When congestion pricing went into effect in New York City almost a year and a half ago, residents in the South Bronx, which has some of the highest asthma rates in the United States, expressed concern about the consequences for air quality. Some predicted that drivers, in an attempt to avoid the toll to enter Manhattan, would take detours through their neighborhood, which is chock-full of major highways and bridges.

Now, a Columbia University study, relying on data from 19 sensors across the South Bronx, shows that overall fine particulate matter — tiny, toxic particles produced by burning fossil fuels — has increased since the start of the tolling program. According to Alexander De Jesus, a Ph.D. candidate and an author of the study, a 2 percent increase in particulate matter was detected in the South Bronx from 2024 to 2025, the first year of congestion pricing.

Researchers from Columbia and other universities worked with data from the South Bronx sensors over two years, comparing the 12 months before congestion pricing with the same period after the program started. They found elevated particulate matter levels throughout most of the neighborhood, especially near major expressways. Two sensors, one near a community garden, showed a decrease in particulate matter levels.

“While New York City’s congestion pricing policy has improved air quality in the congestion pricing zone, it worsened air quality in surrounding areas such as the South Bronx, probably due to traffic diversions,” said Markus Hilpert, an associate professor of environmental health sciences at Columbia’s Mailman School of Public Health and an author of the report.

A spokesman for the Metropolitan Transportation Authority, which oversees congestion pricing, vigorously questioned the study, saying it has yet to be peer reviewed and did not take into account smoke from wildfires that affected the city for about six days in 2025. (The study is still going through the peer-review process, according to its authors, who said they had controlled for factors such as wildfire smoke.)

“Reducing air pollution has always been one of the core goals of New York’s congestion pricing program,” Janno Lieber, the chief executive of the M.T.A., said in a statement. His remarks were released on Tuesday by Mayor Zohran Mamdani, who announced that the transit agency had dedicated $20 million to address asthma in the South Bronx.

According to an abstract of the South Bronx study, which is not yet available in its entirety, the increase in fine particulate matter “was statistically significant, although there was substantial variability in estimates across monitor sites.”

The study does not make a definitive link between the introduction of congestion tolling and the increased readings in particulate matter. But its authors said they had controlled for other factors that contribute to fine particulate matter pollution in the South Bronx, such as building heat, seasonality, weather fluctuations and traffic patterns. What was left, they said, was that 2 percent increase, which they attribute to the congestion pricing program.

Measuring air quality is difficult, scientists say, because of variability in atmospheric conditions. At least one year of data tracking weather fluctuations across four seasons is necessary to have a snapshot of air quality shifts. Even then, every year is unique, which makes it challenging to compare one year with another.

The city’s Department of Health conducted a three-month study that compared the spring of 2024 with the spring of 2025, before and after the start of the tolling program, and found “no significant change” in fine particulate matter around the region.

In a report released this year, the M.T.A. said that highway traffic had mostly decreased during the same time period covered by the Health Department study, including in the South Bronx.

In New York City, traffic accounts for just 14 percent of fine particulate matter; most of the pollution comes from buildings and other sectors. “The South Bronx is a densely populated area,” Dr. Hilpert said. “Very often you see schools and residential high-rises located just next to highways, so even a modest increase in air pollution can have significant public health impacts.”

The South Bronx is one of the poorest areas in New York City, with a median household income of about $32,000 and little green space. In contrast, the neighborhood has an outsize number of waste transfer stations and industrial warehouses, including Hunts Point, one of the largest food distribution centers in the United States, with almost 13,000 trucks coming and going daily. Asthma afflicts one out of five children in the South Bronx.

Congestion pricing, which charges most drivers up to $9 to enter Manhattan 60th Street and below, is funding about $70 million of mitigation efforts in the South Bronx. They include subsidizing asthma programs in the borough and replacing refrigerated diesel trucks that serve Hunts Point with hybrid versions or vehicles that run on cleaner fuels. In 2025, tolls generated more than $578 million in revenue for the M.T.A., which is using the money to upgrade subways and buses that many in the South Bronx rely on, the spokesman said.

Heralded as a success by political leaders and many environmental activists, congestion pricing has reduced the number of cars entering the central business district by 11 percent, or 73,000 vehicles, with the remaining traffic moving faster and more people opting for public transit. Air quality improvements are harder to discern. Some studies show much cleaner air, while others have found little to no difference.

For people in the South Bronx, any decrease in air quality compounds an already challenging pollution situation, according to neighborhood advocates and researchers, who want state and city authorities to adopt measures to mitigate any increase in particulate matter.

“We are calling on the M.T.A. to treat congestion pricing as a living policy, one subject to continuous, transparent evaluation in dialogue with the communities bearing its costs,” South Bronx Unite, a nonprofit focused on social, economic and environmental issues, said in a statement released on Tuesday. “To declare it a success while communities like ours see air quality getting worse is premature and unjust.”

Stefanos Chen contributed reporting.