New England experienced fewer days with harmful levels of ozone pollution this summer than in 2003, the Environmental Protection Agency announced this week.

During the summer of 2004, Maine experienced only one day with unhealthy levels of ozone, which can cause breathing problems for sensitive people and exacerbate asthma attacks. When combined with particulate pollution like soot, ozone is known as smog.

Last year Maine had five days with harmful ozone levels, and in 2002, 17 days.

The cold, cloudy weather that has prevailed for much of the past two summers is likely a major factor in the decline, as ground-level ozone can only be created when air pollutants, like the nitrogen oxides in vehicle exhaust, come into contact with solar radiation.

Robert Varney, regional EPA administrator, also cited recent policy changes, including tougher auto emissions standards and revised standards for diesel engines used in construction, agricultural and industrial equipment, that could have reduced ozone creation.

“When we look back to the air quality conditions a generation ago, we can feel proud of the advances we’ve made in reducing pollution,” Varney said.