Flashing signs along Interstate 81 told everyone driving through the Roanoke Valley that Wednesday was an air quality action alert day. Which was sort of true.

Particle pollution triggered a code orange alert Tuesday evening, and the Department of Environmental Quality expected code orange conditions to continue Wednesday.

On a code orange day, what the DEQ calls "sensitive groups" -- children, the elderly and anyone with heart or lung problems -- should reduce strenuous activities and stay indoors.

But air quality was in the moderate range Wednesday. And no one has designated any actions to combat particle pollution anyway.

Mark McCaskill, a senior planner with the Roanoke Valley Alleghany Regional Commission, sent out an e-mail encouraging people to do some things they would do on an ozone action alert day, saying some of those things help with particle pollution, too.

The flashing signs instructed people to wait until evening to refuel -- an anti-ozone measure, but not one that will help particle pollution.

Ozone has been the Roanoke Valley's problem, and valley governments have devised a plan that goes into effect whenever ozone reaches unacceptable levels. That hasn't happened since 2003.

But particle pollution triggered three code orange days in 2005.

"The tables seem to have turned here for some reason," said Tom Ballou, director of the Department of Environmental Quality's office of air data analysis. "I'm not sure we know why."

Ozone is a predictable adversary. It's seasonal, much worse in summer than in winter.

Heat and sunlight exacerbate the problem.

Ozone, a greenhouse gas and a key component of smog, forms when nitrogen oxide and volatile organic compounds, such as gasoline vapors, combine in bright sunlight. It causes respiratory problems, particularly for children and the elderly, and aggravates respiratory conditions such as asthma.

"Particles can be more of a tricky business," McCaskill said.

A hard rain shower can help reduce ozone. Sometimes it helps with particles too.

Sometimes it makes things worse.

Fine particles are associated with respiratory and coronary health problems, including some that can lead to premature death. Fine particles are 2.5 microns or less in diameter. The dot above each "i" in this article is about 600 microns across.

Current standards limit annual exposure to fine particulates to a daily average of 15 micrograms per cubic meter of air, and a maximum of 65 micrograms per cubic meter in a 24-hour span. The average adult breathes about 11 cubic meters of air a day, barring stressful exertion.

According to an EPA staff report, current standards allow "thousands of premature deaths per year." The EPA has proposed changing the allowable daily exposure from 65 micrograms per cubic meter to 35. It does not plan to change the annual limit.

The valley is on the verge of violating the annual limits, but is well within the daily standards.

But if the proposed daily standards are adopted, the valley could be bumping up against them, too. Exceeding them could trigger EPA mandates similar to those the valley avoided on the ozone front.

In 2002, valley governments faced the prospect of a federally mandated anti-ozone program. Those restrictions could have included emissions tests on vehicles, specially formulated paints, cooking sprays and deodorants, and stringent emissions standards for industry.

The voluntary plan local governments devised to head off the mandates -- along with favorable weather and new federal regulations on coal-fired power plants -- have worked so well that the EPA is expected to declare the valley back within acceptable ozone levels at the end of 2007.

"We may well lick the ozone problem and then focus on the particles," McCaskill said. "After the ozone season, that's the focus we're going to shift to."

Some things play a role in both problems. Traffic is the largest single contributor to the valley's ozone, pouring 45 tons of ozone-producing pollutants into the air each day.

Traffic, particularly diesel engines, also contributes to particle pollution. So do locomotives.

But the main source of particle pollution is coal-fired power plants.

Federal regulations on those are scheduled to get tougher in 2009.

Dr. Luis Matos, an allergy and immunology specialist at the Asthma & Allergy Center in Roanoke, said even healthy people probably should reduce or postpone jogging and other strenuous activity when the air is unhealthy.

Rick Dietz, back at Roanoke's Kirk Family YMCA after a four-mile noontime run, said he hadn't heard about the orange alert.

"I don't pay attention to that," said Dietz, 62. He's been jogging for nearly 40 years. "I just listen to my body.

"I've had much worse days."

Staff writer John Cramer contributed to this report.