The true impact of air pollution has been obscured by the failure to consider people’s exposure as they move around during the day, according to a new study that has mapped the hotspots of New York’s air pollution based on where people gather for work or recreation.

The research cites air pollution as “the world’s single largest environment and human health threat” but laments that the problem has not previously been “considered spatially and temporally”, with most studies basing a person’s pollution exposure on where they live.

Using information obtained from cellphone towers, the researchers were able to build up a picture of millions of New Yorkers’ movements over the course of 120 days. They used a formula that charts population density as well as pollution levels to look at how people’s exposure to PM2.5, an airborne particle linked to an array of health problems, varies according to where they typically congregate during the day.

The researchers found that areas of midtown and lower Manhattan, which have some of the highest PM2.5 levels, saw large numbers of people exposed during the day, but less so at night. Conversely, areas of southern Brooklyn with high PM2.5 levels peaked in pollution exposure at night as people returned to their neighborhoods after work. Areas of Queens near LaGuardia airport remained elevated throughout the day.

Concentrations of PM2.5 also vary throughout the day and affect the identification of hotspots, depending on factors including power generation and traffic.

Illustration: Wonyoung So & Hyemi Song, MIT Senseable City Lab

“Air pollution is linked to respiratory and cardiovascular illnesses but we have previously presumed that people exposed are at home all the time,” said Dr Marguerite Nyhan, lead author and a researcher in environmental health at Harvard University, although all the research was conducted at MIT’s Senseable City Lab.

“We know that’s not a true assessment of exposure as more people are exposed as they go from home to work and when they are socializing. We found that lots of people are being exposed in central Brooklyn and Queens and lower Manhattan, where people work and recreate.

“But that’s not the way cities are typically regulated for air pollution – they just look at highly polluted areas rather than the amount of time people spend in them.”

Disregarding population shifts throughout the day, PM2.5 air pollution in New York is greatest in midtown and downtown Manhattan, parts of the Bronx and areas of Queens and Brooklyn that meet the East River. These areas have PM2.5 levels that are around 14 micrograms per square meter – higher than the annual average advised as acceptable by the World Health Organisation (WHO).

According to WHO, worldwide around 7 million people a year die as a result of air pollution. The US accounts for around 200,000 of these annual deaths, with most linked to fumes from cars, trucks and other transportation, as well as emissions from power plants. Particles of PM2.5, which are around 30 times smaller than a human hair in width, can worsen asthma and heart disease and are linked to reduced lung function.

A study commissioned by New York City found that annual average PM2.5 levels fell by 16% in the six years to 2014 but warned that air pollutants “remain at levels that can be harmful to public health, particularly among seniors, children and those with pre-existing health conditions”.

Asked whether the city considers peoples’ exposure as they move around the metropolis, a spokesman for New York’s mayor, Bill de Blasio, said: “The city’s monitoring study is designed to assess outdoor, street-level air pollutant levels across the city.

“So in a sense, the data can be used to assess exposures at residences, places of employment, or parks and other recreation areas. The study is not designed to assess any single exposure setting.”

De Blasio has set a goal for New York to have the cleanest air of any US city by 2030, pledging to cut PM2.5 levels by 50% in this timeframe by adding electric vehicles to the city fleet, phasing in clean sources of electricity and making buildings more energy efficient.

Nyhan said her research, published in Environmental Science & Technology, could be used if New York wanted to go down the path of “low emissions zones” as London has done. Motorists driving in most areas of Greater London have to pay a daily charge unless their vehicles meet European emissions standards.

“If you identify areas of higher exposure, you can implement low emissions zones there for the maximum health benefit,” said Nyhan, who plans to undertake a similar spatial study in Boston.

“In the last 100 years, air quality has certainly improved but we are still seeing problems in larger cities. We are seeing a transition to electric vehicles but it’s not quick enough – more and more people are residing in cities, which means more people are contributing to and being exposed to pollution in urban areas.”