Chennai metro commuters face elevated air pollution

Chennai: For thousands of city commuters packed into metro coaches every day, the ride may involve more than crowded platforms and long waits. A study on Chennai Metro’s Line 2 (Alandur-Central) found that passengers were exposed to elevated levels of fine particulate matter, bacterial contamination and volatile organic compounds (TVOCs), with underground stations recording the poorest air quality.

The multi-institutional study led by IIT Madras found particulate matter (PM) levels in underground stations were 5-9 times higher than inside coaches, while elevated stations recorded roughly double the PM levels seen in trains. TVOC levels across trains and stations were 2-3 times above recommended limits, while bacterial concentrations crossed safety thresholds inside trains and underground stations. Pollution levels were also higher during morning and evening rush hours, with passenger crowding emerging as a key driver of exposure. CMRL’s average daily ridership has risen to more than three lakh passengers since services began in 2015.

Researchers attributed poor air quality to passenger crowding, outdoor traffic pollution entering stations, enclosed underground spaces, train movement and ventilation patterns, cleaning-product emissions, and air-conditioning systems that were less effective at filtering fine particles and gaseous pollutants such as TVOCs. Peak-hour crowds also increased bacterial contamination and carbon dioxide levels.

For commuters spending 30-60 minutes daily in enclosed coaches and stations, the concern is repeated exposure in compact, airtight environments.

“Although the overall particulate matter concentration in the trains was relatively low, a significant portion (70–90%) of the particle mass consisted of fine particles smaller than 2.5 micrometre. These fine particles pose a health risk, as their small size allows them to penetrate deeply into the respiratory system,” said Amit Passi, first author of the study.

The study found train coaches recorded the lowest PM levels and maintained temperature and humidity within recommended limits, indicating effective air-conditioning and filtration of larger particles, though finer particles were filtered less efficiently, while underground stations showed signs of inadequate AC performance.

Microbial sampling at Alandur station, inside trains and Central station found the highest bacterial and fungal colony counts at Central, followed by trains, and the lowest at Alandur.

“Elevated levels of TVOC and bacteria may be attributed to factors such as high passenger density, metabolic emissions, off-gassing from interior materials, use of personal care products, and the limited ability of AC systems to filter gaseous and biological contaminants,” said Prof S M Shiva Nagendra, one of the study’s authors.

Previous studies cited by the researchers linked elevated bacterial levels to crowding and confined spaces, while fungal growth on HEPA filters in air-conditioning systems could get dispersed during operation.

A Chennai Metro Rail Limited (CMRL) official said trains are cleaned daily and fumigated twice a month at depots, while stations follow cleaning schedules ranging from hourly to quarterly. With ridership increasing, cleaning frequency during daytime operations has been stepped up, particularly during off-peak hours. CMRL said stations use primary and secondary filters in air-handling units, ultraviolet germicidal irradiation systems and CO2 sensors to maintain indoor air quality, while trains are fitted with fresh-air and mixed-air filters.

“Based on the increased passenger movement, the cleaning frequency has been further enhanced during daytime operations, particularly during non-peak hours between 12noon and 4pm, in addition to the regular night time cleaning schedule,” the official said.

Public health experts stressed the need for active indoor air-quality monitoring in underground stations, better-maintained ventilation and HVAC filtration systems, routine sampling and a continuous supply of fresh air to reduce commuter exposure.

Prof Naveen Puttaswamy, Faculty of Public Health at SRIHER, said fine particles such as PM2.5 and PM1 in enclosed metro environments could indirectly raise infection risks because airborne viruses can attach to suspended particles and move through crowded, poorly ventilated spaces. “If metros are charging public money, underground stations should be equipped like airports with efficient ventilation, fresh-air circulation and regular monitoring because PM2.5 particles can carry viruses and commuters inhale them continuously in crowded enclosed spaces,” he said.