Metro's Impact: Pollution Or Progress?

Underground metro systems are among the most polluted places in cities. Measurements from the University of Copenhagen found that ultra-small particle concentrations underground were 10 to 20 times higher than in the city's most polluted stretch of road. The French Agency for Food, Environmental and Occupational Health & Safety found that metros in France had three times the pollution of the outside air. Research on the London Underground discovered ultrafine metallic particles small enough to enter the bloodstream. While it is unclear whether this pollution causes health issues, studies are currently underway to determine the effects on people with sensitive lung conditions. This article will explore the topic of pollution in metro systems and discuss the potential health risks for commuters and transport workers.

Air pollution in metro systems

The primary cause of air pollution in metro systems is the generation of particulate matter, specifically PM2.5 (particles with a diameter of less than 2.5 microns). These particles are created by the friction between metal wheels and tracks, as well as brakes and wheels, which releases tiny metal particles into the air. The composition of these particles varies depending on the environment and can include iron, copper, and other metals. The concentration of these particles can be particularly high in metro stations and tunnels, with levels exceeding recommended limits.

The health risks associated with air pollution in metro systems are not yet fully understood. While there are no widespread signs of severe or acute health issues among commuters and transport workers, there may be more subtle, chronic effects on lung, brain, or heart function. The unique environment of metro systems, with enclosed spaces and limited ventilation, can exacerbate the issue, as pollution is concentrated in a relatively small volume of air.

However, it is important to note that metro systems also play a role in reducing overall urban pollution. The widespread use of metros has contributed to reducing motor vehicle emissions and improving outdoor air quality, especially in densely populated cities. Additionally, some metro systems have implemented initiatives to improve air quality, such as transitioning to cleaner fuels and engines, and electrifying bus fleets.

As the health effects of air pollution in metro systems remain uncertain, individuals concerned about the potential risks can take precautions such as wearing well-fitted filtering masks while using underground transportation.

Health risks of underground railways

Underground railways are used by millions of people every day in many cities. They often have an unusually high concentration of airborne particulate matter due to poor air exchange with the outside environment. The particulate matter in underground railways is primarily composed of metals like iron, copper, and chromium, which are generated from the wheels running on the rails.

The health risks of exposure to airborne particulate matter are well-documented, with links to asthma, lung cancer, heart disease, myocardial infarction, stroke, type 2 diabetes, dementia, and loss of cognitive function. However, the health effects of particulate matter in underground railways specifically are less understood. While in vitro studies suggest that this type of particulate matter may be more toxic than ambient or urban particulate matter due to its metal-rich composition, there is a lack of evidence for these effects being clinically significant.

Some studies have found that pollution levels in underground railways are significantly higher than those found at street level. For example, measurements in the Copenhagen Metro found ultra-small particle concentrations that were 10 to 20 times higher than in the city's Town Hall Square. Similarly, the French Agency for Food, Environmental and Occupational Health & Safety found that metros in France had three times the pollution of the outside air.

Despite these findings, there are no widespread signs of severe or acute health problems among commuters and transport workers who spend significant time in underground railways. However, there may be more subtle, chronic effects on lung, brain, or heart function that have not yet been fully understood or documented.

To mitigate the potential health risks of particulate matter in underground railways, some organizations have implemented strategies to reduce pollution levels. For example, Metro in the United States has adopted a Green Construction Policy that mandates the use of greener, less-polluting equipment and renewable diesel for its infrastructure projects. They have also transitioned to cleaner fuels and engines and are working towards electrifying their bus and fleet vehicles.

Metro construction and air quality

Metro systems have been an essential mode of transport in cities for over 150 years, with new lines still being constructed and expanded globally. While these systems have proven beneficial for urban development, spatial changes in land use, and positive impacts on public health and environmental quality, there are concerns about the air quality within these underground networks and during their construction.

Metro construction activities can generate air pollutants, which can affect the air quality around construction areas and potentially cause health issues for nearby residents and passengers. However, the impact of construction on indoor air quality inside trains and stations requires further investigation, as there is limited data available. For example, a study on the Sydney Metro construction activities in Australia found that they had an insignificant impact on the air quality inside the trains, with PM2.5 and PM10 concentrations slightly higher during the daytime than at night.

Metro systems themselves, especially underground networks, have been associated with higher levels of air pollution. Studies have found that the pollution levels in metro systems can far exceed recommended limits, with ultra-small particle concentrations underground being significantly higher than those found above ground or at street level. The composition of these particles varies and may include metals, rubber, and other pollutants generated by the interaction of train wheels and rails or brakes.

The health effects of exposure to these pollutants are a significant concern. While there are no widespread signs of severe acute health issues among commuters and transport workers, there may be more subtle or chronic impacts on lung, brain, or heart function. The unique environment of underground systems, with pollutants concentrated in a smaller volume of air, may exacerbate these potential health risks.

To mitigate these issues, some metro operators have implemented policies and strategies to improve air quality during construction and operation. For instance, Metro's Green Construction Policy in Southern California mandates the use of greener equipment and renewable diesel, reducing greenhouse gas emissions and air pollutants. Additionally, transitioning to cleaner fuels, electrifying fleets, and improving ventilation and aerodynamic effects can also help address air quality concerns in metro systems.

Fuel consumption and chemical use

While metros are a more environmentally friendly mode of transportation than cars, they still contribute to air pollution. The majority of air pollution in metro systems is usually PM 2.5 (particles with a diameter of less than 2.5 microns). On the London Underground, this pollution is composed of metal particles, primarily iron and copper, which are generated from the wheels running on the rails. A small amount of particulate pollution is also produced by the connection between the trains and the electrified rail.

The composition of underground air pollution differs from that found above ground. Research has found that ultra-small particle concentrations underground were 10 to 20 times higher than in less confined spaces. The French Agency for Food, Environmental and Occupational Health & Safety discovered that pollution levels in French metros were three times higher than outdoor air.

Metro systems are working to improve their fuel consumption and reduce air pollution. Metro's Green Construction Policy (GCP), for example, has improved construction-related air quality in Southern California. By mandating the use of greener equipment and renewable diesel, the GCP reduced greenhouse gas emissions by 45% for one of its major infrastructure expansion projects.

Metro has also been working to improve the fuel efficiency of its bus fleet. Over the past few years, the average fuel economy of Metro's bus fleet has increased by 27%, from 2.96 miles per gallon (MPG) to 3.76 MPG. This has been achieved by replacing older standard diesel buses with newer diesel-electric hybrid, CNG, and clean diesel models. Metro has also committed to electrifying its bus fleet by 2030, which is expected to lower overall NOx emissions by 54% by that year.

Strategies to mitigate emissions

Transition to Cleaner Fuels and Engines:

Recognizing the impact of fuel consumption on air quality, metropolitan areas can transition to cleaner fuels and engines. This includes adopting renewable diesel and gradually electrifying bus and fleet vehicles, reducing tailpipe emissions and improving overall air quality.

Green Construction Policies:

Implementing Green Construction Policies (GCP) can significantly reduce construction-related air pollution. By mandating the use of greener equipment and renewable diesel, metropolitan areas can reduce greenhouse gas emissions and improve air quality during major infrastructure projects.

Improve Energy Consumption and Efficiency:

Addressing the high energy consumption of metro operations is crucial. By improving energy efficiency and transitioning to renewable energy sources, metropolitan areas can reduce their carbon footprint. This includes investing in energy-efficient technologies, optimizing electricity usage, and exploring alternatives like solar or wind power.

Collaboration and Sustainable Infrastructure:

Metropolitan areas should collaborate with various agencies, communities, and utility partners to build sustainable transportation infrastructure. This includes incorporating complete streets concepts, multimodal transportation options, and strategies to mitigate increases in vehicle miles traveled due to highway projects.

Health and Safety Measures:

As research on the health effects of underground air quality continues, implementing precautionary measures can help protect commuters and staff. This includes providing educational resources and encouraging the use of well-fitted filtering masks, especially in areas with high particulate concentrations.

Long-Term Planning:

Metropolitan areas should align their goals with long-term climate objectives, such as achieving net-zero emissions by 2050. This includes setting intermediate targets, such as transitioning to a 100% electric bus fleet and reducing emissions from other sources like power plants and factories.

By adopting these strategies, metropolitan areas can play a crucial role in mitigating emissions, improving air quality, and contributing to the global fight against climate change. These approaches require collaboration and a holistic view of sustainability to create a healthier and more environmentally friendly future for urban residents.

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