A Code Red air quality alert is in effect across the Washington, D.C., and Baltimore areas, including Montgomery County, as smoke from Canadian wildfires drifts across the region, pushing pollution to levels considered unhealthy for everyone, according to the Metropolitan Washington Council of Governments and the Maryland Department of the Environment.

Forecasters expect Friday to be the worst air quality day of the week, with hazy conditions potentially lingering into the weekend. A shift in winds ahead of Saturday thunderstorms should gradually clear the smoke, but it arrives with a second challenge: high humidity and heat indices that could top 100 degrees.

"Air quality is currently considered harmful due to the smoke caused by fires taking place in Canada," said U.S. Navy Lt. Justin Kerbow, chief of the Public Health Department at Walter Reed National Military Medical Center. "This smoke carries with it larger amounts of particulate matter."

Kerbow said events like this can last from hours to days depending on the source, and this one should clear as weather pushes the smoke out of the region within the next day or two.

What the alert means

The Air Quality Index uses color codes to communicate risk. Code Orange, which the region sees several times each summer, means air may be unhealthy for sensitive groups, including children, older adults and people with heart or lung conditions. Code Red means the air is unhealthy for everyone, regardless of health status.

The primary concern during wildfire smoke events is fine particle pollution, known as PM2.5. These particles are small enough to travel deep into the lungs, and prolonged exposure can cause irritation even in healthy people. Kerbow said some people may notice symptoms like these over the next couple of days, and that feeling fine doesn't guarantee the air isn't having an effect.

"The air can affect anyone at any point," Kerbow said. "If anyone feels they are having a reaction that is concerning, they should seek immediate medical attention."

Who should be extra careful

"Everyone should be careful at this moment, especially those with preexisting respiratory conditions and strong reactions to seasonal allergies," Kerbow said.

Patients or staff with specific health concerns or questions about their medications should contact their health care provider or care team. The guidance in this article is general and is not a substitute for individual medical advice.

What to do

During Code Red conditions, the Environmental Protection Agency and regional air quality officials recommend the following: Limit time outdoors, especially prolonged or strenuous activity. If outdoor tasks can be shortened, rescheduled or moved indoors, do so.

Keep windows and doors closed at home. Run air conditioning on recirculate mode rather than settings that draw in outside air. A portable air purifier with a HEPA filter can further improve indoor air. Avoid adding to indoor pollution. Burning candles, frying food and vacuuming without a HEPA filter can all worsen air quality inside the home.

If you must spend extended time outside, an N95 or KN95 respirator can filter smoke particles. Cloth and surgical masks do not provide meaningful protection against PM2.5.

The common thread in what makes things worse, Kerbow said, is time and exertion outdoors.

"Being outside exercising, doing yard work, children playing, all of these things can increase exposure due to increased inhalation," Kerbow said. He recommended canceling outdoor plans entirely until conditions return to normal, rather than simply adjusting timing or intensity.

Balancing smoke and heat

Saturday's forecast presents a tradeoff. The same closed windows that keep smoke out can trap heat in, a particular concern for anyone without central air conditioning.

"To balance heat and smoke without central air conditioning, it would be best to spend as much time at another location that can provide air conditioning and protection from smoke," Kerbow said. "While this is not a requirement, it is a helpful recommendation for those who can do it."

Checking conditions

Air quality can change hour to hour. Before heading out, check current conditions at AirNow.gov or through the free Clean Air Partners app, which provides real-time readings for the metropolitan Washington region. The Metropolitan Washington Council of Governments also posts regional forecasts and alert status.

Kerbow's bottom line for the weekend was simple.

"Stay indoors as much as possible. Limit your time outside," Kerbow said. "A mask is a good precautionary measure, please use it at your discretion. When traveling by vehicle, recirculate the air inside the vehicle, close windows at home and use the A/C."