I’m pretty sure anyone who’s ever ridden on a subway, anywhere, would find this valuable information: how long can a subway car’s air filtration system filter a fart from the air? As far as I can tell, no major subway system has provided such information. Until now.
Yes, the Bay Area Rapid Transit System (BART), the system that serves the San Francisco Bay area, an area well-known for its prodigious and committed flatulators, has publicly given an official estimate regarding how long a fart will linger in a BART subway car:
About 70 seconds, maybe longer if you laid a real monster of a fart or sharted yourself. https://t.co/y2ZxlT7qEJ
— SFBART (@SFBART) August 13, 2020
So, there you go — one minute and ten seconds. This also, I believe, is the first time an official missive from a municipal mass transit system has included the word “sharted.”
The discussion seems to have stemmed from a larger discussion regarding the air filtration systems in BART cars, noting that constant airflow is crucial to preventing the spread of airborne COVID-19 viruses:
As we have demonstrated, BART train cars filter & replace inside air about every 70 seconds. Air flow is critical to minimizing airborne risk of COVID-19.
If you enjoyed the fart BART tweet, you have to read and share this thread: https://t.co/0QUYwlnxG4
— SFBART (@SFBART) August 13, 2020
Here’s a larger version of that diagram:
Each car appears to have an independent HVAC unit under the car floor, which sends air into the cabin via ceiling-mounted vents. Fresh air — making up 30 per cent of the air in the car — is pulled in through exterior vents just below the car’s beltline, and is mixed with recirculated air.
This system allows for all the air in a cabin to be refreshed every 70 seconds, which, as the original tweet suggested, would carry away any lingering farts.
Of course, if the source of the stench is continual — as in the aforementioned shart situation — even with the whole cabin air being replaced every 70 seconds, it could still be contaminated by the shart material as long as its present on the car.
According to my copy of NASA’s Bioastronautics Data Book, humans can generate up to 2800 ml of farts per day, about one and a half two-litre soda bottle’s worth, but the odds of someone unloading the maximum daily volume of flatus in one go on a subway is pretty minimal. It would also probably blow them out of their seat.
Still, it’s good to have this sort of information. I guess it’s useful for COVID-19 reasons, but I like knowing how rapidly farts can be whisked out of the car.
Once, on the LA subway, a dude on the car I was on lit his hair on fire. That smell took a hell of a lot longer than 70 seconds to go away.