- Hey, hi, I have a big old announcement,
all the details are gonna be at the end of the video,
but here's a teaser.
I'm making a class, a Physics 101 course.
It's gonna be freely available
right here on the "Physics Girl" YouTube channel,
so check out the end of the video for all of the details.
Okay, back to your regularly scheduled YouTube video.
In 1977 an Alaska Airlines flight
originating in Homer, Alaska,
was delayed due to maintenance issues.
Its air circulation system was turned off
but the passengers were still onboard the airplane
and sat on the runway for three hours.
Within the next 48 hours,
38 out of the 53 passengers and crew
started experiencing chills, fever,
severe coughing and headaches.
The cause of this sickness
was initially suspected to be a toxic substance
introduced into the passenger compartment by engine failure.
But then,
after dozens of the sick passengers were hospitalized,
one passenger was confirmed sick with influenza.
An epidemiologists later discovered
that every one of those 38 passengers they tested,
came back positive for the flu.
That means without air circulation systems,
a whopping 72% of the people on board
were infected with the viral illness
from one sick passenger.
But recent studies have shown
that the daily reality of airplane travel
is much safer than the Alaska Airlines event.
How safe?
Well, on an airplane with normal and functional circulation
the number of passengers likely to get sick
when one fellow passenger has the flu
is just 0.7 passengers, that is less than one person.
So what has changed since that 1977 flight?
Hey, I'm Diana, you're watching "Physics Girl"
and this case was so intriguing to me
it sent me down the rabbit hole
of this typically overlooked
but incredibly important area of research in science: air.
Specifically ventilation
and how air flow can impact your respiratory safety,
so I called up a researcher at Lund University in Sweden
to talk about air.
- We as humans, we still emit
about a million skin and hair fragment particles per hour.
- Wow.
- [Malin] And if you're 10 people in a surgery room
that will make up a concentration pretty fast.
- Malin has to think about those hair and skin particles
and also bacteria and viruses in the air
because she studies different types of air flow
and ventilation systems, and how they might impact, say,
the spread of respiratory diseases.
So, let's start there,
let's talk about how you even catch respiratory diseases,
diseases like the flu, pneumonia, COVID-19.
The literature on this
refers to three modes of transmission;
number one is self inoculation,
which is when you basically transfer the disease to yourself
by say, touching a dirty door handle,
getting the virus on your hands
and then touching your nose or mouth.
That's why you're not supposed to touch your face.
And then number two, large droplet transmission,
which involves being coughed or sneezed on
or sung to by an infected person
and getting hit by droplets at least five microns in size.
That size is important
and it's actually come up in the news lately.
We'll get to that.
And then there's number three, airborne transmission.
So, the novel coronavirus, SARS-CoV-2,
which is the virus responsible
for the current COVID 19 respiratory disease pandemic,
can be transmitted via the first two modes.
What about airborne transmission, you ask?
Well, scientists are still today debating
whether SARS-CoV-2 is considered to be airborne
but being airborne may not mean what you think it means.
We know you can get the disease from the air,
so when public officials
refer to the virus as being airborne they generally mean
that it can linger in the air in tiny droplets,
at that size of five microns or less.
So, the virus can linger in these tiny particles in the air
even after the person who coughed has left the building,
that might explain this news article that recently came out
about a woman who rode an elevator alone in her building
and went on to be the origin of 71 infected persons.
And also a caveat to all of the information about COVID-19,
the Corona virus that causes it
was only discovered on January 7th of this year,
it was only named on February 11th, 2020.
So all this information is still incredibly new
and scientists are rushing
to understand and contain the virus,
so some of this information can and probably will change.
Now that we understand
a bit about how respiratory diseases spread,
we can talk about the magic that is ventilation research.
Because, for example,
if you study the ventilation in hospitals,
you might be able
to help prevent patients from getting sick
but also protect healthcare workers.
- We go out to hospitals and collect air samples,
and we have done that in three different projects,
and every time we go there and we're collecting,
and the nurses ask, when do we get the answers?
Can you say something?
Can you help us?
'Cause they don't want to have infectious diseases,
they get sick themselves.
- This is such important research.
How do people design the airflow in an operating room
to keep you safe when you're getting open-heart surgery?
Malin worked on a study
looking at different ventilation systems
to see which were the cleanest.
If you wanna understand
the type of research scientists do on ventilation,
this is a great example.
Let's take a look at it.
- So we studied the bacterial concentration in air.
We pull in a lot of air,
you can think of like a vacuum cleaner with a filter on.
So we pull in a lot of air
and then we have a filter there
where the bacteria gets stuck
and then we put this filter on one of these Petri dishes
and then we can cultivate and count the number of bacteria
that was in that air sample.
- This is so simple, by collecting bacteria from the air,
from different places around the room,
they were able to figure out
which was the cleanest ventilation.
And it turns out the cleanest option
was where the output of the air
was right above the operating table.
This seems pretty intuitive
but now we have science to back that intuition.
And I think it's worth mentioning
that they care about cleanliness
but also about temperature and environmental controls.
- 'Cause they are wearing an apron and then a lead vest
and then another apron and a lot of masks and double gloves,
so it's quite sweaty for the surgeon.
While the nurse standing to the side,
she's just wearing short sleeves and a pair of gloves,
so it's very different.
- Yeah. - Yeah, it was well ventilated
where the surgeon is and the bit less high air flows
in the surrounding.
- I just want to say air flow is chaotic.
Malin told me there's still a lot we don't know,
like for example, how far infectious diseases
can travel in little droplets in various spaces.
So that six feet or two meters social distancing rule,
that's not completely understood
and it's not exact in all spaces and situations.
Airflow gets really complicated, real fast,
which brings me to the entire field of study
that we're talking about here: bioaerosols.
Bioaerosols are tiny particles in the air that are living,
or dead but were living.
For example, you can create bioaerosols
by coughing or sneezing out
the viruses and bacteria in your body
and then scientists in Malins field
study those coughs and sneezes.
In fact, Malin is looking to create the perfect sneeze.
I'll explain.
When you cough or sneeze
you eject this hot, moist, turbulent cloud of air
that can travel up to eight meters in a room.
The cloud, which is warmer than the surrounding air,
is buoyant so it moves upward
and it's also comprised of droplets of various sizes.
So, larger droplets fall to the ground faster,
which means they're less of a risk
for transmission of diseases,
but then smaller droplets can hang around longer in the air.
Interestingly, if you sneezed on the moon,
all of the droplets would immediately fall to the ground
no matter the size,
although you wouldn't be able to breathe after you sneezed,
so that's a problem.
So to study these phenomena here on Earth,
Malin has worked on designing a spray bottle
that accurately simulates
the bubbles that humans produce when they cough or sneeze.
The perfect sneeze, my words, not hers.
One study found that bubbles,
like the ones you produce with your snot,
last longer if they contain bacteria.
Researchers found that bacteria
secrete a substance that stabilizes the bubble membrane,
a lot like soap does
and then because the bubble lasts longer,
the membrane gets thinner and then when it bursts,
it creates a lot more tiny droplets
that can transmit disease.
This is a hugely important find for disease transmission.
It means that by the very chemistry
and dynamics of bacteria, they can last longer in the air
and the same might be true for viruses.
That's crazy.
Here's a fun fact, sort of, about coughing.
A cough can travel as fast as 50 miles an hour
and expel almost 3000 droplets.
Now, you know.
So, how do you combat
the transmission of disease in the air?
Well, one massively important method
is to simply replace the air, ventilate.
There's even a unit of speed for replacing the air,
it's cubic feet per minute.
As it turns out, different types of spaces
have different recommended ventilation standards.
For example, the ventilation system in your house
is supposed to replace the air about every three hours.
Did you know
there are standards for ventilation systems of bank vaults
because, you know, if you get stuck in a bank vault
well, you'll have a lot of other problems,
but at least you'll be able to breathe.
In fact, you're supposed to get the same rate of new air
as you would if you were stuck at the office,
about five cubic feet per minute of new air per person.
That's why they always break in
through the ventilation systems.
In weight rooms, air needs to be exchanged
four times faster than the bank vault
because when you work out you produce more carbon dioxide.
So, all those fans at the gym,
I thought that was just for cooling people down
but it turns out it's to move air around the room, as well.
The real lesson here, though,
is that the next time you're stuck in a bank vault
don't do a high intensity workout
because there might not be enough air flow.
Now, bigger spaces
have more space for air flow and exchange.
So what's the biggest space?
- So, in outdoor you have a huge surrounding volume of air.
You may also have some air turbulence and some air flow
and some wind may be going past
and that will dilute the aerosol concentration
very efficiently and very fast.
- Knowing that space size affects ventilation
we can revisit our original question,
whether disease transmission
is particularly worsened on an airplane?
You're in an enclosed space, you're packed like sardines.
Think back to that 1977 study
where 72% of passengers were infected,
well, the ventilation system was off.
when it's running, studies found that outbreaks on flights
for a recent type of flu, the H1N1,
spread to just 1 to 5% of passengers.
But it turns out
airplanes have very active ventilation systems.
Within two to three minutes
they've entirely replaced the air in the cabin
with fresh, albeit dry air.
Airplane systems are continually pumping out
the air that you coughed into, your neighbor sneezed into,
your other neighbors' drool evaporated into,
and they're replacing it with new, filtered, outside air.
That's why it's so important to keep the air flowing.
Although to complicate things,
it matters where you take your air in from.
For example, if the air intake for your ventilation
is on the ground,
you might suck up more particles, more dust and bacteria,
so that's another active area of study for bioaerosols.
And the final lesson is that good ventilation
cannot make a space completely safe.
Malin pointed me to a paper
that showed people in aisle seats
had a higher risk of infection than anyone else
because people walking back from the bathroom
like to touch the backs of seats.
That's why disinfecting surfaces and not touching your face
is so important.
Don't touch your face unless you want a case of the flu.
There are a few more factors
in the transmission of COVID-19 I was curious about
and thought you all might wanna know.
Here's one,
how long does SARS-CoV-2 stick around on various surfaces?
A study published in March
found that the virus stays intact on plastic the longest
and copper the shortest.
The study found
that there were still detectable virus particles
after three days on plastic and stainless steel,
for cardboard, no viable viruses found after 24 hours,
and for copper, no viruses existed after just four hours,
so copper has antiviral properties.
And the last piece of this whole puzzle
that was really interesting to me
had to do with how many virus particles you need
to get infected.
For example, if you were exposed to HIV,
you wouldn't get infected with just one virus particle.
You would need a certain number of virus particles
and that number is the so-called infective dose,
which is defined as, "The lowest number of virus particles
that will cause an infection to 50% of people in a group
if you give them that number of particles."
So for example, for certain strains of HIV
that infective dose is about a thousand particles.
So, what's the effect of dose for SARS-CoV-2?
Well, scientists still lack enough data to say
but based on their understanding of SARS,
which is a similar coronavirus,
one expert says the infective dose should be on the order of
a few hundred or thousand Coronavirus particles
to develop symptoms.
So, we started this video with air ventilation
to learn how diseases spread,
but it turns out that's the tip of the iceberg.
I hope you learned something new though 'cause I know I did.
Thank you so much for watching.
Happy physicking, stay safe
and be careful riding mountain bikes.
Hey, hey, hi, you're still here
which means I get to tell you
all about this big announcement that I have.
I'm gonna be a physics teacher,
at least right here on YouTube.
I'm making a course, it's like a physics 101 course.
It's based off the AP Physics 1 curriculum,
so if you're taking the AP Physics 1 class
and you're gonna take the test, it's a great review
and it's also gonna just be a lot of fun.
It's me deep diving into all the phenomena
that got me so hooked on this subject.
All the lessons are gonna be freely available
right here on my channel, so keep an eye out.
The first lesson and the trailer
is gonna drop within the next couple of weeks
and then the rest of the lessons
will be released throughout 2020.
So, I'm very excited to help you all learn from home
so stay tuned, "Physics 101 with Diana" coming soon.
(encouraging music)