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What Happens to Viral Particles on the Subway

Many New Yorkers are avoiding the subway, fearful of jostling with strangers in crowded cars. Masks and social distancing are essential, but good air flow is also key to reducing the risk of exposure to the coronavirus.

What Happens to Viral Particles on the Subway Aug. 10, 2020
Many New Yorkers are avoiding the subway, fearful of jostling with strangers in crowded cars. Masks and social distancing are essential, but good air flow is also key to reducing the risk of exposure to the coronavirus.

The subway’s ventilation system moves air within train cars more efficiently than restaurants, schools and other indoor settings, according to aerosol experts. But it is not a guarantee to protect against the virus. Here’s how the system works.

At any given moment, 75 percent of the air you breathe in a subway car is recycled. The rest is pulled in from the outside.

Air is constantly sucked up through vents, cooled and filtered before being pushed back through ducts.

At the same time, outside air is pulled into the system, combined with the existing mix and released into the car through the duct panels, which span the ceiling.

This is how air moves through a typical subway car.

Continuous air flow through these vents helps limit viral particles from building up inside a car and infecting people as they inhale.

Most cars have two ventilation units at either end, which help fully replace air in the car with outside air about every three minutes and 20 seconds on average.

Inside the units, fans pull the air through filters and cooling coils.

The filters work to block large and small aerosols before air enters the ducts. But the filters do not catch everything — and some viral particles could slip past and get dispersed into trains.

The way the filters work is determined by their materials and shape. The subway filters have a wave design that increases their surface area, creating more opportunities to trap droplets. The filters are made of fibers that force incoming air to change direction, trapping some particles as air is pulled through the filters.

Filters like these are rated by their ability to block large particles, also known as their minimum efficiency reporting value, or MERV. MERV filters are rated on a scale of one to 20. Experts recommend that indoor spaces upgrade their filters to a level 13 to help ward against the airborne transmission of the coronavirus. Subway cars use filters that are rated MERV-7 and are replaced every 36 days, though they may be replaced sooner if needed.

Still, the subway’s so-called air exchange rate — or how often recirculated air is completely replaced with fresh air — helps minimize the risk of coronavirus transmission for riders. The recycled air on subway cars is replaced on average at least 18 times an hour, far higher than the recommended exchange rate for offices, which is six to eight times an hour, or classrooms, which is three to four times an hour.

Even with a relatively high exchange rate, however, viral particles that slip past the subway system’s filters could — based on ventilation patterns — circulate at least three times in the car over the course of several minutes. For example, if someone in a car sneezes, riders could be exposed to viral particles in the air that don’t get filtered out more than once, underscoring the importance of wearing masks.

Here are two air-flow scenarios in which a rider sneezes — one while wearing a mask, and the other without.

Sneezing with a mask on

Sneezing without a mask

Sneezing with a mask on

Sneezing without a mask

Sneezing with a mask on

Sneezing without a mask

In the mask simulation, some larger droplets escape from the sides of the mask and fall to the floor, while tiny aerosols hover in the air. The ventilation system then pulls those particles into a vent on the ceiling of the car and pushes them through the filters.

In the simulation without a mask, many more droplets coat the floor around the rider and aerosols disperse into the air, eventually moving through the ventilation system.

Because the ventilation system pushes air across the train, rather than down into vents on the floor, those aerosols could come in contact with someone standing between the person sneezing and the ceiling vent.

“You can see how well dispersed that plume is,” said Krystal Pollitt, an assistant professor of epidemiology at the Yale School of Public Health. “Even if you are not standing directly beside the person who coughed or sneezed, you are still in contact with that emission.”

Public health experts caution that not all the particles released from a sneeze — or those shown in these simulations — contain viral matter. And while it is still unclear how much virus is needed for someone to be infected, coming into contact with a few viral particles may not make you sick.

Still the simulations underscore the importance of both good ventilation and passengers wearing masks: Without proper ventilation, those aerosols would stay aloft in the air and build with each sneeze or cough.

To help address that, the Metropolitan Transportation Authority, which runs the city’s subway, is testing out new ultraviolet technology that could be installed inside trains to kill viral matter in the air. This would prevent any viral particles a sick rider exhales from being recirculated through the ventilation system.

New York State has also required subway riders to wear face masks and the state’s transit agency has begun offering free masks. Masks not only help protect riders, but also train conductors, who are sequestered in tiny cabins for long stretches at a time.

Air to the engineer’s cab

HVAC

Engineer’s

cab

Air flow from the

passenger area

Air to the engineer’s cab

HVAC

Air flow from the

passenger area

Engineer’s

cab

To try to safeguard the cabins where conductors work, fans push filtered air from the passenger area into their operating booths before being pulled back out again.

Public health experts say that the high air exchange rate and widespread mask usage on the city’s subways sharply reduces the chances of a so-called superspreader event on trains.

Only 20 percent of New York’s 5.5 million weekday subway riders now use the system. The health experts caution that if more people begin riding the subway, pushing crowding closer to pre-pandemic levels, the ability of the system’s ventilation to lessen the risk of viral transmission diminishes dramatically. Even when riders are wearing masks, it is possible for a passenger to inhale the viral particles exhaled by a sick passenger if they are standing shoulder-to-shoulder.