous year was for ozone. This year the American Lung Association found that some 175 million Americans live in locations with unhealthy ozone levels while a year ago only 92.5 million residents did so.
Numerous studies have shows that ozone exposure, over long periods of time, can harm human health. Last month, a study conducted at the University of California, Berkeley showed that long-term exposure to ground-level ozone is linked to an increased risk of death from respiratory ailments. Other studies have implicated ozone-heavy smog in heart disease.
The number of people affected by high levels of ozone has nearly doubled in part as a result of tighter standards for it adopted by the Environmental Protection Agency (EPA) in March of 2008.
Although the new EPA ozone standards were put in place in 2008, the data collected between 2005 and 2007 was recalculated against the stricter guidelines. And even with these new ozone thresholds, many scientists don't feel they are stringent enough to protect human health.
Still, there are improvements. While Los Angeles led the list of American cities for ozone levels, the city continues to make consistent improvements in reducing this threat.
"I was pleasantly surprised to see that L.A.'s hard work over the years has paid off. They've made concentrated efforts to clear up their ozone and really taken it seriously," said Janice Nolen, assistant vice president of national policy for the American Lung Association. But she admits that Los Angeles will likely remain number one in ozone for some time because of its geography.
"Heat and sunlight make a difference to ozone. And if you add lots of mountains and the traffic in Los Angeles, this allows ozone to cook more and the air there is less likely to move out," explained Nolen.
In fact, warm and sunny locations tend to have more ozone formation and seem to dominate the rankings for this pollutant: Six out of the top 10 cities in the country for ozone were in California, two were in Texas and both Arizona and North Carolina had one of the most polluted ozone communities.