A Code Orange air quality alert, meaning conditions are unhealthy for sensitive groups, is in effect for much of Maryland on Monday, a relatively rare occurrence outside of the summer months.
The alert is because of warming temperatures and a high pressure system that is making the air stagnant over the region, according to the Maryland Department of the Environment. That is causing fine particles to build in the air.
That is different than the common air quality alerts issued during summer months, which are most often linked to high levels of ozone pollution, which can be exacerbated by hot weather.
"In wintertime, it's almost always fine particles," MDE spokesman Jay Apperson said. "That can happen any time of the year."
Code Orange levels were measured both in Western Maryland and the metropolitan Baltimore area Monday morning, according to data from MDE provided to AirNow.gov.
There is, on average, a single day each December with poor air quality of at least Code Orange because of particulate matter, compared with three days each in July and August.
Poor air quality caused by ozone, on the other hand, is typically measured between April and September with a peak of 11 days of at least Code Orange in both July and August.
In Code Orange conditions due to particles in the air, people with heart and lung disease, older adults and children are at greater risk of health concerns, according to AirNow.gov.