The Colorado Department of Public Health and Environment issued an ozone alert Friday for La Plata and Montezuma counties.

“The alert doesn’t mean the ozone level exceeds the standard of 75 parts per billion,” said Mike Silverstein, deputy director of communications at the health department. “It means the level is likely to exceed the standard, and we like to let people know so they can protect themselves.”

The ozone level – an eight-hour average – at 3 p.m. Friday in Cortez was 70 ppb, Silverstein said. The department has no ozone monitor in Durango.

The alert was valid from 4 to 11 p.m. but not necessarily limited to Durango and Cortez, the alert said.

An ozone alert means air quality could be “unhealthy” for certain people with lung diseases such as asthma.

People in this category should avoid heavy or prolonged exertion.

Ozone is formed from the reaction of the ultraviolet rays of sunlight with volatile organic compounds and nitrous oxide in still-air conditions.

On-road vehicles, solvents and non-road equipment contribute most volatile organic compounds.

But the U.S. Environmental Protection Agency lists fires and residential wood burning as sources of volatile organic compounds.

Firefighters in northern New Mexico on Friday continued to battle the Las Conchas Fire, the largest in the state’s history. The fire near Los Alamos was only minimally contained.

Sources of polluting emissions vary from state to state. Wildfires in New Mexico contribute significantly more pollutants than in Colorado.

In 2005, fires in New Mexico contributed 17,000 tons of pollutants; in Colorado, fires contributed 3,800 tons.

The ozone standard was lowered from 85 parts per billion to 75 ppb in 2008. Southwest Colorado remains in compliance with federal ozone standards, but that could change if standards are tightened, as is currently being considered.

daler@durango herald.com