These 10 cities had the highest level of year-round pollution and some of the highest incidences of heart and lung issues as well, according to the American Lung Association's annual report. Located in the southern edge of California's San Joaquin Valley, Bakersfield is hemmed in on three sides by mountains making it a perfect place for pollution to linger and collect. "Light prevailing winds push air south and the farther south you go, the worse the air," said Seyed Sadredin of the San Joaquin Valley Air Pollution Control District. The area is also a major oil producing region, which introduces diesel soot, from well pumps, and chemical fumes into the air. Still, the area has made great strides in recent years. Overall, harmful emissions are down 80% over the past 20 years, according to Sadredin. Future improvement should come from recent Environmental Protection Agency programs that offer cash incentives to replace dirty diesel engines in trucks, tractors and buses. Jared Blumenthal, the EPA's regional administrator, said local farmers have replaced many of their old tractors, as well as diesel-fueled water pumps with electrical ones. 2.Merced, Calif. Criss-crossed by major roads, including California's Route 99 and Route 140, which takes many travelers to Yosemite National Park, Merced has long struggled with ozone and emission-related pollution. In fac,t, some 80% of the area's harmful pollutants can be traced to transportation sources, said Bonnie Holmes-Gen, senior director for policy and advocacy for the American Lung Association in California. However, Merced is making progress in reducing its ozone levels, with the pollutant hitting it lowest level in 14 years, she said. Another issue facing the town is its proximity to San Francisco and Oakland. Jared Blumenfeld,of the EPA, estimates that as much as a quarter of the pollutants in Merced drift in from those metro areas. 3. Fresno, Calif. Fresno's air suffered a setback last year, with an increase in the particle pollutants found in smoke and haze like soot and sulfur dioxide, which can aggravate asthma and heart issues. The Fresno metro area's population sharply increased 16% between 2000 and 2010, spurring the once compact city to sprawl outwards. Now, residents drive much further distances to get to work, leaving a trail of exhaust and emissions. "Air pollution is proportional to vehicular traffic," said John Swanton, an air pollution specialist with the California Air Resources Board. Like other cities located in California's valleys, layers of warm air trap those emissions like a lid on a cooking pot and pollution builds up to unhealthy levels. 4. Los Angeles Los Angeles is still one of the worst cities for all types of pollution, but the air quality is much better than it was in the first State of the Air report released 14 years ago. Days with unhealthy ozone levels have fallen by a third since then, to about 125 days a year. Improvements in auto engines and clean-burning gasoline have made most of the difference, according to Carsten Warneke, a scientist with the Cooperative Institute for Research in Environmental Sciences at the University of Colorado Boulder. "The main emissions source [in Los Angeles] is vehicle exhaust, but the cars we're driving are getting better every year," he said. With those improvements, many of the volatile organic compounds that used to fill the skies of Tinseltown, such as benzene and toluene, have been reduced by about 98%, according to Warneke. 5. Hanford, Calif. A large part of Hanford's pollution problems stems from tiny bits of dust, soot and gasses in the air. Close to half of that mix is from nitrate particles, which have been boosted by the massive cattle feedlots in the area. Mishandled animal waste can release large quantities of ammonia into the air. The nitrogen in the ammonia then combines with auto emissions to form nitrate particle pollution. The town also receives doses of pollution from cities to the North, like Fresno. But even more comes from the traffic along the big North-South superhighways that flank both sides of town. About 40% of all the container freight tonnage in the country comes through the ports of Los Angeles, Long Beach and Oakland and much of it winds up traveling these roads. 6. Modesto, Calif. Situated at the northern end of the San Joaquin Valley, Modesto shares many of the same pollution issues other valley cities face. One of the biggest culprits: Exhaust from California's Route 99, a major truck route for agricultural produce haulers. But Modesto also gets emissions and industrial drift from the nearby San Francisco-Oakland metro area. The American Lung Association's Bonnie Holmes-Gen said reducing particle pollution in Modesto has proven tough on a local level as well. Many area residents burn wood for heat in the winter and the area is prone to droughts. In July, August and September, virtually no rain falls in most years. Wildfires can break out and keep skies smoky for weeks. 7. Visalia, Calif. Residents of Visalia can't rely on summer winds and rains to wash away high ozone and particle pollution. On average, less than a third of an inch of rain falls between June 1 and September 30. And since the city is located hard up against the Sierra foothills, air flowing eastward hits the mountains and stays there, building up pollutant levels. Forest fires and smoke are a bigger problem here than elsewhere in the San Joaquin Valley. Visalia is close to national forests in the mountains and the U.S. Forest Service holds "prescribed burns" to keep the underbrush under control, according to Seyed Sadredin of the San Joaquin Valley Air Pollution Control District. "Even worse, we have wild fires, which totally overwhelm the control in place," he said. In a particularly bad year, like 2008, he said, there can be 3,000 wildfires in the area. 8. Pittsburgh The air in this steel town was once so polluted with coal and coke soot that streetlights were sometimes turned on at high noon. Now, much of Pittsburgh's pollution comes from Ohio, West Virginia and further west, according to Neil Donahue, who studies transport pollution at Carnegie Mellon University. While coal-burning power plants strung for hundreds of miles along the Ohio River are still a problem, many plants can switch back and forth between coal and natural gas. So when gas prices drop, as they did last year, the plants rely more heavily on that cleaner-burning fuel, which accounts for some of the city's improved air quality. With its deep valleys, however, Pittsburgh's topography works against it, especially during the hazy days of summer, according to Donahue. That's when warm blankets of air bottle up pollution in the city's deep river valleys for days at a time. 9. El Centro, Calif. El Centro's air quality has several factors working against it, namely its hot, dry climate and the border it shares with Mexico, which has much less stringent clean air laws, according to Brad Poiriez, air pollution control officer for the region. The city tries to keep dirt roads and farm drives watered or covered with crushed asphalt to stop dust from flying, but there's still plenty of open land where strong winds can stir it up. The steady stream of traffic to and from Mexico only makes it worse. Often the line is more than two hours long, which means cars idle in the hot desert sun and emit a steady stream of exhaust, said Poiriez. Many of those cars are registered in Mexico and are older models with engines that burn dirtier than newer cars. The air pollution control office has been working with Mexican authorities to require vehicles that cross the border to meet U.S. emissions standards. It has also gotten Mexican authorities to limit uncontrolled burns on farms. That helped the area reduce its pollution in 2012, but it still has a way to go. 10. Cincinnati With three major interstate routes meeting in the metro area, much of Cincinnati's air pollution comes from vehicle exhaust. Megan Hummel, spokeswoman for the Southern Ohio Air Quality Agency, said the agency is spearheading a campaign to reduce engine idling in the city. The program uses posters and pubic service announcements aimed at parents waiting to pick up schoolkids, reminding them to turn off engines within 30 seconds after stopping. It also managed a program to retrofit 519 school buses to burn fuel cleaner and took 63 older, dirtier buses out of service entirely. Their efforts are paying off: The city has made steady progress reducing its smog and ozone levels, said Hummel. "We routinely exceeded ozone standards in the 1990s and early 2000s and now we rarely do," she said.