Just How Bad Was the Pollution in New York?

The air in New York City on Wednesday wasn’t just bad by the city’s standards. It was historically bad.

The chart above shows the concentration of a particularly dangerous kind of air pollution according to provisional data measured by the New York City Community Air Survey and the state’s Department of Environmental Conservation from a network of sensors around the city.

Those sensors measured levels of particles 2.5 microns in diameter or smaller, brought by wind from wildfires in Canada. Wednesday’s daily average was the highest since recording in New York began in 1999, according to data compiled by the Environmental Protection Agency. (The second-highest level was on Tuesday.) Wednesday’s level of pollution was also higher than the worst daily average record in San Francisco — on Nov. 16, 2018, when wildfires were blazing in Northern California — and, at its peak, it approached average pollution levels recorded in Portland, Ore., on Sept. 13, 2020, during the worst of its pollution from nearby wildfires.

The city’s pollution sensors are calibrated differently from those used by the Environmental Protection Agency to record measurements in those other cities, according to Patrick Gallahue, a spokesman for the city’s Department of Health. On Friday, the city reduced the levels reported by the sensors by nearly 50 percent, citing a “calibration algorithm.” Our chart above has been updated to reflect those new levels.

The small particles measured by the sensors — each about a 30th as wide as a human hair — are dangerous to human health because they are tiny enough to penetrate into the lungs and bloodstream. The Environmental Protection Agency closely tracks pollution of this type from smokestacks and vehicles.

Thirty-five micrograms of the particles per cubic meter of air is considered a safe average level in the air for one day, according to the agency. Wednesday’s average level in seven stations across the city was around 200, nearly 6 times as much as that threshold.

Wednesday’s pollution, of course, was not caused by a power plant or vehicles, but by major wildfires in Canada, mostly in Quebec. Fires of this magnitude are unusual in the eastern part of North America, said Eric James, a research associate at the University of Colorado, who works on the model used by the National Oceanic and Atmospheric Administration to predict the movement of smoke from wildfires. The East Coast tends to have more rain in the summer than the western half of the continent, where wildfires are so common that they have a season. An unusual weather pattern pushing the pollution down the coast is also a culprit, he said.

How long the air will stay dangerous depends on what happens with the fires, the weather in Canada and the wind. Weather forecasters trying to predict the air quality struggle to make estimates with much precision beyond a 48-hour window, Mr. James said. Their models rely on satellite measurements of the magnitude of wildfires, but do not predict whether the fires will get better or worse over time, beyond a typical pattern of growing slightly stronger during the hot part of the day and slightly weaker at night.

Predicting the wind that moves the smoke around is also a challenge, particularly during the summer. Wind during the winter tends to be more predictable. Summer winds are often produced by thunderstorms, which are small, local and typically short-lived. By Friday, though the fires continued to burn, air in the city had improved considerably.