Why is it so smoky now?

Hundreds of wildfires have raged across northern Quebec for the past few weeks, driven by unusually warm and dry conditions.

Then on Monday, a low-pressure system surrounded by counterclockwise winds moved just off New England’s coast. Its strong, dry gusts fanned the flames in Canada, then pushed the resulting smoke and ash into the eastern United States.

Intense Canadian fires are unusual this early in the season, but the fires this year were fed by a warm spring and drought-like condition, Kent Moore, an atmospheric-physics professor at the University of Toronto, told me. Because the fires took hold before the vegetation could “green up” for the spring, the conflagrations grew more rapidly than they normally would, he said.

The dry conditions also helped the fires ignite the soil itself. “Because the soil was so dry — because of the dry winter and the dry spring — the fire got into the ground,” he said. “The vegetation below ground started to burn, and those are much harder to put out.”

How does the current crisis compare to previous events?

This is likely one of America’s worst days for air pollution in several decades, in terms of the number of people affected and the severity of the exposure, Marshall Burke, an economist and sustainability professor at Stanford, told me.

“It’s pretty off the chart,” Burke said. Wildfire smoke is affecting New York City much more acutely — and much earlier in the year — than it has at any time since 2006, when contemporary air-pollution data began to be kept.

What’s the connection between the 2023 Canadian wildfires and climate change?

It is difficult to attribute a single unprecedented event to climate change, and the climatology of wildfires in eastern North America is particularly challenging. (Climate change has more clearly worsened wildfires out West.) “This is probably an unlucky year for Canada, as far as wildfires go,” Moore, the atmospheric physicist, said.

But wildfires will become more likely across Canada as the climate changes, he said. And while climate change should broadly increase rainfall across Canada, it will also increase the likelihood of heat waves and more extreme spring and summer temperatures, which can make wildfires more likely.

“Nova Scotia has always had wildfires. It’s just they’ve had more wildfires this year than they have on average for the whole year,” he said. “There’s always going to be wildfires, but there’s going to be more of them.” As Canadian cities sprawl into previously uninhabited woodland areas, he added, the human impacts of wildfires will increase — even if the number and intensity of wildfires does not.

What should I do about the air pollution?

First, consult the air quality index at AirNow.gov. The AQI is a unit-less index of air quality running from zero to 500; any reading above 100 would rank as unusually polluted in the United States. On Wednesday afternoon, some air sensors in New York and New Jersey indicated that the AQI exceeded 400.

If it’s higher than 100, then the most vulnerable groups of people — including children, the elderly, anyone with a cardiopulmonary condition, and pregnant women — should limit strenuous activity outdoors. If higher than 150, then people should generally try to limit their outdoor activity; at levels above 200, the air is considered unhealthy and everyone should try to go outside as little as possible.

If you think you might be especially vulnerable, err on the side of caution. Recently, a body of new and large-scale studies have shown that air pollution is generally worse for the body and brain than previously thought.

Then, if you’re in an area with hazardous air pollution, consider how to limit your exposure to the air as much as possible. Keep your doors and windows closed. An air filter outfitted with a HEPA filter can improve the air in a home or apartment. It should generally be closer to the most sensitive people.

“Any pregnant moms — if my wife or anyone I knew was pregnant right now — I would be texting them to stay inside and sit by an air filter,” Burke told me. Exposure to severe air pollution during pregnancy has been shown to increase the risk of preterm birth, and it can reduce a child’s lifelong earnings, cognitive performance, and other indicators, he said.

A KN95 or N95 mask — the type of high-filtration mask used to prevent COVID infection — can also significantly limit your exposure to soot and ash. If you are in a sensitive group or are worried about air pollution’s health effects, you could consider wearing a mask inside if you have no other way to filter the air.