People throughout North Idaho checking the weather on their mobile devices were recently hit with a new and ominous warning that simply read: Air Stagnation Advisory.  

But what exactly is an air stagnation advisory? What causes it and who might it affect?  

Emily Flores, airshed coordinator with the Idaho Department of Environmental Quality, said this particular event was caused by low ceiling heights, low transport winds and high pressure lingering over the area.

“High pressure leads to generally low wind speeds which do not allow particulates in the air to move out and away from their sources," she said.

In terms of air quality, a low ceiling means that the cloud base is lower than usual — generally 1,000 feet or lower — which keeps pollutants or particulate matter in the local airshed. According to the National Weather Service, the main contaminants of concern during an air stagnation advisory are PM2.5 and ground-level ozone. These include gaseous pollutants and fine particles from sources such as vehicle emissions, older wood-burning stoves, industrial emissions and wildfires.  

“Any pollution produced during the stagnant period is likely to become trapped nearer to the ground, where people are,” Flores said. “When you start to see elevated particulate levels, people who are affected most readily are those with preexisting respiratory issues, the elderly and children.” 

As the pollution levels increase, the number of people who could be negatively affected by it increases. 

Unlike the more familiar air quality index, air stagnation levels aren’t clearly defined. Instead, people in sensitive groups must keep an eye on the advisory and monitor how they are feeling.  

The good news, according to Flores, is that the situation should be clearing up over the next few days as a low-pressure system moves through the region. This should bring increased wind speeds, precipitation and better ventilation.  

“As conditions improve, trapped particles will start to move out and you should notice an improvement on the monitors,” Flores said.  

Air Stagnation Advisories are issued by the National Weather Service and can be found at www.alerts.weather.gov.