An air pollution advisory has been issued through Tuesday for portions of Jackson and Josephine counties.
Noting that the National Weather Service predicts a strong temperature inversion will shroud the region through that period, the Oregon Department of Environmental Quality warned Wednesday that stagnant conditions may trap fine-particle pollution near the ground, causing air quality to deteriorate in parts of the two counties, as well as in areas of Douglas, Klamath and Lake counties.
As a result, air quality in some of the affected areas may be as bad or worse than during the forest fires last August, warned DEQ spokesman William Knight.
"Woodstoves and fireplaces are the No. 1 problem, followed by driving," he said. "We are asking people to seek alternatives if they can until we get through this. Give your neighbor a break."
The weather forecast for Jackson and eastern Josephine counties calls for foggy conditions each morning, followed by cloudy conditions in the afternoons until Tuesday.
"We are in a high-pressure ridge right now," observed Jim Bunker, a hydro meteorological technician with the National Weather Service station at the Medford airport.
"What happens is that it sets up a temperature inversion," he said. "The cold air is trapped in the valleys at night. That doesn't allow for any mixing of the air. It traps pollutants from wood stoves and automobiles. It'll stay there until the next system comes in and bumps that ridge out."
The good news is that another system is predicted to move in from the west either Monday night or Tuesday, he said.
"It'll probably be enough to clear things out," he added.
Although the air quality in the Medford area was considered good Wednesday afternoon, the air quality tends to worsen as the temperature drops later in the day, Knight said.
"If you look at the hourly concentration, as we get into the evening hours, the particulates go up," he said. "There is definitely a noticeable spike."
That is likely the result of residents starting fires in their woodstoves or fireplaces when they get home, as well as increased driving as people head home from work, officials said.
"It'll taper off overnight, then spike again early in the morning," Knight said.
Older adults, children and people with heart or lung disease are at the greatest risk, he said.
Moreover, exercise and physical activity cause people to breathe faster and more deeply, inhaling more pollution into their lungs, he added.
"People out there exercising are going to breathe in a lot more pollution," he said. "If they go out for that run, they expose themselves more than the average person."
For an hourly update on fine particulates in this area, see www.deq.state.or.us/aqi/index.aspx.
Reach reporter Paul Fattig at 541-776-4496 or firstname.lastname@example.org.