Covid-19: As metro services set to resume, air flow poses biggest health challenge
To mitigate this risk in part, the Delhi Metro Rail Corporation (DMRC) has decided to increase the rate at which fresh air is pumped into train coaches and open ventilation vents at underground stations more often to ensure stale air escapes.
Delhi Metro officials have made widespread changes to how air conditioning and ventilation systems work inside trains and at underground stations in an attempt to reduce the risk of Covid-19 infections. But the way the air flows, particularly in coaches, and how the virus floats around, continues to pose a threat for users.
The risks also depend on how well people are able to maintain distance from one another and whether they adhere to Covid-safe precautions such as wearing masks.
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To mitigate this risk in part, the Delhi Metro Rail Corporation (DMRC) has decided to increase the rate at which fresh air is pumped into train coaches and open ventilation vents at underground stations more often to ensure stale air escapes. 
Inside a metro coach, this will result in the air being refreshed every 2 minutes, 33 seconds. Two recent studies show contrasting results on how close environment public travel risks can be mitigated and, conversely, allowed to result into a major outbreak.
The first, published in Jama Network Open on September 1, showed the transmission of the virus from one person to 23 others on a bus packed with 67 people in China. None of the passengers wore a mask.
The second, published on September 5 in the Emerging Infectious Diseases journal, showed that in a flight full of 310 people, six asymptomatic Covid-19 infected people spread the virus only to 1 other person. All passengers wore masks except for when they had meals and used lavatories.
Researchers have previously pointed to how air flows are a possible factor. In a bus, much like in a metro train, overhead ducts create a bigger area of air circulation than in flights in which air conditioning vents on the top and bottom create a restricted, vertical draft.