As the 2010 summer ozone season ends, the Environmental Protection Agency reported Oct. 1 that New England experienced an increase in the number of poor air quality days this year, compared to 2009.

Based on preliminary data collected between April and September, there were 28 days when ozone monitors in New England recorded concentrations above the standard, the EPA said, in a press release. By contrast, in 2009 there were 11 days over the standard.

However, over the longer term, air quality in New England continues to improve, the federal environmental agency said.

The number of unhealthy ozone days in each state this summer was:

24 days in Connecticut (compared to 5 in 2009);

15 days in Massachusetts (7 in 2009)

6 days in Rhode Island (1 in 2009);

6 days in Maine (3 in 2009);

9 days in New Hampshire (2 in 2009)

none in Vermont (none in 2009).

Ground-level ozone, the main ingredient of smog, is considered unhealthy when average concentrations exceed 0.075 parts per million (ppm) over an 8-hour period. In January, 2010, the EPA proposed to strengthen the ozone standard to a level within the range of 0.060-0.070 ppm. A final decision is expected in the near future.

The increase in the number of days with unhealthy air this year was directly related to the increase in the number of hot days this year. Sunlight and high temperatures speed the formation of ground-level ozone smog. Many areas of New England had more hot days this summer than last summer. July and August were especially hot, dry and sunny for much of New England.

For example, at Bradley Airport in Connecticut, there were 34 days this summer when the temperature was at or above 90°F, compared with 11 such days in 2009. Although warm temperatures this summer led to an increase in unhealthy days, over the long-term, New England has experienced a decreasing number of unhealthy ozone days and peak ozone concentrations have decreased significantly over the last 30 years. In 1983, New England had 113 unhealthy days, compared with 28 this summer.

Another measure of air quality in New England is the geographic extent of the unhealthy air quality. This is determined by counting the number of air quality monitors that recorded exceedances of the ozone standard. A higher number of monitor exceedances means a more extensive area of unhealthy air quality. When comparing the 2010 ozone season to the 2002 ozone season, a New England summer with temperature data similar to this summer, the total number of monitored exceedances dropped from 860 in 2002 to 101 this past summer. This is an approximately 88 percent decrease in the number of areas in New England exceeding the standard over the eight year period.

Exposure to elevated ozone levels can cause serious breathing problems, and aggravate asthma and other pre-existing lung diseases. It can also make people who are vulnerable more susceptible to respiratory infection.

Ground-level ozone (smog) is formed when volatile organic compounds and oxides of nitrogen (NOx) chemically react in the presence of sunlight. Cars, trucks and buses give off the majority of the pollution that makes smog. Fossil fuel burning at electric power plants, which run at high capacities on hot days, gives off significant amounts of smog-making pollution. Gas stations, print shops, household products like paints and cleaners, as well as gasoline-powered lawn and garden equipment, also contribute to smog formation.

Since model year 2004, new cars, sport utility vehicles, pickup trucks, and minivans are meeting new emission standards, resulting in vehicles that are 77 to 95 percent cleaner than older models. Also, beginning in 2007, EPA’s standards for new diesel engines for trucks and buses have contributed to the reduction of NOx and particulate matter emissions from those new vehicles by 90 percent over older models.

The EPA has proposed the Transport Rule, which is expected to further improve air quality in the eastern U.S. by reducing power plant emissions from 31 states and the District of Columbia. This rule will require significant reductions in sulfur dioxide and nitrogen oxides emissions.

The daily air quality index forecast will continue to be available at epa.gov/ne/aqi/. The public can also sign up at this address to receive air quality alerts. These alerts are issued by e-mail, whenever necessary, to notify program participants when high concentrations of ground-level ozone or fine particles are predicted to occur, in their area. Historical charts of unhealthy air days from 1983 through 2010 are available for each state at epa.gov/ne/airquality/standard.html. A preliminary list of the unhealthy readings recorded this summer by date and monitor location, and corresponding air quality maps for each day, can be found at epa.gov/region1/airquality/o3exceed-10.html