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What's Really Living in the Subway
What's Really Living in the Subway
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0:00
Some metro systems have a reputation for being clean as a whistle.
0:04
Others …I’m looking at you, New York City… have reputations for being subterranean cesspools.
0:09
We get it. With 58 billion annual rides taken, the world’s subway systems are bound to host some germs,
0:15
even in the cleanest trains and stations.
0:17
And in the dirty ones… it’s frightening to think about what could be festering in there.
0:21
But more than a few scientists have studied the microbial environments of subways,
0:25
and they’ve made some fascinating discoveries about the differences between them.
0:28
So hold on tight and stand clear of the closing doors.
0:31
You’re about to learn what’s riding along with you on the subway.
0:34
[♪ INTRO]
0:38
Listen, microbes are everywhere and you’re picking up a bunch when you
0:41
ride the T or L or the underground or whatever you want to call it.
0:44
We know because scientists in Mexico City swabbed
0:47
passengers’ hands before and after they rode the metro.
0:50
By the end of a trip, the assortment of microbes they collected was more diverse than at the start.
0:56
So you’re picking up quite a variety of germs.
0:59
And you’re getting them from the guy down the car picking his nose.
1:02
…Or maybe from the lady who shared a section of her newspaper with you instead.
1:05
Based purely on vibes, I’d rather have her bacteria all over me.
1:08
But based on science, we know that after a ride, the swabbed passengers’ microbial
1:12
milieu ended up looking a lot more like their fellow commuters’ microbes.
1:16
And it’s not just a Mexico City thing.
1:18
One of the most famous studies on metropolitan microbes was set
1:21
in the New York City subway system: one of the oldest and busiest in the world.
1:26
It’s one of only a handful of systems globally that runs 24/7,
1:30
and has a long-standing reputation for being gross.
1:33
Yes, I am picking on New York, but we have a lot of Complexly staff
1:37
who live in New York, and so I am saying this with love.
1:39
Even the senior author of the New York study was worried about how gross it might be.
1:43
He said he was inspired to discover the microbial underbelly of the City that
1:47
Never Sleeps when he caught his young daughter licking a New York City subway pole and was,
1:52
understandably, concerned for her health.
1:54
So he and his team got to work swabbing more than a thousand benches, turnstiles,
1:59
handrails, and other places in every single subway station in the Big Apple.
2:03
They collected tiny bits of microbial DNA and then ran it through a process called sequencing.
2:09
First, you identify the order of A’s, T’s, G’s, and C’s in parts of that DNA.
2:13
Then you run that sequence through a database to see if it matches known organisms.
2:18
And they got more than a few matches.
2:20
I’ll start with the good news: Of the top 10 most common bacterial species they found,
2:25
most were bacteria usually found on skin or in soil.
2:29
None of them normally cause disease in humans, although several can cause
2:33
infections in people who are already sick or have had a weakened immune system.
2:38
So maybe New York City deserves a little more
2:40
credit for their relatively harmless subway microbiome.
2:43
But New Yorkers weren’t the only ones interested in the
2:45
microscopic stowaways of their transit systems.
2:48
Researchers studying the T in Boston found microbes all over that system, too.
2:53
But they noticed that the type of surface, like a hand grip versus a bench,
2:57
was the biggest determinant of what kind of microbes would be there.
3:00
Hanging grips close to face level had more oral microbes,
3:04
and seats had more vaginal-associated microbes.
3:07
And before you raise too much concern over what goes on in Boston transit,
3:11
those researchers pointed out that vaginal-associated microbes can be transmitted through clothing.
3:16
So you can end up with different microbes from riding with different people and
3:19
from coming into contact with different surfaces.
3:22
But scientists in Shanghai found that microbes
3:24
in the air at subway stations can also vary with the seasons.
3:29
In their study, about 500 bacterial species were only detected during the springtime
3:35
and more than 6,000 were only found in the summer.
3:38
Meaning: there’s no one subway microbe.
3:41
Every trip has a different bunch of microscopic passengers along for the ride.
3:45
But for the most part, the microbes come from us.
3:47
And, importantly, researchers haven’t found a ton of pathogens, or germs that make you sick.
3:52
…Although note that I said they didn’t find a ton of pathogens.
3:56
It wasn’t unheard of for them to be lurking.
3:58
But overall, things are looking a lot less scary for public transit users
4:01
than you might have worried about when we started this episode.
4:04
And on top of helping us all sleep easy at night,
4:07
these microbes are telling some pretty cool stories about their specific neighborhoods.
4:11
But before we can talk about all that, all science needs funding, including ours.
4:15
So, here’s an ad.
4:17
Thanks for watching this SciShow video!
4:19
And thanks for supporting us on SciShow by commenting, sharing, and joining us on Patreon.
4:24
There’s a lot of online ways to keep fact-checked information,
4:28
like what you find in these videos, around.
4:30
And now there’s a way to do that in person too!
4:33
Complexly, the 501c3 non-profit that makes SciShow, PBS Eons, Crash Course,
4:39
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4:42
It’s called Something Worth Knowing and it’s live at the Wilshire Ebell theatre
4:46
in Los Angeles California on September 16th, 2026.
4:50
This first-ever Complexly in-person event is a fundraiser to support
4:54
our mission of inspiring curiosity and lowering barriers to knowledge building
4:59
…because curiosity builds understanding, and understanding builds a better world.
5:03
You can get your ticket at somethingworthknowing.org.
5:06
See you there!
5:08
In one publication summarizing 60 cities’ worth of swabbing and sequencing, each city had its own
5:14
unique microbial fingerprint determined by which species were found there and in what amounts.
5:19
Those researchers identified more than 4,000 different kinds of microbes.
5:23
And most of them were specific to less than a quarter of the samples.
5:27
That means part of each city’s microbial
5:29
fingerprint comes from some super regional microbes.
5:32
Each fingerprint was so distinct that a machine
5:35
learning algorithm could accurately identify cities based on microbial data.
5:39
And that’s even more impressive when you consider how much of
5:42
the cities’ microbiome still overlapped!
5:45
More than 97% of the samples contained the same 31 bacterial species.
5:50
They might not have been the most common microbes in each city,
5:54
but they did show up in almost every single one.
5:56
Those 31 bacteria formed what the researchers call the “core urban microbiome,” shared across
6:02
places as diverse as Singapore, Rio de Janeiro, London, Barcelona, and Denver!
6:07
About half of those core microbes are known to hang out with humans, like some that cause acne,
6:12
some that cause dental plaque, and others that make your feet stink.
6:15
These include staph and strep, which might sound scary, but there are lots
6:18
of bacteria in those families that don’t typically make people sick.
6:22
And even the ones that do usually only cause problems under specific conditions.
6:26
The other half of the core microbes were bacteria commonly found in the environment.
6:30
They tend to be harmless to you and me, but can cause serious infections if, say,
6:35
you managed to get some in contact with a catheter inside your body.
6:39
…Which is unlikely to happen on the subway.
6:42
I just had to mention that they’re not totally harmless.
6:45
And then there are the weird ones.
6:46
The microbe researchers found extremophiles that live in and around rocks, and can
6:52
survive conditions that make other microbes shake in their tiny boots …if they had them.
6:57
I’m talking about exposure to heavy metals, ionizing radiation, and UV light.
7:02
And, yes, all of these species tell a story.
7:04
You know how Sherlock Holmes can see flecks of mud
7:06
on someone’s clothes and know exactly where they’ve been?
7:09
Now that microbial fingerprints have been established for so many major cities,
7:13
we might have a higher tech version of that.
7:15
By sequencing the microbes on someone's clothes or shoes,
7:18
detectives might be able to figure out where they’ve been!
7:20
Another helpful hint these microbes give us is in relation to antimicrobial resistance.
7:25
When pathogens evolve workarounds to the chemicals we use to eliminate them,
7:29
they can end up becoming superbugs that are very hard to kill.
7:32
This often happens when people misuse antibiotics, like taking them when you
7:35
don’t actually have a bacterial infection or not finishing your complete course.
7:39
At that point, you’re just giving a competitive
7:41
advantage to bugs with mutations that allow them to survive the antibiotic.
7:45
So it’s not great if we find a bunch of
7:47
antimicrobial resistance in our global subway microbiome.
7:51
But at least we’d know what we’re up against.
7:53
And when it comes to knowing what we’re up against, this
7:56
survey of subway microbiomes was super helpful.
7:59
It identified almost 11,000 viruses that aren’t in databases!
8:03
…Meaning we may never have seen them before!
8:06
We’re learning about a totally new world of subway microbes.
8:09
And they could turn out to be good guys that only infect and kill harmful bacteria.
8:13
So yes, they uncovered a giant amount of microbes in our subway systems.
8:16
But no, you don’t need to be that worried about getting sick from riding public transit.
8:20
Remember, the overwhelming majority of microbes
8:23
identified in subway systems are not microbes known to cause disease in humans,
8:27
but rather microbes that are already living in and on human bodies without making us sick.
8:32
But for those that do make us sick, a study published in 2020 across 121 US
8:37
cities investigated if riding public transit was associated with influenza and pneumonia deaths.
8:43
Those researchers compared the death rates from those
8:46
causes to the percentage of people in that city who use public transit.
8:50
If public transit were a major cause of disease spread, they figured that cities
8:54
with high proportions of transit users would have the highest number of deaths.
8:59
But they didn’t.
9:00
In fact, New York City, which had the highest rate of public transit users at 57% of the population
9:06
over age 16, was among the 10 cities with the lowest rate of influenza and pneumonia deaths.
9:12
Despite the close quarters, the authors think transmission was lessened by
9:17
typical practices like not talking to or touching strangers when you can help it.
9:21
And a simulation of an influenza epidemic in New
9:24
York City predicted that only 4.4% of all transmission would happen on the subway,
9:29
compared to 30% of transmission at home and 24.5% at school.
9:34
Those are the places you probably spend more of your time anyway.
9:37
So nasty viral infections don’t seem to be super associated with riding the subway in general.
9:42
But the giant caveat to that statement is that
9:45
there’s only so much we can learn from DNA sequencing.
9:48
A DNA sequence alone can’t tell us whether a microbe was even alive at the time of sampling.
9:54
After all, DNA is a remarkably stable molecule,
9:57
so it can stick around long after the organism it belongs to is dead.
10:00
But even if the microbes are alive, sometimes that’s a good thing too.
10:04
Many studies have shown that exposing children to microbes
10:07
early in life is really important for training a healthy immune system,
10:10
and might even prevent issues like allergies and asthma later on in life.
10:14
For adults, those microbes can also become part of a healthy microbiome,
10:18
where there is often strength in diversity.
10:20
So if you, like that researcher’s kid,
10:22
licked a subway pole, it seems pretty unlikely you’d get sick.
10:25
But keeping your hands and arms inside the car and your tongue
10:28
inside of your mouth is still probably a good idea as you zip around the city.
10:33
[♪ OUTRO]
