Wildfire smoke drifting more than 1,000 miles from Ontario and northern Minnesota pushed the Boston metro area air quality index, which includes Brookline, to around 130 on Wednesday, July 15. That reading lands in the "unhealthy for sensitive groups" category and prompted the Massachusetts Department of Environmental Protection to issue a formal air quality alert.

The DEP said the air is forecast to be unhealthy for people with lung or heart disease, older adults, and children because of elevated fine particulate matter descending from the smoke plume. The warning covers Brookline and the broader region.

The smoke had been visible in hazy skies since Tuesday, July 14, but stayed high in the atmosphere. That changed Wednesday. The National Weather Service Boston office said in its forecast discussion that near-surface smoke models indicated the smoke could be smelled at ground level.

"I think based on the near-surface smoke progs that you could smell this smoke today," the NWS wrote in its Wednesday morning discussion.

The thickest concentration of smoke Wednesday settled across central and southern New England, especially along and south of the Mass Pike, according to WBUR. A second wave of smoke is projected to push back into the region Thursday afternoon, July 16, the NWS said. No all-clear date beyond Thursday has been confirmed.

Roughly 850 wildfires are burning across Canada, with more than a dozen ablaze in northern Minnesota, according to Canadian and U.S. fire agencies as reported by Forbes. The largest single out-of-control fire in western Ontario covers approximately 130,000 acres. A jet stream pattern funneled the smoke plume directly into New England.

Fine particulate matter from wildfire smoke can penetrate the lungs and bloodstream. Children, older adults, pregnant people, and anyone with pre-existing heart or lung conditions such as asthma or COPD face the greatest risk, according to the NWS and DEP advisories.

Meteorologist Matthew Cappucci wrote on social media Wednesday that residents should consider wearing N95 masks to filter fine particulate matter from the smoke.

Brookline installed roughly 65 air quality sensors in partnership with Northeastern University's iSUPER project as of 2025. Residents can check real-time conditions at the MA DEP's MassAir forecast portal.

The NWS expects smoke to continue through Thursday, July 16. Residents with respiratory or heart conditions should limit outdoor activity until conditions improve.