For cities in the Northeastern U.S., large fires across Quebec are particularly hazardous because they’re near enough that smoke will be transported close to the ground — and close to people — rather than high in the atmosphere, as when western wildfires send east a haze that filters sunlight and creates dramatic sunsets.

In 2016, Mickley teamed with Francesca Dominici, the Clarence James Gamble Professor of Biostatistics, Population and Data Science at the Harvard T.H. Chan School of Public Health, and colleagues at Yale to publish a study finding that hundreds of counties across the West will likely experience greater effects from wildfire smoke in the coming decades. They coined a new term, “smoke wave,” to describe the phenomenon, which Mickley said applies to what the Northeast experienced last week.

A “smoke wave” is defined as two or more consecutive days of extreme levels of fine particles, called PM_2.5, emitted specifically by wildfires. Fine particles are thought to be especially dangerous because they’re tiny enough — about 30 times smaller than the width of a human hair — to penetrate deep in the lungs and even cross into the bloodstream.

Powerful protective measures exist, notes Joe Allen, an associate professor of exposure assessment science at the Chan School. Using a high-quality mask like an N95 can help, as can limiting time outdoors and avoiding strenuous activity, he said. However, those steps alone aren’t enough because outdoor pollution penetrates indoors.

Portable air purifiers can clear the air inside homes, Allen said. In office buildings, pandemic-era HVAC filtration upgrades should also protect against wildfire smoke. In fact, Allen’s research team monitors air quality in many buildings on the East Coast and, despite outdoor levels of PM_2.5 higher than 200 micrograms per cubic meter in the past week, they saw levels below 10 micrograms per cubic meter inside buildings with upgraded filtration.

“A key point to keep in mind is that even though levels are lower indoors, because we spend so much more time indoors, most of the outdoor air pollution people breathe can happen indoors,” Allen said. “It’s really important for people to know that these are not hard or expensive solutions.”

As far as tracking tools, Allen gave high marks to the federal government’s six-grade Air Quality Index, which dates to the 1960s but has gained more prominence in the past week. Allen said the index, which rates air quality from green for good to maroon for hazardous, is particularly helpful because it simplifies complex variables that change from pollutant to pollutant, and links them to physiological impacts.

The information is vital because the consequences of pollution exposure can be so wide-ranging and severe.

“We see health effects from short-term exposure to levels 10 times lower than what we saw in New York City,” Allen said. “Beyond the well-known lung and heart health effects, my team just published a study of indoor PM_2.5 exposure and simultaneously measured cognitive function of office workers around the world. We saw a decrease in performance when PM_2.5 levels increased.”