A persistent temperature inversion is trapping air, and pollutants such as woodsmoke, in Baker Valley and other areas across Oregon, according to the Oregon Department of Environmental Quality.
The air quality index in Baker City crept into the "moderate" category this morning.
As of 10 a.m. the index was 57, slightly above the threshold of 51 for moderate. Numbers of 50 or lower are in the "good" category. The moderate category runs from 51 to 100.
During a temperature inversion high pressure in the atmosphere traps colder air, which is heavier, in valleys and other low-lying areas. The condition can last for more than a week, and is relatively common in autumn and winter.
The National Weather Service is not predicting any major changes in the weather through the rest of the week.