Thread by @nytimes

Subway ridership is way down in New York City — 20% of the city’s 5.5 million weekday riders are using the system. Many fear the confined spaces of subway cars will lead to coronavirus infections.

We looked at how safe the air is on the subway.
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Air flow is key in preventing the spread of the virus on subways, along with masks and social distancing.

The subway’s ventilation system moves air within the train cars more efficiently than in restaurants and other indoor settings, experts say — but it's not perfect.

75% of the air in a subway car is recycled. It’s sucked into vents, cooled and filtered.

25% is pulled from the outside and combined with the existing air.

Here’s how air flow through this venting system helps limit viral particles from building up inside a car.

The ventilation helps fully replace air in the car with outside air about every 3 or 4 minutes.

Filters work to block viral particles, but some could slip past and get dispersed into trains.
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If viral particles slip past the system’s filters, they could circulate at least 3 times in the car over several minutes.

If someone sneezes, riders could be exposed to viral particles that don’t get filtered out — which is one reason why wearing masks is important.

The ventilation system pushes air across the train rather than down into floor vents. So someone standing between a person without a mask who sneezes or coughs and the ceiling vent could come into contact with aerosols.
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Experts say that the high air exchange rate and wearing masks sharply reduce the risk of spreading the virus.

But if more people are riding the subway, it's harder for ventilation to lessen the risk of transmission.

See more.

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