Southern Maryland spent much of last week choking on smoke from wildfires burning hundreds of miles away in Canada, a haze thick enough to shutter public pools, cancel an outdoor concert and push air pollution in Calvert, Charles and St. Mary’s counties into the most severe category on the federal government’s health scale.
The smoke, drifting south from more than 800 fires burning across Ontario and other Canadian provinces, sent the region’s Air Quality Index above 200 on Friday, July 17, triggering Code Purple alerts — the second-highest of six tiers, meaning conditions were considered “very unhealthy for everyone.” All three Southern Maryland counties registered Code Purple readings that morning, with the day settling into a mix of Code Red and Code Purple as smoke lingered over the D.C. region.
The St. Mary’s County Department of Emergency Services notified residents that the alert stemmed from more than 100 fires burning in western Ontario, compounded by ozone pollution and above-normal temperatures. Charles County issued its own Code Purple health advisory over social media that morning, warning that pollution levels were unsafe even for healthy adults.
The disruption reached into daily life across the region. The YMCA-operated Great Mills Pool in St. Mary’s County closed for the day because of the smoke, and St. Mary’s College of Maryland canceled its River Concert Series performance that evening. In Calvert County, all outdoor Parks and Recreation facilities — including Breezy Point Beach and the Chesapeake Hills Golf Course — shut down at 1 p.m. as officials urged residents to stay inside.
Gov. Wes Moore’s office declared a statewide Code Red Air Quality Alert for Friday, warning that smoke drifting in on prevailing northwesterly winds could affect “everyone, not just sensitive groups,” and singled out people with asthma, chronic obstructive pulmonary disease or heart disease, along with older adults, pregnant women, young children and outdoor workers as most at risk. Western Maryland reached Code Purple as well. State health officials urged residents to keep a week-to-10-day supply of asthma and other respiratory medications on hand and to check on relatives with chronic conditions.
It was not Southern Maryland’s first bout of smoke this summer, nor its worst in recent memory. Forecasters with the National Weather Service office that covers the region, based in Sterling, Virginia, said the smoke had been building since wildfires ignited in western Ontario and northern Minnesota in mid-July, with more than 180 fires burning in Ontario alone by July 14. Meteorologists tracking the event described Friday’s air quality as the worst the Washington region had experienced since June 2023, when Canadian wildfire smoke last blanketed the East Coast. That earlier episode was linked to a 20% jump in heart- and lung-related medical visits across the University of Maryland Medical System compared with prior years, according to hospital researchers.
Relief arrived over the weekend. A cold front pushed through Saturday, bringing severe thunderstorms that knocked out power to tens of thousands of Maryland customers — more than 20,000 in Anne Arundel County alone — but also washed much of the smoke out of the atmosphere. By Sunday, air quality across most of the state had returned to moderate levels, though Western Maryland remained under a Code Orange advisory for sensitive groups. Forecasters cautioned the reprieve might not last, noting that shifting winds could carry more smoke back into the region depending on how the Canadian fire season, which has already burned an estimated 3.5 million acres, continues to develop.
Scientists tracking the smoke’s path have relied in part on satellite modeling to understand its composition and movement. NASA’s Goddard Earth Observing System, which blends satellite, aircraft and ground-based data with weather information, has tracked brown carbon — the organic particles that give wildfire smoke its yellow-orange tint — separately from human-generated pollution since a February 2026 update allowed the model to distinguish fire-driven organic carbon from anthropogenic sources such as vehicle exhaust. Brown carbon is a major contributor to the fine-particle pollution, known as PM2.5, that public health officials say poses the greatest risk to cardiovascular and respiratory health during smoke events like the one that settled over Southern Maryland last week.
State and local officials say residents should monitor conditions through the Maryland Department of the Environment’s air quality forecast page, AirNow.gov or the regional Clean Air Partners forecast before spending extended time outdoors, since conditions in Southern Maryland can differ significantly from those in Baltimore or the District depending on wind patterns. With Canada’s fire season expected to continue for weeks, health officials say the region should be prepared for additional smoke events into the late summer.
