The public transit system wasn’t designed for social distancing. Crowded cars of jostling strangers makes six feet of distance between others impossible. Yet millions of Americans, especially in hubs like New York City, rely on the subway, trains, and buses for their transportation.
In the wake of COVID-19, many people are avoiding the subway, fearful that even with masks, the close quarters and inefficient ventilation system will allow the coronavirus to spread freely.
This makes it more important than ever before to understand how the ventilation system works on the subway and how public transit authorities are taking unprecedented steps to ensure passenger safety.
How Does the Subway’s Ventilation System Work?
Believe it or not, the subway’s ventilation system moves air within train cars more efficiently than restaurants, schools, and other indoor settings that most of us assume are far safer.
75% of the air in a subway car is recycled, and 25% is pulled from the outside. The recycled air is continually sucked into vents, cooled, filtered, and then pushed back into the subway cars through air ducts along the ceiling. The outside air, meanwhile, is pulled into the system and combined with existing recycled air.
Since this ventilation system maintains continuous air flow, viral particles don’t have much of an opportunity to build up inside of subway cars. This is believed to minimize the likelihood of passengers becoming infected with COVID-19 as they inhale.
The filtration system is designed to block large and small aerosols before they enter the air ducts. Filters used on subway cars include a wave design that increases their surface area. This makes it possible to trap droplets and pathogen particles as the incoming air is forced to change direction.
However, the filtration system isn’t perfect. Some viral particles do inevitably slip past and enter the subway car’s air flow. Based on ventilation patterns, some viral particles could circulate at least three times in a subway car over the course of several minutes.
How Does the Subway Ventilation System Compare to a Classroom or Office?
It’s easy to feel paranoid about catching COVID-19 while riding the subway, but this fact might surprise you: the recycled air on subway cars is replaced about 18 times per hour. That’s about triple the frequency used in an office building and six times more often than done in schools!
However, subway filters aren’t as powerful as they could possibly be. They’re rated by their minimum efficiency reporting value, or MERV. Experts recommend that all indoor spaces upgrade their filters to a level 13 to proactively combat airborne transmission of coronavirus. At this time, the subway system uses filters rated MERV-7.
The Importance of the Face Mask
Considering all of these factors, the importance of wearing face masks on the subway is clear. Simulations compare what happens when a masked and unmasked person sneezes. When a mask is on to block the sneeze, tiny aerosols hover in the air and some larger droplets escape from the sides of the mask and fall to the floor. The ventilation system quickly pulls those particles through the filtration system.
The simulation without a mask is much different. A higher volume of droplets fall to the floor and disperse through the air. The ventilation system eventually sucks them up, but it’s more likely for nearby passengers to be exposed.
There’s no one simple answer, but the more we learn about the coronavirus, the better we can prepare to make public places safer. The Metropolitan Transportation Authority and other transit systems in large cities are doing their part by testing new ultraviolet technology to kill viral material in the air and maximize rider safety.
With superior air filtration, UV technology, masks, and other strategies, it may be possible to once again ride the subway without fear.