A haze hung over Bangor on Thursday and Friday as the city enjoyed some of the first hot, humid days of the season.

But for those with asthma, heart disease or emphysema, the weather was less than ideal. Maine’s Department of Environmental Protection this week issued the year’s first warning that air pollution levels could be high enough to pose a health risk for the elderly, young children and people with chronic respiratory problems.

Forest fires have been smoldering in Quebec, and a wind from the northwest has been carrying the smoke directly into Maine. On Thursday, ozone levels in the air atop Cadillac Mountain in Acadia National Park reached unhealthy levels in excess of state scientists’ predictions.

“Certainly the last couple days, you couldn’t see too far at all,” said Paul Nichols, who runs the air monitoring station on Cadillac.

State meteorologist Martha Webster said Friday that the air mass hovering over Maine is moving very slowly, and she expects the weekend to bring similar weather and similar worries for sensitive people.

“We’re probably going to continue to see these hazy conditions right on through the weekend,” agreed Larry Gabric, a meteorologist at the National Weather Service office in Caribou.

Wind patterns will likely continue to bring smoke into the upper atmosphere, he said. However, a weak cold front is expected to move in from the north this weekend, causing thunderstorms and clearing the air at ground level.

“Where there are more clouds, there’s less ozone,” Webster said.

The state’s Friday afternoon forecast predicted “moderate” concern about soot statewide with the same warning about ozone for all but far northern Maine, Down East and the Western mountains, areas where the cold front is expected to have the most impact. In the case of “moderate” air pollution, sensitive groups should avoid strenuous exercise and limit their exposure to cleaners, paints and other products that can irritate their lungs, according to DEP.

It’s been several years since Maine last felt the effects of Canadian forest fires, but the state struggles with bad air quality every summer. Typically, Maine’s bad air is solely a result of high ozone levels. The same gas (composed of three oxygen molecules) that blankets the Earth with a protective layer high in the atmosphere is a problem at ground level where it makes the air difficult to breathe and contributes to smog.

Ozone is created when gases from air pollution – primarily car exhaust – react with ultraviolet light. On sunny days when the tourist traffic in Southern and coastal Maine is at its highest, ozone levels can be so high that they exceed the federal standards laid out in the Clean Air Act. In fact, during 2004, nine of Maine’s counties violated the act at least once, according to federal Environmental Protection Agency data. There haven’t been any violations recorded yet this year, but it’s too soon to know what impact the last few days may have had.

Industrial pollution from the South and West, carried in by the prevailing winds, does contribute to Maine’s air quality problems, but according to DEP scientists, the biggest source of pollution is right in our back yard – the Boston-to-Portland corridor.

Acadia National Park – frequently described as being at the “end of the tailpipe” – hosts two of the state’s air quality monitors and reports many of Maine’s highest pollution levels. In fact, the park is perennially listed on the environmental group Appalachian Voices’ list of the most polluted national parks, coming in at No. 4 last year using 2003 data despite recent improvements in air quality.

In 2004, Maine experienced just one day in which air pollution levels were considered unhealthy for sensitive people as a result of high ground-level ozone concentrations, according to EPA data.

But it was likely the cold, cloudy weather of the past two summers, not a significant reduction in pollution, that impeded ozone formation and kept the air relatively clear, state meteorologist Tom Downs said Friday.

During the drought of 2001, problems peaked at nine unhealthy days in Hancock County, according to EPA data.

This year, with a normal summer expected, should be somewhere in between, Downs said.

“We can’t say the ozone problem is over,” he said.