Four days into the summer and already the Wasatch Front has seen a "yellow" day on the air quality scale, though the milestone arrived a lot later than last year.

Ozone levels reached the moderate range on Wednesday, so air quality control regulators urged residents in Utah, Salt Lake and Davis counties to park their vehicles at home and choose mass transit or consolidate trips to avoid contributing to the problem.

Ozone pollution becomes more troublesome on hot summer days and can make it difficult for people to breathe. The summertime culprit happens when naturally occurring ozone interacts with sunlight and byproducts of automobiles, gasoline vapors, refineries and other industries.

The heat of the afternoon traps the ozone on the valley floor, with the result being a brownish-yellowish haze.

Last year, the federal Environmental Protection Agency tightened the ozone standard, reducing the allowable amount of ozone from 80 parts per billion to 75.

This year's yellow day came a month later than in 2008, said Department of Environmental Quality spokeswoman Donna Spangler.

"Last year we had a much hotter May, while this year the rain has tempered that," she said.

There were five no-drive days in May 2008 and nine in June that year. Air quality officials are hopeful that additional storms headed this way will keep June's numbers lower this year as well.

For more information, click on airquality.utah.gov to find a color-coded three-day forecast for Salt Lake, Davis, Cache, Weber and Utah counties.

"Green" means air quality is good and people are encouraged to make clean air choices to help keep air-pollution levels low. "Yellow" means the pollution is building and people are encouraged to take proactive steps by voluntarily carpooling, consolidating vehicle trips and avoiding idling or mowing the lawn in the middle of the day. "Red" means pollution levels are critically high, and residents should avoid using gasoline- and diesel-powered engines.

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