How risk from ozone is measured

Recent ozone alerts in Rio Blanco County were based on forecasts that ozone could reach an average of 75 parts per billion (ppb) over an eight-hour period.

That same threshold is used to determine when regions must take steps to reduce ozone levels, although the Environmental Protection Agency is expected to lower the threshold.

The EPA considers ozone over 75 ppb to be unhealthy for sensitive groups. Ed Brotsky, air-quality specialist for Mesa County, said the county did not exceed the good range this winter. Occasionally, it moves into the moderate range for ozone in the summer months, but that’s still below the EPA threshold.

However, the EPA is expected to drop the threshold this year to between 60 and 70 ppb, which is one reason health officials in area counties are watching the ozone situation so closely. For a region to be in violation, its fourth-highest, eight-hour average would have to exceed the threshold for three years in a row.

“It’s definitely on our radar because they’re probably going to lower the standard, and we’d be right on the cusp,” Brotsky said.

At a 70 ppb standard, Mesa County probably would be OK. At 65, it would be on the edge, and at 60 it probably would be in violation, he said.

A violation would require the state to come up with an EPA-approved action plan, which could include automobile emissions testing, Brotsky said.


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