If you found it a lot more comfortable to be indoors than out this week, it might not have been the heat. Or the humidity.

It might have been the ozone.

Apologies to the Chamber of Commerce, but the air in places like Acadia National Park, Port Clyde, Cape Elizabeth and Kennebunkport was anything but crisp and clean on Wednesday.

Pollution from tailpipes and smokestacks, a southwest wind and strong sunshine combined to make the air along the Maine coast potentially hazardous to your health. High ozone levels were detected Tuesday, and spread across the entire coastal region Wednesday before the weather shifted and cleared the air again.

Maine sees at least one high-ozone day each summer, and we used to have a lot more of them back in the 1980s.

But this summer is not off to a very good start, with four high-ozone days before July 1.

“We already have more ... this year than we’ve had in the last three summers. And we’re just getting into ozone season,” said Martha Webster of the Maine Department of Environmental Protection.

Maine’s ozone season ends in October, when the sun’s rays are too weak to cook up the chemical ingredients: nitrogen oxides and volatile organic compounds. The pollutants come from our cars and trucks and from power plants and industries that burn fossil fuels. In strong sun, they react and form ozone, an invisible and odorless gas.

Way up in the stratosphere, ozone protects us from harmful ultraviolet rays. But down here where we live and breathe, it can cause respiratory problems and aggravate lung and heart diseases.

The levels detected Wednesday were considered unhealthy for young children, the elderly and other sensitive people. A lot of others also probably felt the effects in one way or another, most likely as a tightness or a burning in the chest.

“It’s almost like getting a sunburn on the inside of your lungs,” said Ed Miller, executive director of the American Lung Association of Maine.

The return of ozone this year is a function of wind patterns. While Mainers do contribute, most of the ingredients that cause our bad air days drift here from more populated and industrial areas to the south and west. That’s a big reason Maine is supporting tighter federal regulation of the pollutants that form ozone.

Ozone levels tend to peak along the coast, in Acadia and other unlikely places, because the gas gets concentrated when it runs up against the cool air coming off the ocean.

It’s clearly a good thing that high ozone levels have become more rare in Maine, if only because lots of Mainers seem to think they’re immune.

In parts of the country with more frequent ozone alerts, people stay indoors and relax, said Dora Ann Mills, director of the Maine Center for Disease Control and Prevention. Mills said she was scratching her head Wednesday because of all the people out jogging or playing sports despite the bad air.

We tend to think it’s just a little heat and we can handle it, she said. And maybe we needed a reminder to not take this clean sea air for granted.

To check air quality conditions, go to this DEP site.

And take a look at the air quality in Acadia National Park on this National Park Service site.

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