Scores of Chicagoans and suburbanites retreat from the dog days of summer by heading to bucolic vacation spots around Lake Michigan.

But there is no escape from city pollution.

During the hottest days of June, July and August, exhaust from automobile tailpipes, diesel engines and factory smokestacks blows out over the lake and is baked by the sun into ground-level ozone, a hazy soup of pollution commonly known as smog.

Trapped between warm air and cold water, the lung-damaging pollution swells above the lake until the wind shifts and carries smog back to shore — just in time for the afternoon cocktail hour.