Regulators have come to expect this pattern. But, unlike last year, there is no talk of cracking down on fireworks-caused pollution.

"I don't think we are going to legislate Fourth of July fireworks," said Bob Dalley, who oversees monitoring for the Utah Division of Air Quality.

The U.S. Environmental Protection Agency (EPA) bases its pollution limits on public health. It also oversees the state's pollution-control programs. So, when sooty pollutants are five to 11 times the level considered acceptable, regulators take a harder look.

For the past few years, EPA and state regulators have been at odds about calling Utah's big fireworks holidays "exceptional" events. The state points out that the pollution isn't always bad in Utah and Weber Counties - just the nighttime celebrations of the Fourth and Pioneer Day in July.

The EPA sees it differently, says Richard Long, who directs air pollution programs for the agency's Denver office. "It's hard to say a Fourth of July celebration is not a recurring event."

This year, the squabble is less pressing than it has been before. That's because the average midnight-to-midnight pollution levels were not as bad as in 2002 and 2003, in particular.

Regulators look at two types of soot, called PM 2.5 and PM 10. The smallest particles, PM 2.5, are about one-fortieth the diameter of a human hair.

The larger, PM 10, is about one-tenth the diameter of a human hair. Both are considered harmful to health, especially, the old, the young and people with respiratory diseases.

This year, as in past years, Ogden had the highest readings for both types of soot. In one hour, PM 10 reached an average concentration of 808 micrograms per cubic meter of air. EPA calls anything above 150 micrograms "unhealthful."

Also in Ogden, PM 2.5 reached 751 micrograms per cubic meter - more than 11 times EPA's healthful level.

But EPA cares much less about an average in one hour than an average over the day. So, with a daylong average of 102 at that Ogden monitor, EPA is not terribly worried about the PM 10, said Long.

Meanwhile, PM 2.5 in Ogden averaged 77.5 at that station over the day. That means the state will have to count the Fourth as a significant pollution day for Ogden.

A pollution-measuring station in Lindon, in Utah County, had a high reading of 659, also in the 10 o'clock hour. But, like Salt Lake City, the daily average was below EPA's level of concern. Air quality experts can't explain why Salt Lake's fireworks pollution levels are so much lower than in Ogden and Lindon.

Utah's fireworks fight figures into a 10-year pollution control plan the state is developing for the EPA.

State regulator Cheryl Heying said fireworks spikes like these skew annual pollution data. It would be useless for EPA to mandate year-round controls for drivers or industrial plants because of the holiday increases, she added.

At the same time, the state wants to understand fireworks pollution better. Heying said virtually nothing is known about short-term health impacts of fireworks smoke. "We're not blowing it off either," she said. "We're trying to get more information."

fahys@sltrib.com