Railway and metro systems, though considered eco-friendly, face significant air pollution challenges from diesel emissions, mechanical wear, and poor ventilation in enclosed stations. Diesel locomotives release NOx, CO, PM, SO₂, and black carbon (BC), contributing to urban air pollution and severe health risks, particularly in underground metro stations where pollutants accumulate due to restricted airflow. Even electric rail systems generate secondary pollution through brake wear, wheel-rail friction, and dust resuspension.

Enclosed railway environments exacerbate exposure to PM2.5, ultrafine particles, and VOCs, increasing risks for commuters and workers. Studies highlight that diesel trains have 35x higher ultrafine particles, 6x higher black carbon, and 8x higher NOx compared to electric alternatives. Additionally, metro systems can generate ozone (O₃) due to high-voltage electrical components.

Real-time air quality monitoring using IoT-enabled sensor networks, AI-driven analytics, and advanced filtration is crucial for mitigating these issues. Technologies such as Selective Catalytic Reduction (SCR), Diesel Particulate Filters (DPFs), and smart ventilation systems significantly improve air quality. High-precision NDIR, electrochemical, and laser-based sensors enable real-time pollutant tracking, allowing authorities to implement adaptive strategies.

Future advancements, including hydrogen-powered trains, AI-driven pollution control, and hybrid rail systems, will be key to achieving cleaner transit environments. With tightening regulatory frameworks, real-time air quality monitoring and proactive mitigation strategies will be essential in ensuring compliance and safeguarding public health in railway ecosystems.

Imagine stepping into a bustling metro station during rush hour. You take a deep breath, expecting a clean, conditioned environment, but what if that air is filled with tiny, invisible particles that could harm your health over time? Air pollution in railways and metros is a hidden challenge, and here’s why it matters. Railways and metro systems are the backbone of urban transportation, moving millions daily while reducing road congestion and emissions. But have you ever considered the air quality inside these transit systems?

Rail is usually considered a green mode of transport compared with road and air in terms of its relative impact on climate change. However, rail services, particularly those operated by diesel-powered trains, also emit air pollutants. From diesel locomotive emissions to fine metal particles from brakes and poor ventilation in underground stations, air pollution in railways and metros is a hidden but serious concern. In enclosed metro tunnels, pollutants like PM2.5, nitrogen oxides (NOx), and carbon dioxide (CO₂) can accumulate, leading to health risks for passengers and workers alike. Electric metro systems aren’t entirely pollution-free; friction from brakes, wheel-rail interactions, and resuspended dust contribute to air quality issues.

This is where advanced air quality monitoring steps in. With real-time sensors, IoT-based networks, and AI-powered analytics, transit authorities can detect pollution hotspots, optimize ventilation, and ensure safer breathing environments. In this blog, we’ll explore the sources of air pollution in railways and metros, its impact, and the latest monitoring technologies that help keep the air clean for millions of daily commuters. Let’s dive in!

Rail services contribute to air pollution by emitting harmful pollutants that affect human health. Particulate matter (PM10 and PM2.5) is a major concern, primarily generated from mechanical wear due to friction between train wheels and rails, pantographs and catenary wires, and braking systems (Salma et al., 2007). These non-exhaust emissions occur regardless of the train’s power source and contribute significantly to airborne particulates in railway environments.

In addition to mechanical wear, diesel-powered trains emit combustion-related pollutants, including carbon monoxide (CO), carbon dioxide (CO₂), nitrogen oxides (NOx), sulfur dioxide (SO₂), and black carbon (BC). These pollutants originate from fuel combustion and include ultrafine particles that fall within the PM2.5 and PM10 size ranges (Chong et al., 2015; Givoni et al., 2009; Jaffe et al., 2014; Jeong et al., 2017; Moreno et al., 2015).

Among these emissions, black carbon (BC) is a key air pollutant, predominantly originating from diesel exhaust, especially in developed economies (Jaffe et al., 2014). Research estimates that diesel-powered trains contribute approximately 2.0% of total NOx emissions, 2.8% of PM2.5, and 2.5% of black carbon emissions from mobile sources. These pollutants pose serious health risks, particularly in enclosed railway environments, where prolonged exposure can impact respiratory and cardiovascular health.

To mitigate these emissions, rail networks must adopt low-emission technologies, advanced braking systems, and effective air filtration strategies to improve air quality in and around railway corridors.

1. Train Operations and Combustion Emissions

Rail transport, particularly diesel-powered locomotives, contributes significantly to air pollution due to fuel combustion and mechanical wear.

• Diesel Locomotive Emissions: Diesel-powered trains release nitrogen oxides (NOx), carbon monoxide (CO), and particulate matter (PM) due to incomplete fuel combustion in internal combustion engines. NOx emissions react with atmospheric pollutants to form ground-level ozone (O₃) and secondary PM, exacerbating urban air pollution.
• Braking System Wear: Mechanical friction between brake pads, wheels, and tracks generates fine metallic particulates, primarily in the PM10 and PM2.5 size ranges. These particles, composed of iron, copper, and other metals, contribute to indoor and outdoor railway air pollution.

Did you know this? Recent studies show that 50-70% of brake wear material in rail transport becomes airborne particles, mainly PM10 and PM2.5. These particles, rich in iron and copper, come from friction between brake pads, wheels, and tracks.

• Electric Trains and Secondary Pollution: While electric rail systems produce no direct exhaust emissions, they contribute to secondary pollution through power generation processes. Electricity generation, depending on the energy source, can release CO₂, NOx, and SO₂, indirectly impacting air quality.

A Danish study found that pollution levels inside and around diesel trains are much higher compared to electric trains. Inside diesel trains, ultrafine particles were 35 times higher, black carbon was six times higher, nitrogen oxides (NOx) were eight times higher, PM2.5 (tiny particles small enough to enter the bloodstream) was twice as high, and benzo(a)pyrene was six times higher.

2. Tunnel and Station Environments

Enclosed spaces, such as underground metros, exacerbate pollutant accumulation due to limited ventilation and confined air circulation.

• PM Accumulation in Underground Stations: Subway systems are the veins and arteries of cities, moving people where they need to go. Their speed, accessibility, and affordability offer an alternative to often chaotic city streets. Poor ventilation and restricted airflow in underground metro stations trap PM10 and PM2.5, leading to elevated pollutant concentrations. Tunnel structures limit natural dispersion, making airborne particulates more persistent.

Did you know this? A study in Barcelona found that subway air had higher PM2.5 levels (43 µg/m³, range: 37-49 µg/m³) than trams or walking outdoors.

• Dust Resuspension and Indoor Pollution: Passenger movement and train-induced turbulence disturb settled dust, causing resuspension of PM and other airborne particles. This effect increases occupational exposure risks for station workers and commuters.
• Ozone (O₃) Formation in Metro Systems: Metro and railway systems utilizing high-voltage electrical components can contribute to ozone (O₃) generation due to the interaction of electrical discharges with air pollutants. Ozone formation in enclosed spaces can worsen respiratory conditions and degrade indoor air quality.

3. External Environmental Factors

Railway stations and yards are subject to external sources of pollution, which impact air quality inside and around railway infrastructure.

• Emissions from Railway Yards and Maintenance Depots: Volatile Organic Compounds (VOCs) are emitted from fuel storage tanks, engine maintenance activities, cleaning agents, and lubricants. VOCs contribute to ground-level ozone formation and air toxics exposure.
• Pollution Infiltration from Surrounding Areas: Railway stations near high-traffic roads and industrial zones are exposed to NOx, PM, and SO₂ infiltration. These pollutants enter metro stations and train carriages through ventilation systems and passenger movement.
• Meteorological Influence on Pollutant Dispersion: Wind speed, humidity, and temperature variations affect the distribution of pollutants. Enclosed metro systems experience higher pollutant retention, while open railway stations depend on natural airflow for pollutant dispersion.

Pollution levels in diesel vs. electric trains:

While rail transport is an eco-friendly mode of travel, it generates significant noise pollution, especially in densely populated areas. In countries like the Netherlands, managing railway noise is crucial for infrastructure planning and capacity management. Strict noise regulations impose limits on railway operations, sometimes restricting certain train types or routes. One of the primary sources of railway noise is the wheel-rail interaction, where mechanical contact produces vibrations and sound emissions.

Did you know this? Sound levels above 85 dB can cause hearing damage over time. Passing trains often exceed this limit, making railway noise a major concern for nearby communities.

Although the energy used to produce railway noise is only a tiny fraction of the energy required to power a train, the impact is substantial. Key sources of railway noise include:

• Wheel-Rail Interaction: Friction between wheels and tracks generates continuous rolling noise.
• Traction Noise: Diesel engines, traction motors, gearboxes, turbochargers, and fans contribute to mechanical and exhaust noise.
• Warning Signals: Train horns and level crossing alarms add to peak noise levels in urban areas.
• Maintenance Equipment: Track grinding, tamping, and other railway upkeep operations create high-decibel noise.
• Shunting and Coupling: The impact noise from connecting or moving railcars is a major disturbance in rail yards.

How does the noise from passing trains affect the surrounding environment?

The noise emitted by passing trains can have detrimental effects on the surrounding environment and its inhabitants. It can disrupt wildlife, disturb residents, and even impact the quality of life in affected areas.

1. Noise from trains can disrupt wildlife in the surrounding areas.
2. Residents living near train tracks may be disturbed by the loud sound.
3. Quality of life in areas affected by train noise can be negatively impacted.

Air pollution in railways and metros poses health risks to commuters and workers and impacts the environment.

Health Effects on Commuters and Workers

Exposure to diesel engine fumes poses significant health risks. The International Agency for Research on Cancer (IARC) has classified diesel exhaust emissions as carcinogenic, linking them to an increased risk of lung cancer (Benbrahim-Tallaa et al., 2012; WHO-IARC, 2012). Diesel exhaust primarily emits fine particulate matter (PM2.5), which has been associated with acute respiratory (Lin et al., 2011; Paunescu et al., 2019; Wang et al., 2022) and cardiovascular diseases (Kirrane et al., 2019; Nichols et al., 2013; Song et al., 2022).

Black carbon (BC) is a key indicator of incomplete fuel combustion and serves as a reliable tracer for diesel emissions (Jaffe et al., 2014). Enclosed train stations with diesel-powered trains are considered high-exposure zones for diesel exhaust, significantly affecting both commuters and railway workers. Since station workers spend prolonged hours in these environments, assessing their occupational exposure to diesel emissions is critical.

Additionally, station design and operational factors influence pollution levels. Evaluating airflow, ventilation systems, and train operations can help develop effective strategies to reduce workforce exposure to diesel emissions, ensuring a safer indoor air environment.

Environmental Impact

Railway station air quality is influenced by both outdoor and indoor pollution sources. Traffic emissions, industrial activities, and regional air pollutants enter stations through ventilation systems. However, additional pollutants originate within the station itself, including emissions from diesel and electric trains, food outlets, and passenger activities (Chong et al., 2015; Font et al., 2020). These indoor pollution sources compound the overall air quality burden, leading to higher particulate and gaseous pollutant concentrations.

Rail and metro operations further contribute to urban air pollution. Diesel locomotives release nitrogen oxides (NOx), sulfur dioxide (SO₂), and carbon dioxide (CO₂), accelerating ground-level ozone formation, acid rain, and climate change. Braking system wear generates metal-rich particulate matter (PM10 and PM2.5), polluting both indoor air and the surrounding environment. Additionally, electric rail systems indirectly contribute to emissions, as power generation processes release greenhouse gases (GHGs) depending on the energy source.

Regulatory Compliance and Legal Implications

Air pollution in rail systems is subject to strict regulatory standards, including ambient air quality limits set by WHO, EPA, and national pollution control boards. Operators must monitor emissions, comply with workplace exposure limits, and adopt cleaner technologies to meet legal mandates. Non-compliance can result in fines, legal action, and reputational damage, emphasizing the need for proactive air quality management in railway infrastructure.

Monitoring is the first step in improving air quality, as it helps assess current pollution levels before deciding on the best mitigation measures. The Air Quality Monitoring Network (AQMN) can measure air pollutants for railways and metros. It is a comprehensive and systematic monitoring program for measuring air quality at railway stations to provide high-quality and robust pollutant concentration data.

Sensor-Based Real-Time Monitoring Systems

Modern railway and metro systems deploy sensor-based air quality monitoring systems to measure particulate matter (PM1, PM2.5, PM10), nitrogen oxides (NOx), carbon monoxide (CO), volatile organic compounds (VOCs), and black carbon (BC). These high-precision sensors provide real-time pollutant data, enabling authorities to detect pollution hotspots and ensure compliance with environmental standards. Advanced sensors use NDIR, electrochemical, and laser-based techniques for accurate pollutant detection.

Real-World Impact: Ahmedabad Railway Station

Oizom’s Polludrone Smart was deployed at Ahmedabad Railway Station to monitor air quality, providing crucial insights into pollution levels caused by train emissions and passenger movement. The system helped authorities identify pollution trends and implement data-driven mitigation strategies. Read the full case study here.

IoT-Enabled Air Quality Monitoring Networks

The Internet of Things (IoT) enhances air quality monitoring by connecting multiple sensors across railway stations, tunnels, and train cabins. IoT-enabled networks enable remote monitoring, predictive analytics, and automated alerts for air pollution levels. The IoT Air Pollution Monitoring systems integrate with cloud platforms to analyze trends, helping transport authorities implement real-time mitigation strategies such as ventilation adjustments or train speed modifications to reduce emissions.

Smart Ventilation and Filtration Integration

Efficient ventilation and filtration systems are crucial for controlling indoor air quality in enclosed metro, railway stations, and tunnels. Smart ventilation systems use pollution sensors and automated controls to optimize airflow, reducing particulate buildup. High-efficiency particulate air (HEPA) and activated carbon filters effectively capture PM, VOCs, and gaseous pollutants, improving air quality for passengers and workers. With Oizom’s smart air quality monitoring solutions, they can measure real-time data of CO₂ levels, temperature, and humidity, offering actionable insights to ensure optimal environmental conditions. Using Oizom’s technology, railway and metro authorities can Automate HVAC systems by integrating real-time data and starting mitigation systems when pollutant levels exceed safe thresholds.

AI and Machine Learning in Pollution Control

Artificial intelligence (AI) and machine learning (ML) are transforming railway pollution management. Predictive models analyze historical air quality data, identifying pollution trends and optimizing mitigation strategies. AI-driven automation adjusts ventilation settings, recommends maintenance schedules for emission-heavy components, and enhances real-time decision-making to minimize exposure risks.

Monitoring air pollution in railway and metro systems is complex due to multiple emission sources, restricted access, dynamic train movements, and microclimate variations within tunnels and stations. Ensuring accurate measurements requires highly sensitive monitoring equipment, real-time data integration, and adaptive strategies to account for diverse environmental conditions.

Key Technical Challenges:

• Diverse Emission Sources: Railway air pollution originates from diesel engines, braking systems, wheel-rail interaction, and high-voltage electrical components. Identifying the exact pollution source is challenging due to overlapping emissions.
• Limited Accessibility for Monitoring Equipment: Installing air quality sensors in tunnels, tracks, and confined station spaces is difficult due to restricted access, safety concerns, and operational disruptions.
• Dynamic Train Movements and Emission Fluctuations: Train operations involve acceleration, braking, and varying speeds, leading to irregular emission patterns. High-frequency sensors are needed to capture real-time variations in pollution levels.
• Fine Particulate Matter (PM2.5) Detection: Railway emissions contain ultrafine particulates, requiring advanced laser-based and optical sensors for precise detection and differentiation from background pollution.
• Microclimate Variability in Tunnels and Stations: Air quality within underground metros differs significantly from outdoor environments due to ventilation systems, passenger density, and pollutant accumulation. Localized sensor placement is essential for accurate monitoring.
• Data Integration and Standardization: Merging air quality data from various monitoring locations requires consistent measurement techniques and unified data formats to create a comprehensive pollution map across the network.
• High Costs and Maintenance Requirements: Deploying and maintaining a robust monitoring network requires significant investment in sensor calibration, data transmission, and long-term operational reliability.

Rail operators can implement several strategies to improve air quality for passengers and workers, focusing on both train-side and station-side interventions.

Train-Side Solutions

• Cleaner Fuels & Engine Efficiency: Switching from diesel to electric trains and using biofuels (biodiesel, renewable diesel) can reduce greenhouse gas (GHG) emissions by up to 86%.
• Advanced Filtration & Exhaust Treatment: Diesel particulate filters (DPFs) capture most soot particles, while Selective Catalytic Reduction (SCR) technology reduces NOx emissions using chemical reactions.
• Hybrid Diesel-Electric Conversion: Hybrid trains use electric traction modules and energy storage systems to recover energy from braking, reducing fuel consumption by 20% and carbon dioxide emissions by 2,400 tonnes per year.
• Electrification: Electric trains emit 20-30% less GHG per passenger-kilometer and are 20-45% cheaper to manufacture, maintain, and operate compared to diesel trains.

Station-Side Solutions

• Improved Ventilation & Exhaust Systems: Installing high-capacity exhaust fans and optimizing station design can improve airflow, reducing pollutant buildup in enclosed metro stations.
• Operational Optimization: Reducing engine idling time and relocating loading/unloading areas away from congested zones can minimize localized air pollution.
• Real-Time Air Quality Monitoring: Using sensor-based monitoring helps track pollution trends and adjust station ventilation and scheduling to reduce exposure.

Health Impact of Railway Air Pollution

• Black Carbon Exposure Risks: Commuters generally have lower exposure to pollution than road traffic, but black carbon remains a concern. Long-term black carbon exposure increases lung cancer mortality, with railway workers facing higher risks.
• Short-Term Pollution Risks: High pollution levels can also contribute to kidney disease and respiratory issues, highlighting the need for air quality control measures in railway environments.
• For Commuters: Choose carriages with better ventilation (often at the front or rear). Wear a high-efficiency mask (N95) during peak hours in enclosed metros. Limit exposure by standing away from doors where outdoor pollutants enter.

Many cities worldwide have acknowledged the issue of rail-based air pollution and taken proactive measures. Here’s how different regions compare:

Many metro systems worldwide are incorporating smart environmental monitoring to improve commuter health. Explore more with our use case how a robust air quality monitoring for railways and metro solution helps in detecting pollution hotspots and ensuring a cleaner transit environment.

Air pollution in railways is subject to different standards across countries, with varying levels of enforcement.

Air pollution in railways and metros is a critical environmental and public health concern, driven by diesel emissions, brake wear particulates, poor ventilation, and confined station infrastructure. Effective mitigation requires a data-driven, multi-layered approach combining electrification, advanced exhaust treatment, real-time air quality monitoring, and AI-driven ventilation control.

Deploying high-precision monitoring systems such as Oizom Polludrone enables rail operators to continuously track key pollutants, including PM1, PM2.5, PM10, NOx, SO₂, CO, and VOCs. These insights facilitate proactive air quality management, allowing for real-time adjustments in ventilation, filtration, and operational schedules. Additionally, diesel after-treatment solutions like Selective Catalytic Reduction (SCR) and Diesel Particulate Filters (DPFs) help minimize NOx and particulate emissions, while hybrid and electric rail systems significantly reduce black carbon exposure.

A holistic, technology-driven approach is essential for enhancing sustainability, reducing health risks, and creating a cleaner rail transport ecosystem. Future-ready solutions such as hydrogen-powered trains, AI-driven filtration, and smart tunnel ventilation will be critical in reducing railway pollution. As regulatory demands tighten, real-time monitoring and adaptive air quality management will be pivotal in shaping the future of pollution control in railway networks.