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Wildfire Smoke: Haze Begins to Lift in Northeast as It Pushes South and West

Dec 28, 2023Dec 28, 2023

Relief might be coming to millions in the Mid-Atlantic as the polluting plume moves on. Driven by wildfires in Canada, the smoke triggered alerts from to the Midwest and south to the Carolinas.

Source: AirNow · Data as of 6 a.m. Eastern.

Follow our updates on the wildfires, smoke and air pollution.

Liam Stack, Kevin Williams and Judson Jones

While wildfires continued to burn across eastern Canada and send a polluting haze into the United States, the worst appeared to be over for now in the big cities along the Mid-Atlantic, where for two days smoke had blotted out the sun and an acrid smell hung in the air.

A New York Times analysis of atmospheric computer models showed that poor air quality peaked in some major metro areas along the East Coast by late Wednesday or early Thursday, and conditions have been steadily improving in New York, Philadelphia, Washington, D.C. and elsewhere.

But smoke could remain dense in other parts of the Northeast through at least Friday. And the noxious air was covering a wider swath of the country as it spread further south and west, triggering air quality warnings as far west as Indiana and as far south as the Carolinas. On the other side of the Atlantic, aerosols were registering in Norway.

The polluted air disrupted life around the Northeast on Thursday, delaying flights and forcing the postponement of graduations and Pride events. Millions in the region woke up to unhealthy levels of air pollution, one day after New York City registered its worst air quality readings in decades. In Philadelphia, public schools were shifting to remote learning on Friday.

New research from Stanford University scientists showed that Wednesday was by far the worst day on record in the United States for wildfire smoke since 2006, which means the largest number of Americans experienced a day when air quality was deemed to be unhealthy for all age groups. Tuesday was the fourth worst.

The source of the smoke was not abating, as a string of fires in eastern Canada continued to blaze, forcing tens of thousands of people from their homes. About 250 wildfires burned out of control there as of early Thursday, the authorities said, about 150 of them in Quebec. Some have been burning for weeks.

Here's what else to know:

The smoke that choked New York on Wednesday pushed southward on Thursday and was expected to spread west, into the Ohio River Valley, on Friday, the National Weather Service said. It is expected to appear as a widespread haze that could dip as far south as Florida, and not the dense mass of smoke that affected New York on Wednesday.

A storm system swirling off the coast of Nova Scotia in recent days blew smoke from the fires south into the United States. Forecast models for Thursday showed that thick smoke could return to New York later in the day if sea breezes pushed smoke currently hovering off the coast back inland.

The effects of the fires are expected to be noticeable even on the other side of the Atlantic Ocean. Scientists at the Climate and Environmental Research Institute in Norway who are tracking the smoke through the atmosphere said it has moved over Greenland and Iceland since June 1, and observations in southern Norway have confirmed increasing concentrations of aerosols.

The level of pollution Wednesday was higher than the worst day in San Francisco after major wildfires in 2018.

Eduardo Medina

In St. Louis the air quality is forecast to be unhealthy on Friday for older people, children and those with heart or lung conditions on Friday, according to the Weather Service. Similar conditions are expected farther east in Louisville, Ky., where smoke from the Canadian wildfires will continue to seep into the region on Friday, causing hazy skies, the Weather Service said.

Dani Blum

A lot is still unknown about the toll wildfire smoke takes on your health. But most adults and children without pre-existing conditions will likely recover quickly from the effects of short-term exposure to the smoke passing over the eastern United States, said Jeffrey Brook, an associate professor of occupational and environmental health at the University of Toronto Dalla Lana School of Public Health.

The smoke that people have encountered this week is one of many exposures to pollution our bodies will take in over time, he said — it's not likely that we’ll be able to identify a health problem in the future and definitively pin it on a few days of wildfire smoke.

"The brevity of this exposure for this period of time shouldn't have significant long-term effects for the general population," said Dr. Panagis Galiatsatos, a pulmonary and critical care medicine physician at Johns Hopkins Medicine. But there's limited data on how to assess the health effects from a "onetime big burst of smoke," said Mary Prunicki, director of air pollution and health research at the Sean N. Parker Center for Asthma and Allergy Research at Stanford Medicine.

"What happens to the person that doesn't have any outward symptoms from this brief exposure? Probably there are changes in their bloodstream, but maybe that's transitory. We don't actually know," she said. "If they have no significant impacts acutely from the smoke, probably they’re going to not have long-term impacts. But research hasn't actually shown that either way."

Isolating the long-term effects of wildfire smoke in general is difficult — it's tricky to determine how exposure to smoke can impact cognitive performance years later or other consequences, said Laura Corlin, an assistant professor at Tufts University School of Medicine. And we also don't know the exact threshold for just how much exposure is likely to have a long-term impact, said Dr. Raj Fadadu, a researcher at the University of California, San Francisco School of Medicine who has studied the health effects of wildfire smoke.

What we do know is that even minutes of exposure to wildfire smoke can trigger inflammation in the body, said Dr. Brook. Inflammation can lead to a cascade of downstream health effects; the longer it persists, the more it raises the risk for cardiovascular issues and strokes. A few days or a week of enhanced inflammation is most likely not enough to lead to detectable health problems in the future, he said. "But inflammation is inflammation, and it is bad."

We also know that wildfire smoke is particularly dangerous for people with underlying lung or heart conditions. Smoke can exacerbate symptoms of asthma and chronic obstructive pulmonary disease. It can put babies, children, older people and pregnant women at risk of severe health effects. Smoke also poses significant risk to fetuses. For otherwise healthy people without pre-existing conditions, even brief exposure to wildfire smoke can lead to stinging eyes, irritated sinuses, wheezing, shortness of breath, headaches, itchy skin and coughing.

If you go outside, wear a tightfitting mask like an N95, and pay extra attention to your body for the next hour or so that follows, said Dr. Emily Pennington, a pulmonologist at the Cleveland Clinic — watch out for symptoms like intense coughing and chest tightness. If you are struggling to breathe or experiencing chest pain, seek medical attention. Continue to monitor your health over the next few days, and make sure you’re staying hydrated and getting enough sleep, which might help you feel better, advised Dr. Corlin. And take whatever precautions you can to minimize your level of exposure — namely, staying inside as much as possible.

Eduardo Medina

The Indiana environmental management department issued an air quality alert for Friday, when conditions are expected to be unhealthy for sensitive groups. The stagnant weather pattern has continued to move the Canadian wildfire smoke across the state, and officials warned that the air quality may continue to be polluted over the weekend.

Smoke from Canadian wildfires continues to impact Indiana. See current conditions and air quality forecasts at More info:

Roni Caryn Rabin

As an orange plume of smoke wafted through New York yesterday, health experts warned pregnant women, among other vulnerable people, to stay inside. But why single them out?

It's mostly the fine particulates, the microscopic particles inhaled in every breath of tainted air. They are so small that they pass through the lungs and into the bloodstream, then circulate throughout the body. They can even accumulate in the placenta.

"These very small particles, less than the width of a hair, get into the body in some very invasive ways," said Dr. Nathan DeNicola, lead author of the recommendations from the American College of Obstetricians and Gynecologists on reducing prenatal exposures to toxic environmental agents.

"It affects cardiovascular health and is biologically active in the placenta," he added. "It creates inflammation and triggers other processes."

Dr. DeNicola was a senior author of an analysis of 68 studies that contained data on 32.7 million births in the United States. Exposure to heat, ozone and particulates significantly increased the risk of poor outcomes, particularly preterm births and babies with low birth weight.

Black mothers and those with asthma were more likely to experience these problems, according to the analysis, which was published in 2020 in JAMA Network Open. Other studies have linked exposure to air pollution to spontaneous abortion and stillbirth.

Dr. Leonardo Trasande, a professor of pediatrics at NYU Grossman School of Medicine, said that roughly 4 percent of preterm births were attributable to fine particulates in the air. And a growing body of evidence suggests that prenatal exposure to air pollution contributes to range of ailments later in a child's life, among them obesity, damaged respiratory systems, impaired immunity and compromised heart health.

"What I’m telling patients right now is to download the EPA Air Now map, which gives you the air quality index up to the minute, on a scale of ‘generally safe’ to ‘hazardous,’ and for pregnant women — and anyone susceptible to respiratory or heart disease — to stay inside," Dr. DeNicola said. "This is really a time to avoid going outdoors."

He also suggested other precautions, such as wearing masks. N95s that filter small particles are ideal. Air purifiers with HEPA filters can be used to clean indoor air.

People vulnerable to particulates, including pregnant women, should also check their air-conditioners to make sure they are not bringing in air from the outside, Dr. DeNicola said.

A.C.O.G. also has urged pregnant women to follow health alerts about local air quality and to avoid exercising outside.

Eduardo Medina

As smoke continues to emanate from hundreds of raging wildfires in Canada, scores of events were being postponed or canceled in the United States on Thursday and into Friday.

Here's a partial list of events affected by the smoky conditions across the eastern United States and Canada:

The Temiskaming Hospital in Ontario said that all surgeries and obstetric procedures scheduled for Friday have been canceled because the air quality around the hospital was expected to "significantly deteriorate.".

The 54th annual Special Olympics Summer Games in Pennsylvania, which were scheduled to begin Thursday and end on Saturday, have been canceled, disrupting the plans of about 2,000 athletes, 800 coaches and 1,000 volunteers from across the state. The organization said that "while canceling events is truly disappointing for athletes who have trained for months to compete," the love of sport "doesn't compare to the importance of protecting the health and safety of all involved."

A workday event to remove nonnative and invasive plants on Friday at Montgomery Parks in Maryland has been canceled.

The Public Theater canceled performances on Thursday and Friday of its Free Shakespeare in the Park production of "Hamlet" in Central Park.

The Prospect Park Carousel in Brooklyn, a typically busy activity for children in the warmer months, will be closed on Friday.

"Rock N’ Roll Lot" parties scheduled to be held at four McDonald's parking lots in central Pennsylvania on Friday were postponed to later in the month, according to Penn Live.

At a groundbreaking celebration for a mixed-use housing and business development on Friday in Harrisburg, Pa., N95 masks were set to be distributed to attendees.

Emiliano Rodríguez Mega and Gabriela Sá Pessoa

As smoke from Canadian wildfires continues to drift south, some big cities across the world stand as examples of how challenging it can be to control air pollution.

Still, despite inadequate policies and limited progress in some cases, the efforts to cut air pollutants has led to some important lessons — from limiting the number of cars on the street to cracking down on violations.

Once singled out as the world's most polluted city in the 1990s, Mexico City has since turned things around. Along with other measures, including air quality monitoring and forecasting, authorities have for more than three decades implemented a strict license-plate based program that bans drivers from using their cars one weekday per week. Those restrictions can be expanded if the air quality deteriorates.

The program, Hoy No Circula, or No Circulating Today, began to include Saturdays in 2008 — although scientific studies have suggested the expansion didn't help improve air quality.

Other cities adopted similar driving restriction programs, with mixed results.

In 1997, Brazil's largest city, São Paulo, introduced its own car rotation policy to mitigate traffic and air pollution. "These objectives were only fulfilled in the short run, as the drivers started buying a second car to bypass the restrictions," said Hannah Machado, a member of the Breathe Coalition, which aims to improve air quality and combat climate change in Brazil. "That put more cars on the streets, reversing the outcomes."

Some measures have been more effective at reducing air pollution.

Santiago, Chile, has limited the number of cars across the city since the late 1980s. The driving bans, enacted during winter when the use of fossil fuels and firewood for heating increase, have been an effective way to curb emissions. The city has also been also working on electrifying its bus fleet, which could further reduce air pollutant levels by 2050.

An unexpected case of success came in 2021, when, for the first time this century, Beijing's air quality met China's national standards. Though hazardous smog continues in the city, the apparent success came after authorities clamped down on factories and reduced coal burning, in part by shifting from coal to natural gas.

Air quality policies do not always translate into more breathable air.

In Asia, some cities use drones and helicopters that spray water to clear the air, as well as cloud seeding, which involves spraying clouds with chemicals designed to produce rain, all with mixed results. The northern city of Xian, China, even built a tower that stood more than 300 feet high and was touted as the world's largest air purifier.

India's ambitious National Clean Air Program, launched in 2019, sought to reduce air pollutants up to 30 percent by 2024 across more than 130 cities that exceed national standards. Policy missteps, lack of funding and a poor design have hindered implementation, however. To date, experts warn, progress remains uneven.

Lauren McCarthy

As a smoky haze chokes American cities across the country, people are left wondering what effect the peculiar phenomena might have on their health, including their skin, the body's largest and most exposed organ.

Skin disease was first linked to wildfire smoke by a study published in 2021 assessing air pollution from the California Camp Fire in 2018.

Dr. Maria Wei, an author of the study, said researchers know that wildfire pollution "flares certain inflammatory skin diseases such as eczema and psoriasis, and likely will have some effect on other inflammatory diseases too, such as acne and rosacea."

Some people also experience itching from ambient pollution, she said. If you are impacted, which may not be immediately after exposure, she recommends consulting a dermatologist.

When researchers talk about pollution exposure, they’re generally talking about particulate matter smaller than 2.5 microns. (A human hair is at least about 20 microns in diameter.)

This matter can get in the bloodstream, affect every organ and have a range of potentially harmful implications. Dermatologists, though, are interested in how these particles land on the skin — and possibly penetrate it — and create a cascade of inflammation, accelerating signs of aging and causing disease.

Studies have already shown that air pollution increases wrinkles and age spots. Because there's not enough research to recommend with certainty the best course of action for the skin after wildfire pollution exposure, Dr. Wei said, "at this point, prevention is the best approach."

She recommends common sense procedures that people can easily take: Stay indoors to minimize exposure, wear long sleeves, long pants and a mask to diminish exposure. It's good to have an air purifier, too, she said.

An emollient might help, Dr. Wei said, but that's not perfectly clear because "it's always possible the moisturizer could hold the pollution on to your skin," a factor her team is studying, along with other prevention methods.

Dr. Wei added: "It can't hurt to take a shower when you come in, certainly, you would want to wash off any particular matter that would settle on your skin."

But for everyday use, doctors and scientists agree that you almost certainly do not need to buy anything labeled "pollution protection." In part, perhaps, because buying too many beauty products has contributed to the prospect of a hotter future.

Madeleine Ngo

School districts in the Northeast are used to handling snowstorms, but wildfire smoke? Not so much. And they had various responses to the crisis.

Some canceled outdoor after-school activities or closed early. Others, such as Newark Public Schools and Irvington Public Schools in New Jersey, canceled classes entirely. New York City announced it would hold classes remotely on Friday because of poor air quality.

One thing that did not seem to be a factor was the new air filtration systems that were installed in response to the coronavirus pandemic.

Districts have spent billions of dollars in federal relief funds on upgrading heating, ventilation and air conditioning systems in schools. In a March analysis of more than 7,000 districts by Burbio, a company that tracks school data, nearly half planned to use some relief funds for air filtration and HVAC upgrades — spending that was projected to total about $8.9 billion.

Some district officials said they were not significantly worried about the risk to students’ health, in part because they had recently upgraded air filters and HVAC systems with the stimulus funds, but they closed schools anyway.

For instance, in Morris County, N.J., the Mount Olive Township School District sent all elementary and middle school students home about two hours early on Wednesday because of the smoke conditions. High school students, who are normally released earlier, followed a regular schedule.

Sumit Bangia, the district's acting superintendent, said she had decided to send younger students home early because some were going to the nurse's office and complaining about the smell of smoke, which had resulted in some experiencing headaches and itchy eyes. The odor was also making it difficult for students to concentrate, she said.

Still, Ms. Bangia said district officials were not extremely concerned about the smoke's impact on students’ health, in part because the district had recently spent federal relief funds to upgrade to higher-quality air filters.

"We have a top-notch air filtration system in our schools, so it was never a matter of the kids not being safe," she said.

Glenn Miller, the district's director of facilities, maintenance and custodial services, said the district had spent about $300,000 of its roughly $3.2 million in relief funds on upgrading air filters and units. He said all of the district's filters were changed to "hospital grade" ones by May 2020. Although those filter out more pollutants and irritants, Mr. Miller said they did not filter out the smell of smoke.

Ms. Bangia said air conditions had improved on Thursday and that the district planned to follow a regular release schedule.

On the West Coast, school district officials have long had to deal with poor air quality from wildfire smoke.

Kelli Hays, the superintendent of the Moorpark Unified School District in Ventura County, Calif., said district officials monitor the county's air quality index to determine whether they should keep students inside or close schools. Officials would keep students inside if the county deemed the air quality to be potentially unhealthy or if ash were falling, she said.

Ms. Hays said it was rare for the district to close schools, since that could be disruptive for students and families. The district will typically cancel classes only if there is a threat of a fire approaching the schools or if ash fall is thick and could result in students’ breathing in harmful particulate matter.

"It really needs to be very bad before we close the district," she said.

She said the district's indoor air quality was especially strong now, since officials had used some of their pandemic relief funds to purchase new HVAC systems and higher-quality air filters.

Christoph Koettl

A satellite image captured thick haze over New York, as the city experienced its worst air quality on record on Wednesday. The image shows a heavy layer of smoke covering the entire city, compared to an image taken a week earlier.

Sophie Downes

In a Facebook post, Connecticut's Department of Energy and Environmental Protection thanked the members of the state's Interstate Fire Crew who traveled to Nova Scotia to assist the Canadian teams battling the wildfires. "Their selfless commitment and the sacrifices they make to protect communities and natural resources are an example for all," the department said.

Campbell Robertson

Philadelphia's public schools will shift to remote learning on Friday, the superintendent of the school district said in an announcement Thursday afternoon, explaining that there was "no indication that air quality will significantly improve for the remainder of this week." Indoor graduation ceremonies will go on as scheduled, the announcement continued, adding that outdoor ceremonies will take place "but at alternative locations and, in some cases, at adjusted times."

Aishvarya Kavi

In Washington, blocks from the National Mall, a group of high schoolers had set up a volleyball net in a plaza across the street from their school and were engrossed in their usual after-school game. With only a light haze in the air and a water fountain streaming in the background, it seemed to be a normal June afternoon.

Xander Sehgal, 16, and Megan Giles, 15, said they had worn masks in the morning but that the smoke didn't bother them anymore.

"I mean, you can kind of feel it in your throat," Mr. Sehgal said, "but volleyball is more important."

Judson Jones

The worst of the wildfire smoke is likely over for millions of people across the Northeast, after two days of orange-tinted skies and the smell of burning forests filling the air. But smoke could remain dense in other parts of the region through at least Friday, according to a New York Times analysis of atmospheric computer models.

Poor air quality peaked in some of the Northeast's most populous cities late Wednesday or early Thursday, and conditions have been steadily improving in places including New York, Philadelphia and Washington, D.C. Instead of a concentrated mass of thick smoke streaming into some of the nation's densest urban regions, the plume from Canadian wildfires became more widespread Thursday, spreading haze into the Deep South.

A swath of more concentrated smoke still hung over portions of the Mid-Atlantic, from Virginia into New Jersey, on Thursday afternoon, with forecast models showing it lasting through the evening. And New York could see some smoke return, blown back in from the Atlantic Ocean by sea breezes. But conditions in the city were likely to improve and stay that way on Friday, forecasters with the National Weather Service said.

By the time the sun rises on Friday morning, forecasters expect it to lose much of its reddish hue — caused by light scattering from wildfire smoke — across much of the Northeast, though a swath from the Washington region through Western Pennsylvania could still experience some gloom throughout the day.

Forecasters said they hope that air quality will continue to improve as the weekend progresses, with a new weather pattern blowing the Canadian smoke elsewhere — though exactly where is hard to predict. Anticipating smoke patterns is more difficult than some other forms of weather forecasting, in part because the amount of smoke produced by wildfires can be difficult to predict.

That means the current air quality forecast could quickly change. The New York Times is tracking it here.

Christopher Maag contributed reporting.

Eduardo Medina

New Jersey's health commissioner, Judith Persichilli, said at a news conference that 143 people sought care on Wednesday for asthma-related problems, the highest number since the state had high pollen levels two months ago. The air quality remained mostly unhealthy in New Jersey as of Thursday afternoon. Gov. Phil Murphy echoed a message repeated by other officials across the Northeast: "Unfortunately, this is our new reality."

Emily Anthes

As heavy layers of smoke descended on the East Coast this week, public health officials have been united in their message: Stay inside.

It's good advice, said Joseph Allen, an indoor air quality expert at the Harvard T.H. Chan School of Public Health. "But it's incomplete," he said. "It's a little bit of a sense of false security that indoors is always a place of refuge."

One pollutant of major concern in smoke is what is known as fine particulate matter. These tiny airborne particles are capable of traveling deep into the lungs and are associated with a wide range of adverse health effects, including coughing, asthma attacks and heart attacks.

Some fine particulate matter originates indoors — it is often produced by cooking or burning candles, for instance — but it also seeps into our homes and offices from outside. "Most people don't realize just how much outdoor air pollution penetrates indoors," Dr. Allen said.

Studies have shown that when levels of particulate matter spike in outdoor air, they rise in indoor air, too. And because people spend so much time indoors, most of our exposure to the particulate matter that originates outside actually happens in indoor environments, scientists have found.

Fine particulate matter is too small to see with the naked eye, but people can track the levels indoors with portable air quality monitors; many of the low-cost carbon dioxide monitors that became popular during the pandemic will also measure levels of fine particulate matter, Dr. Allen said.

The same high-quality air filters that public health officials have recommended during the pandemic will also reduce exposure to fine particulate matter indoors. "The good news here is that the way to protect yourself should be familiar to people right now," Dr. Allen said. Portable air cleaners equipped with these filters are a good option for people who cannot add better filters to a central ventilation system, he added.

People who are especially concerned about the indoor air and have underlying conditions that put them at high risk might benefit from wearing a high-quality mask during this brief emergency period, Dr. Allen said, though that is not a long-term solution.

The moment might even call for a little bit of relaxation. "The amount of particles you inhale is also a function of your breathing rate," he said. "This is not the time for strenuous activity, even indoors."

Matt Richtel

Poor air quality threatens everyone's lungs, but the risk is amplified among infants and children, whose lungs are still developing and who breathe faster than adults. At a news conference on Thursday, New Jersey's health commissioner, Judith Persichilli, noted that young people breathe "more air relative to their size."

When fine particulate matter is drawn into the air sacs, or alveoli, of lungs, it causes inflammation that narrows the airways and can trigger an asthma attack, or lead to a future one, said Dr. David Cornfield, a professor of pediatrics at the Stanford University School of Medicine. And the damage from severe lung irritation can be lasting.

"If there's damage to the airways, it can prevent normal development," Dr. Cornfield said.

Compared to adults, infants and children have far fewer of the air sacs that are central to gas exchange in the lungs. Infants have about 30 million, Dr. Cornfield said, whereas adults have between 300 million and 500 million. Over the first two decades of life, he said, children gain more of these air sacs, such that a 10-year-old probably has more than 200 million, but early-life lung inflammation can curtail the development of full lung capacity.

Also, infants and children breathe faster than adults, leading them to inhale more micro-particulate matter for their size. Dr. Cornfield also referred to "standard wisdom" that children, because they are lower to the ground where particles settle, face greater inhalation risk.

In general, asthma attacks are slightly more common among children than adults; in 2020, 40.7 percent of adults reported having at least one asthma attack in the past year, compared to 42.7 percent among people 18 and under, according to the Asthma and Allergy Foundation of America. That figure rose to nearly 53 percent for children under 5.

On average, 300 people under the age of 24 and 5,000 adults above that age die from asthma each year in the United States. According to one estimate, 237,000 children worldwide under the age of 5 died in 2015 from asthma and other conditions associated with air pollution, mostly in Asia and Africa.

The research on the effects of wildfires on lung disease is still developing; traditionally industrial pollution has been the more common concern. But researchers say that the differences between the two might not be significant.

The risk posed by pollutants, including wildfire smoke, can be diminished by staying indoors, Dr. Cornfield said. He generally recommended that adults and children stay inside when the air quality index is 100 or above, and that they wear a mask when venturing outside. Indoor conditions can also be made safer through use of filtering systems, like air conditioning, Dr. Cornfield said.

In Thursday's news conference, Ms. Persichilli recommended that children avoid strenuous athletic activity while the air quality remains poor. She added: "Be aware of the signs of distress in your children."

Joseph Goldstein

Source: NYC Department of Health and Mental Hygiene

The Canadian wildfire smoke that has blanketed New York City led to a small spike in emergency room visits for asthma and wheezing on Wednesday, when the air quality in the city was the worst it has been in decades, according to city data.

In all, there were 309 emergency room visits on Wednesday in which the chief complaint involved asthma, wheezing or chronic obstructive pulmonary disorder, according to data from the city's Department of Health. That was a noticeable uptick from the preceding days, when there were about 135 to 200 such visits a day.

Wednesday's tally was the highest number of asthma-related emergency room visits since May 23, 2022. On that day there were 339 asthma-related emergency room visits, coinciding with a seasonal rise in tree pollen levels.

"While we are seeing higher than usual asthma-related visits to the Emergency Department, these visits and calls are still in the low hundreds, and our hospital and health care systems are fully able to respond to patients," Pedro Frisneda, a spokesman for the city's Health Department, said on Thursday. "This is an important reminder to stay indoors as much as possible, with the windows closed. If you must be outside, a high-quality mask may help."

Though the air quality had improved slightly since Wednesday, the concentrations of fine particulate matter in the air were still at levels deemed "unhealthy" on Thursday afternoon. And the air quality was expected to worsen again later in the day as a sea breeze blew smoke back on shore and over the city.

The emergency room data comes from the city's syndromic surveillance program, which funnels information from emergency room visits to the Health Department for analysis. The data indicates clusters of worsening asthma on Wednesday in Harlem and East Harlem, Far Rockaway, the northern Bronx, Bushwick and Brownsville, among other neighborhoods.

In New York City, asthma disproportionately affects Black and Hispanic children and those living in poorer neighborhoods, according to city data.

The increase in emergency room visits for asthma does not come as a surprise. Wildfire smoke "can lead to breathing difficulties and trigger exacerbations of their disease" in people with asthma and chronic obstructive pulmonary disorder, according to the federal Environmental Protection Agency. The city's health commissioner, Dr. Ashwin Vasan, has said that older adults and children are particularly vulnerable.

Long-term exposure to the fine particulate matter in wildfire smoke (known as PM2.5 because each particle is smaller than 2.5 micrometers across, about one-thirtieth the width of a strand of hair) has also been linked to developmental problems in children and cognitive impairment in older adults.

A recent study examining data from California estimated that emergency room visits for asthma increased 110 percent in the week following extreme levels of wildfire smoke.

Some New Yorkers with asthma said that although they have not had to go to the emergency room, their symptoms have been far worse than normal the past few days.

"I’ve been constantly coughing and wheezing, not being able to catch my breath at all," said Loretta Fleming, 54, a diabetes health educator in the Bronx who has had asthma since she was 12. She said she had been using her inhaler five or six times a day since wildfire smoke settled over New York City.

Aishvarya Kavi

Conditions in Washington, D.C., improved incrementally throughout the day, and the haze has lifted slightly over the city. But as evening rush hour began, more people were wearing masks — even on the metro. The transit authority said that trains are running with advanced filtration today.

Christopher Maag

Air quality is once again deteriorating in the New York area this afternoon as wildfire smoke approaches the city from opposite directions, according to John Cristantello, a meteorologist for the National Weather Service. Smoke directly from Canada is pouring in from the northwest, and a sea breeze is pulling more in from the east over the Atlantic Ocean. The result: darker skies, though nothing as ominous as Wednesday. Conditions should begin to improve tonight, he said, with a new weather pattern.

Somini Sengupta

Wildfire smoke has reversed the gains the United States made in cleaning up its air, with the wildfire smoke that penetrated New York City Wednesday being the latest and worst example. A special issue of the Climate Forward newsletter examines how the United States cleaned up air pollution in previous decades and what can be done in the coming years about wildfire smoke pollution.

Benjamin Hoffman

With the Air Quality Index in the Bronx having dropped to 164 — less than half of where it was last night — the Yankees started their doubleheader against the Chicago White Sox at the scheduled time of 4:05 p.m. Visibility isn't perfect at Yankee Stadium but it is nowhere near as bad as it had been over the previous two days.

Joe Drape

With the 155th running of the Belmont Stakes scheduled for Saturday, Gov. Kathy Hochul of New York announced enhanced measures to protect horses at the state's racetracks as they deal with poor air quality because of smoke from wildfires in Canada.

"We must all work to ensure that animals — including these peak-performance equine athletes — are protected," Ms. Hochul said in a statement. "The measures being implemented at tracks across New York State are effective steps to keep all those who participate in the sport safe now and into the future."

If the air quality index reaches 201 on Saturday, an amount considered "very unhealthy," there will be no training, and the Belmont races will be canceled. If it is between 151 and 200, considered unhealthy, horses will be allowed to work out or race only after their attending veterinarians certify that they won't be harmed. If a horse develops a respiratory problem, it must be declared unfit and scratched without penalty.

The index in Elmont, N.Y., the home of Belmont Park, was 143 on Thursday afternoon, down from more than 260 on Wednesday evening.

The new, permanent guidelines for the state's harness and thoroughbred tracks were issued by Dr. Scott E. Palmer, equine medical director for the New York State Gaming Commission. Pat McKenna, a spokesman for the New York Racing Association, the operator of Belmont Park, said the guidance was "grounded in our shared efforts to provide the safest possible environment for training and racing thoroughbred horses."

Mr. McKenna said the racing association remained optimistic about the chances for training and racing to resume on Friday.

Aishvarya Kavi

The air quality in Washington, D.C., has improved somewhat since this morning, but outdoor activities and events are still being canceled. Capital Pride has called off two events previously scheduled for tonight after the White House postponed a pride event until Saturday.

Ian Austen

Sitting on a park bench in downtown Toronto, Maria Lee flicked through the photos on her phone to show how smoke had occluded the view from her high-rise apartment near Lake Ontario.

"I like a blue sky, no pollution and sunshine," Ms. Lee said, pointing at the gray smudge on the horizon in the pictures. "Now everything is dirty."

As the deep smog lifted over Ottawa, the national capital, on Thursday, changing winds caused the air quality in Toronto, Canada's largest city, to visibly deteriorate. The government agency Environment and Climate Change Canada gave downtown Toronto a "moderate risk" rating on its air quality scale at midday Thursday, but forecast that conditions would worsen as the day continued and move the city to the "high risk" level.

But that was not as severe as had been anticipated. Steven Flisfeder, a warning preparedness meteorologist at the weather service of Environment and Climate Change Canada, said that the intensity of the smoke wafting from the northeast part of the province had been curbed by increased humidity and a bit of rain and cloud cover near wildfire areas.

"That's going to help flush out the contaminants from the air a little bit," Mr. Flisfeder said. "That should improve the conditions on the air quality side."

Ms. Lee said that while the city looked less appealing, she had found that Thursday's milder temperatures meant that the smoke actually had less of an effect on her breathing than it had earlier in the week. Nevertheless, she had an N95 mask slipped over one of her wrists. And she said that the smoke has exacerbated the asthma of one of her close friends.

In the vast square in front of Toronto's city hall, Patrick Junior Bradley, a bicycle messenger who lives in the suburban district of Scarborough, noted the absence of the usual flocks of tourists. It was unclear whether that was related to warnings from health officials to stay indoors. Few people on downtown streets were wearing masks, however.

And Mr. Bradley, who is known as P.J., said he had no qualms about cycling through the haze.

"It doesn't worry me," he said. "I just hope wherever the fire is that people are safe."

Sarah Graham

Wednesday was the worst day on record in the United States for wildfire smoke since 2006, by a lot. The largest number of Americans experienced an "extreme smoke day," which is defined as air quality deemed unhealthy for all age groups. And Tuesday was the fourth worst. Highly populated areas were hit with record levels of pollution, according to research from Stanford University scientists. This chart shows how bad it was in New York City

June 7th was the worst wildfire day on record in the US since 2006, by far. June 6th was the 4th worst. Just a massive, awful event, with highly populated areas getting hit with unprecedented levels of pollution. Great data work by @StanfordECHOLab @minghao_qiu Jessica Li.

Thomas Fuller

As plumes of smoke from Canadian wildfires engulf the East Coast and President Biden sends firefighters to Quebec, California officials are embracing a preventive approach that involves selectively cutting trees and intentionally lighting small blazes before the next catastrophe strikes.

Years of smoke-filled summers in California helped spur a change in how the state manages its forests, including spending hundreds of millions of dollars on programs to reduce forest growth.

It remains to be seen whether such efforts will be enough to combat the impact of climate change on California's forest landscape. But if they are, they could hold promise for containing similar wildfires in Canada, where prescribed burns have been rare.

The notion of selectively burning forests to save them has required a major shift in the way that California thinks about its vast wildlands. In March last year, state and federal authorities released a plan to increase prescribed burns in California from around 80,000 acres in 2020 to 400,000 annually by 2025.

In a state that is home to national treasures like Yosemite National Park and the towering redwood groves along the Pacific Coast, the shift in forest management has come with some resistance. Prescribed burns send smoke into communities and risk burning out of control, as was dramatically illustrated last year in New Mexico when two burning operations by the U.S. Forest Service combined to form the largest wildfire in New Mexico's recorded history. The fire burned 341,000 acres and destroyed hundreds of homes, causing the forest service to suspend its burning operations across the country for four months.

Yet in a marked shift from the decades of total fire suppression policies, there is now a consensus among forest managers in California that prescribed burns, when managed properly, condition the forests to be more resilient and help prevent the hot-burning megafires.

"The system is incredibly out of whack from human decisions to suppress fire," said Malcolm North, an expert in fire ecology at the University of California, Davis. "You can't just say nature can take care of itself because when we do have fires now, they don't burn in ways that are ecologically beneficial."

Prescribed burning in Canada is much less common than in the United States, according to Robert Gray, a wildland fire ecologist based in British Columbia. Despite recommendations by expert committees over the past two decades for more intentional burning, Mr. Gray calculated that prescribed fires burn less than 50,000 acres each year across Canada annually, in a nation that has more than 25 times as much forestland as California does.

"By and large, it never took off," he said.

Mr. Gray said policymakers in Canada have been wary of prescribed burns because they are "scared to death of liability" — of an intentional fire escaping and burning out of control. Funding is also an obstacle, he said.

The current round of fires has led to renewed calls for more intentional burning to make the landscape more resilient.

"Prescribed burns definitely would have helped," Mr. Gray said of the current fires.

But even California's goals might be hard to meet. Scott Stephens, an expert in wildland fire science at the University of California, Berkeley, said the state has the funds for prescribed burns but lacks enough qualified people to carry out the work.

"It's aspirational," he said of the 400,000-acre target. "It's a great goal but it's going to be challenging."

Experts in California believe forests need to be thinned through a combination of prescribed burns and logging. Officials at Yosemite National Park, for example, are trying to selectively cut down hundreds of trees in hopes of saving its ancient sequoia groves from destruction.

The embrace of intentional fire is also a return to the Native American practices that sculpted areas like Yosemite before the arrival of Europeans.

This shift toward more prescribed burns came after several years of witnessing the costs and destruction of megafires. In 2020 alone, wildfires killed more than 30 people in California, caused $12 billion in damages and leveled thousands of homes. It was also the year that wildfire smoke blotted out the sun and the skies in Northern California turned apocalyptic orange, similar to the choking effects experienced on the East Coast this week.

Jesus Jimenez

Around this time on Wednesday, the sky over New York City was a remarkable orange hue. The strange color is gone, but it's still quite hazy in the city. Air quality has improved, at least for now. The Air Quality Index is 179, which is considered "unhealthy," but far better than the levels over 400 that New York recorded on Wednesday.

Aishvarya Kavi

More than 100 people have gathered outside the White House for a protest against fossil fuels and the Mountain Valley Pipeline that was recently approved by the Biden administration. The demonstration was planned long before the wildfire smoke descended on the city, but protesters say it makes their demands all the more relevant.

"We are here despite the smoke," said Lawrence MacDonald, 68, a volunteer with Th!rdAct, a group of seniors that campaigns for environmental sustainability. He added, "You don't have to look far for the impacts from fossil fuel. They’re all around us, everyday."

Michael D. Shear

President Biden said Thursday that he had spoken to Prime Minister Justin Trudeau of Canada to offer more assistance for the wildfires raging in the country. And he said that communities in the United States affected by the smoke should listen to updates from state and local officials. He directed people to, a website where they can find out if the air in their area is healthy.

Michael D. Shear

Biden also said in a statement that "since May, more than 600 U.S. firefighters, support personnel and firefighting assets have been deployed, working alongside Canadian firefighters to tackle what is likely to be the worst fire season in Canadian history."

Rich Griset

At the University of Richmond, it looks as though a bonfire is taking place somewhere among the idyllic greenery and gothic buildings of red brick and blue-gray slate. Marcos Hendler, a chemistry and classical civilizations major who is on campus for the summer to do research, trekked to the student commons overlooking Westhampton Lake under bright but hazy skies. "When I was walking here, it just felt difficult breathing," he said.

Rich Griset

Hendler said he was trying to stay inside as much as possible, and that meant his laundry would have to wait. His apartment lacks laundry facilities, so that would be another trip. "I thought that I’d limit the time that I’m outside, and only go when I need to," he said.

Campbell Robertson

Air quality that reached dangerously unhealthy levels on Thursday added to the challenges facing those already struggling to breathe — including children in Philadelphia, who have asthma at more than triple the national rate, according to the Philadelphia Regional Center for Children's Environmental Health

"Philadelphia is a very poor city, the housing stock is very old, there are lots of environmental contaminants and so the air pollution rates are pretty high here," said Dr. Rebecca Simmons, one of the center's directors. "This haze just adds another layer of environmental exposure."

Children growing up in old and poorly-maintained homes are routinely exposed to mold, dust and lead. The city's aging infrastructure doesn't help: At least half a dozen city school buildings, which are on average around 70 years old, were shut down in the spring because of asbestos contamination.

The pandemic prompted the installation of fans and air filtration systems in some schools, but, Dr. Simmons said, "not to the extent that it is needed."

Beyond urging people stay inside and use air purifiers if they have them, Dr. Aimin Chen, another director of the center, said there is not much more the city could be doing in the immediate crisis. The chronic problems run so deep that acute responses are inevitably limited.

The Air Quality Index in the city topped 400 overnight, which is deemed "hazardous" by the Environmental Protection Agency. By midday it had dropped to 189 — an improvement, but still firmly in the "unhealthy" zone.

Dr. Simmons said she was concerned that there could be higher admissions rates to hospitals and more people visiting emergency rooms. There had not yet been a spike, but she said that's not surprising, "given how high our baseline rate is anyhow."

Meagan Campbell and Dan Bilefsky

The wildfires convulsing Canada have helped focus experts on a vexing and potentially lifesaving question: How long would it take to evacuate residents when out-of-control blazes intensify?

The wait would be worryingly long in some cases, according to a leading expert who has modeled the projected duration of a mass evacuation of central Halifax, Nova Scotia — a maritime city where a wildfire late last month forced more than 16,000 people to evacuate and destroyed about 150 homes in a suburban area.

Such is the importance of whisking citizens to safety that firefighting authorities in British Columbia have asked people to prepare a go-bag and an evacuation plan.

But Ahsan Habib, director of the School of Planning at Dalhousie University, said his model showed that evacuating about 100,000 people from central Halifax at 10 a.m. on a weekday would take an estimated 23 hours. If there were traffic collisions, he added, it would take longer.

"We were surprised," said Dr. Habib, a transportation professor and expert on travel behavior analysis. "We double checked multiple times," he added. "In a very small time window, we are trying to move so many cars, and our city historically has narrow roads. We do not have any drills or even any kind of preparedness for how to do these things safely in general."

That warning comes as Canadians have been grappling with the issue of preparedness. Canada is a sprawling country where wildfire emergency response management is handled by each of the 10 provinces and three territories, making coordination sometimes difficult. The challenges of evacuation have become all the more apparent at a time when firefighting resources are severely stretched.

Dr. Habib said the chaos during an evacuation could also lead to secondary fires or other incidents. "When things of this kind of scale happen, people get panicked," he said. "Disaster brings disaster."

The recent wildfires in Nova Scotia have been in relatively low-density population areas. But Dr. Habib warned that evacuating areas with larger populations would be far more difficult. He added that evacuating a university city like Halifax, where many students use bikes or public transportation, would also pose difficulties.

"What do we do with people who do not have cars? Younger folks — millennials for example, students for example," he asked. "It's a university town. We found there's a tremendous gap in how to evacuate those particular groups who don't have cars."