There is a group of people who, out of sheer curiosity, carry scientific instruments with them to measure the amount of potentially harmful particles in the air around them. Harmful air particles are tiny clumps of matter that can be inhaled in the air. Terry Gordon, an environmental health scientist at New York University, said: "We're a lot like nerds who study air pollution, right? ”
A few years ago, a colleague of his walked into the New York subway and found readings on a particulate concentration detector astonishingly high. Gordon recalled: "He thought the instrument was broken." But this is not the case, and such high readings have sparked a lot of buzz. So the scientists conducted a study investigating particulate concentrations in various subway stations in New York, New Jersey, Philadelphia and elsewhere in the northeastern United States.
This is just one of many papers on particulate pollution in subway air, and recent publications on particulate pollution in subway systems around the world reflect growing concern about the health risks that urban commuting environments can pose. Previously, prosecutors in Paris opened a criminal investigation into allegations that air pollution in the local subway endangered people's lives. Not only that, but allegations that RATP , the operator of the underground rail system in Paris, deliberately underreported the concentration of particulate matter in the subway air, has been denied by the company.
It is undeniable that the measured concentration of air particulate matter in subways is usually several times that on the street. The friction of the metal wheels of subway trains on the tracks, as well as the braking brakes, produces tiny metal particles that are swept into the air as the train moves. The question is how particulate concentrations in different subway systems compare, and whether science reveals whether there are real health risks for people traveling or working in these environments. Long-term inhalation of harmful air particulate matter can cause various heart and lung problems and even lead to premature death.
Gordon, however, was surprised to hear the charges by Paris prosecutors. "Paris is nothing compared to the concentration of particulate matter in the air of the London Underground," he said, adding that, at least according to his research, there are more particulate matter in subway stations in and around New York than anywhere else.
Gordon studied some subway stations in the northeastern United States and detected the highest ever measured air pollution. For example, stations in Manhattan and New Jersey measured about 1,700 micrograms (μg) of particulate matter 2.5 microns in diameter or less per cubic meter of air – commonly known as PM2.5. This reading is an average taken over two relatively short, about one-hour long windows. PM2.5 is considered particularly dangerous because these very fine particles can penetrate deep into people's lungs and possibly even enter people's bloodstreams. The World Health Organization recommends that the average amount of PM2.5 a person is exposed to in a day should not be exceeded.
The Port Authority Trans-Hudson (PATH), which belongs to the transit system of the stations in Manhattan and New Jersey, said air quality at the stations was within guidelines set by the U.S. Occupational Safety and Health Administration in an independent assessment. "We will continue to take appropriate measures to ensure that the air quality of our PATH system remains within regulatory standards," PATH said. ”
Another study published last year detailed PM2.5 levels across all nine lines of the New York City Subway and 341 platforms at 287 stations. Although they fluctuate, with some readings as high as about 600, the average concentration on the platform is around 120 to 300.
A spokesperson for the Metropolitan Transportation Authority in New York said: "We have previously conducted air quality tests on subway trains operated by our system and have not found any health risks. He added that ensuring the safety of customers and employees is the highest priority for the Transportation Authority.
By comparison, an official study of particulate matter concentrations in the air in the Paris metro, published last year, found that PM2.5 concentrations in the Paris metro were much lower than previously measured, but still about three times the concentration of PM2.5 outdoors in Paris. Between 2010 and 2018, PM2.5 concentrations gradually declined from 70. According to another study, the average PM2.5 concentration in the Stockholm Metro between 7 a.m. and 7 p.m. on weekdays is 260, while the PM2.5 concentration on the Seoul subway platform is about 129.
It is important to note that the studies differed in several ways. Scientists use different monitors, measure at different times of the day, and calculate averages of periods of varying length, so the results cannot be directly compared. Not only that, but seasonal differences can also affect readings, as a study on particulate concentrations in Shanghai's subway system pointed out, with average readings of inhalable particulate matter much higher in autumn and winter, possibly due to winter haze affecting many parts of China.
But these studies as a whole do provide insight into the high concentration of particulate matter in the metro network. And some older, deeper subway stations, such as those in New York, tend to have relatively higher overall particulate matter concentrations.
The London Underground is one of the most studied metro systems in the world. The London Underground originated in 1863 and is the oldest underground in the world. Hassan Aftab Sheikh, a doctoral student at the University of Cambridge in the United Kingdom, recorded particulate concentrations at multiple subway stations and noted that the deepest, oldest lines tend to be the most polluted. He noted that the average concentration of particulate matter measured was around 300-400, but that it could soar to around 1,000 when the local rail rushed into the platform and blew up dust on the ground and around the tracks. This concentration that rises in a short period of time will soon decrease again.
Old-fashioned subway, image source network
Last year, Sheikh and his colleagues studied the relatively high levels of iron oxide in the London Underground particulate matter. A chemical composition analysis of particulate matter in the London Underground can show that it has most likely been around for many years. "This is not the usual iron oxide" and "because it has been in the metro system for too long, it has been further oxidized", said Sheikh.
The London Underground system is in contrast to the Paris Underground system, which has trains with rubber wheels on several lines. Sheikh also noted that the Paris metro system is relatively new and has better ventilation than the London Underground.
A spokesman for Transport for London, which operates the Underground Tube, noted that safety was the agency's top priority and that staff had been working for years to reduce dust in the tube system. She added: "This includes the use of industrial piggyback dust [vacuum] cleaners as part of our multi-million pound Underground cleaning programme".
The last thing to know with certainty is whether these particulate substances actually cause health problems. In many cases, millions of commuters have traveled on the subway for many hours, five days a week, for many years, and thousands of railroad workers have spent even longer in the tunnels. However, in these populations, even though air pollution concentrations in the subway exceeded the recommended standards, no serious or acute health problems were found in a wide range of people. However, it is uncertain whether subway particulate matter has more subtle chronic effects on human health or effects on lung, brain and heart function.
Matthew Loxham, an air pollution toxicologist at the University of Southampton in the UK, said: "We certainly can't rule these factors out. It's just that, based on the current evidence, there don't seem to be clear and clear health risks, at least in the groups surveyed. He co-authored a review of evidence on health risks in 2019, which reached this conclusion. He doesn't know if there will be any new evidence to re-overturn it.
He added that metal particles are ubiquitous in subway air particles, especially ferrous metal particles. Since metals are generally considered toxic, this fact may cause public concern. And the particulate composition is also sometimes soluble, meaning that these metal particles can dissolve in people's lungs and enter the body's cells.
"It's bad, but it's also possible that those soluble ingredients are easier to remove than solid particles," Rocksham said, suggesting that some lumpy particles may or may not settle in people's lungs, which may or may not cause health problems in the future.
High concentrations of iron are indeed a problem, he added, but our bodies process iron all the time; It is a key component of hemoglobin in our blood, so we have mechanisms in our body that regulate it. He stressed that it is not yet possible to determine whether these regulatory mechanisms have an effect on subway air particles. In addition, it is difficult to link the occasional detection of high concentrations of inhalable particulate matter in subway air to specific negative health outcomes, so don't jump to conclusions.
If people are concerned that air particles in the subway will endanger their health, they can wear masks that can filter air particles. Where practical, metro operators may consider installing screen doors on platforms to reduce the concentration of dust raised by running trains to passengers. But even with this approach, there are other risks. A study on the use of such screen doors in Seoul subway stations found that while it sometimes reduces particulate content on platforms, it also increases particulate content in trains.
Gordon said it's hard to say whether subway airborne particulate matter actually poses a health risk to humans, but he stressed the need for further research, including long-term studies of the health of subway workers for years, even in their retirement.
BY CHRIS BARANIUK
Translation: Sweeping Monk
Reviewer: Tibetan idiot
Translated content represents the views of the author only
It does not represent the position of the Institute of Physics of the Chinese Academy of Sciences
Editor: Tibetan idiot