Face Masks: Much More Than You
Wanted To Know
There’s been recent controversy about the use of face
masks for protection against coronavirus. Mainstream
sources, including the
CDC and
most
of the media say masks are likely useless and not
recommended. They’ve recently been challenged, for
example by Professor
Zeynep Tufekci in the New York Times and by Jim
and Elizabeth on Less Wrong. There was also some
debate in the comment
section here last week, so I promised I’d look into it in
more depth.
As far as I can
tell, both sides agree on some points.
They agree that N95
respirators, when properly used by trained
professionals, help prevent the wearer from getting
infected.
They agree that
surgical masks help prevent sick people from
infecting others. Since many sick people don’t know
they are sick, in an ideal world with unlimited mask
supplies everyone would wear surgical masks just to
prevent themselves from spreading disease.
They also agree
that there’s currently a shortage of both surgical
masks and respirators, so for altruistic reasons
people should avoid hoarding them and give
healthcare workers first dibs.
But they disagree
on whether surgical masks alone help prevent the
wearer from becoming infected, which will be the
focus of the rest of this piece.
1. What are the
theoretical reasons why surgical masks might or
might not work?
Epidemiologists
used to sort disease transmission into three
categories: contact, droplet, and airborne. Contact means you only get a
disease by touching a victim. This could be
literally touching them, or a euphemism for very
explicit contact like kissing or sex. Droplet means you get a disease
when a victim expels disease-laden particles into
your face, usually through coughing, sneezing, or
talking. Airborne means you get a disease
because it floats in the air and you breathe it in.
Transmission via “fomites”, objects like doorknobs
and tables that a victim has touched and left their
germs on, is a bonus transmission route that can
accompany any of these other methods.
More recently,
scientists have realized that droplet and airborne
transmission exist along more of a spectrum.
Droplets can stay in the air for more or less time,
and spread through more or less volume of space
before settling on the ground. The term for this new
droplet-airborne spectrum idea is “aerosol
transmission”. Diseases with aerosol transmission
may be spread primarily through droplets, but can
get inhaled along with the air too. This concept is
controversial, with different
authorities having different opinions over which
viruses can be aerosolized. It looks like most
people now believe aerosol transmission is real and
applicable to conditions like influenza, SARS, and
coronavirus.
Surgical masks are
loose pieces of fabric placed in front of the mouth
and nose. They offer very good protection against
outgoing droplets (e.g.
if you sneeze, you won’t infect other people), and
offer some protection against
incoming droplets (e.g.
if someone else sneezes, it doesn’t go straight into
your nose). They’re not airtight,
so they offer no protection against airborne
disease or the airborne component of aerosol
diseases.
Respirators are
tight pieces of fabric that form a seal around your
mouth and nose. They have various “ratings”; N95 is
the most common, and I’ll be using “N95
respirator” and “respirator”
interchangeably through most of this post even
though that’s not quite correct. When
used correctly, they theoretically
offer protection against incoming and outgoing
droplet and
airborne diseases; since aerosol diseases
are a combination of these, they offer generalized
protection against those too. Hospitals hate the new
“aerosol transmission” idea, because it means they
probably have to switch from easy/cheap/comfortable
surgical masks to hard/expensive/uncomfortable
respirators for a lot more diseases.
Theory
alone tells us surgical masks should not provide
complete protection. Coronavirus has
aerosol transmission, so it is partly airborne.
Since surgical masks cannot prevent inhalation of
airborne particles, they shouldn’t offer 100% safety
against coronavirus. But theory doesn’t tell us
whether they might not offer 99% safety against
coronavirus, and that would still be pretty good.
2. Are people
who wear surgical masks less likely to get
infected during epidemics?
It’s unethical to
randomize people to wear vs. not-wear masks during a
pandemic, so nobody has done this. Instead we have
case-control studies. After the pandemic is over,
scientists look at the health care workers who did
vs. didn’t get infected, and see whether the
infected people were less likely to wear masks. If
so, that suggests maybe the masks helped.
This is an
especially bad study design, for two
reasons. First, it usually suffers recall bias – if
someone wore a mask inconsistently, then they’re
more likely to summarize this as “didn’t wear masks”
if they got infected, and more likely to summarize
it as “did wear masks” if they stayed safe. Second,
probably some nurses are responsible and do
everything right, and other nurses are irresponsible
and do everything wrong, and that means that if
anything at all helps (eg washing your hands), then
it will look like masks working, since the nurses
who washed their hands are more likely to have worn
masks. Still, these studies are the best we can do.
Gralton
& McLaws, 2010 reviews several studies
of this type, mostly from the SARS epidemic of the
early 2000s. A few are underpowered and find that
neither surgical masks nor respirators prevent
infection (probably not true). A few others show
respirators prevent infection, but do not
investigate surgical masks (probably right, but
useless for our purposes). Two seem relevant to the
question of whether surgical masks work:
Rapid
awareness and transmission of SARS in Hanoi French
Hospital, Vietnam was conducted in a poor
hospital that only had surgical masks, not
respirators. In the latter stages of the epidemic, 4
workers got sick and 26 stayed healthy. It found
that 3 of the 4 sick workers hadn’t been wearing
masks, but only 1 of the 26 healthy workers hadn’t.
This is a pretty dramatic result – subject to the
above confounders, of course.
Effectiveness
of precautions against droplets and contact in
prevention of nosocomial transmission of SARS is larger and more
prestigious, and looked at a cluster of five
hospitals. Staff in these hospitals used a variety
of mask types, including jury-rigged paper masks
that no serious authority expects to work, surgical
masks, and N95 respirators. It found that 7% of
paper-mask-wearers got infected, compared to 0% of
surgical-mask and respirator wearers. This seems to
suggest that surgical masks are pretty good.
The meta-analysis
itself avoided drawing any conclusions at all, and
would not even admit that N95 respirators worked. It
just said that more research was needed. Still, the
two studies at least give us a little bit of
evidence in surgical masks’ favor.
How concerned
should we be that these studies looked at health
care workers specifically? On the one hand, health
care workers are ordinary humans, so what works for
them should work for anyone else. On the other,
health care workers may have more practice using
these masks, or may face different kinds of
situations than other people. Unlike respirators,
surgical masks don’t seem particularly hard to use,
so I’m not sure health care workers’ training really
gives them an advantage here. Overall I think this
provides some evidence that surgical masks are
helpful.
I was able to find
one study like this outside of the health care
setting. Some people with swine flu traveled on a
plane from New York to China, and many fellow
passengers got infected. Some researchers looked
at whether passengers who wore masks throughout the
flight stayed healthier. The answer was very much
yes. They were able to track down 9 people who got
sick on the flight and 32 who didn’t. 0% of the sick
passengers wore masks, compared to 47% of the
healthy passengers. Another way to look at that is
that 0% of mask-wearers got sick, but 35% of
non-wearers did. This was a significant difference,
and of obvious applicability to the current
question.
3. Do surgical
masks underperform respirators in randomized
trials?
Usually it would be
unethical to randomize health care workers to no
protection, so several studies randomize them to
face masks vs. respirators. But a few others were
done in foreign hospitals where lack of protection
was the norm, and these studies did include a
no-protection control group.
MacIntyre
& Chugtai 2015, Facemasks For The
Prevention Of Infection In Healthcare And Community
Settings, reviews four of these. Two of the four are
unable to find any benefit of either masks or
respirators. The third finds a benefit of
respirators, but only if nobody tested the
respirators to see if they fit, which doesn’t make
sense and suggests it’s probably an artifact. The
fourth finds a benefit of respirators, but not
masks. It seems unlikely that respirators don’t
help, so this suggests all these studies were
underpowered.
In other words,
respirators are better than masks are better than
nothing. It would be wrong to genuinely conclude
this, because it’s not statistically significant.
But it would also be wrong to conclude the studies
show masks don’t work, because they mostly show
respirators don’t work, and we (hopefully) know they
do.
Overall these
studies don’t seem very helpful and I’m reluctant to
conclude anything from them. In section 6, I’ll talk
more about why studies may not have shown any
advantage for respirators.
4. Do surgical
masks prevent ordinary people from getting
infected outside the healthcare setting?
The same review
lists nine randomized trials with a different
design: when the doctor diagnoses you with flu, she
either asks everyone in your family to wear masks
(experimental group), or doesn’t do that (control
group), and then checks how many family members in
each group got the flu.
How did these go?
That depends whether you use intention-to-treat or
per-protocol analysis. Intention-to-treat means that
you just compare number of infections in the
assigned-to-wear-masks group vs. the control group.
Per-protocol means that you only count someone in
the study if they actually followed directions. So
if someone in the assigned-to-wear mask group didn’t
wear their mask, you remove them from the study; if
someone in the control group went rogue and did wear a mask, you remove
them too.
Both of these
methods have their pros and cons. Per protocol is
good because if you’re trying to determine the
effect of wearing a mask, you would really prefer to
only be looking at subjects who actually wore a
mask. But it has a problem: adherence to protocol is
nonrandom. The people who follow your instructions
diligently are selected for being diligent people.
Maybe they also diligently wash their hands, and
diligently practice social distancing. So once you
go per protocol, you’re no longer a perfect
randomized controlled trial. Only intention-to-treat
analyses carry the full weight of a gold standard
RCT.
According
to intention-to-treat, the studies unanimously
found masks to be useless. But there were
a lot of signs that intention-to-treat wasn’t the
right choice here. Only about a fifth of people who
were asked to wear masks did so with any level of
consistency. The rest wore the mask for a few hours
and then get bored and took it off. Honestly, it’s
hard to blame them; these studies asked a lot from
families. If a husband has flu, and sleeps in the
same bed as his wife, are they both wearing masks
all night?
Of the three
studies that added per-protocol analyses, all three
found masks to be useful (1, 2, 3) . Does this prove
masks work? Not 100%; per-protocol analyses are
inherently confounded. But it sure is suggestive.
The review author
summarizes:
The routine use
of facemasks is not recommended by WHO, the CDC,
or the ECDC in the community setting. However, the use of
facemasks is recommended in crowded settings
(such as public transport) and for those at high
risk (older people, pregnant women, and those
with a medical condition) during an outbreak or
pandemic. A modelling study suggests that the use
of face-masks in the community may help delay and
contain a pandemic, although efficacy estimates
were not based on RCT data. Community masks were
protective during the SARS outbreaks, and about
76% of the population used a facemask in Hong
Kong.
There is
evidence that masks have efficacy in the
community setting, subject to
compliance [13] and early use [12, 18, 19]. It has
been shown that compliance in the household
setting decreases with each day of mask use,
however, which makes long term use over weeks or
months a challenge […]
Community RCTs
suggest that facemasks provide protection
against infection in various community settings, subject to
compliance and early use. For health-care workers,
the evidence suggests that respirators offer
superior protection to facemasks.
Parts of this
summary are infuriating. If the big organizations
recommend that especially vulnerable groups wear
masks, aren’t they admitting masks work? But if
they’re admitting masks work, why don’t they
recommend them for ordinary people?
It
looks like they’re saying masks work a little,
they’re too annoying for it to be worth it for
normal people, but they might be worth it for the
especially vulnerable. But then why don’t
they just say masks work, and let each person decide
how much annoyance is worthwhile? I’m not sure. But
it looks like the author basically ends up in favor
of community use of surgical masks in a pandemic,
mostly on the basis of per-protocol analyses of
community RCTs.
5. How do
surgical masks and respirators compare in hokey
lab studies?
Our source here is
Smith et al 2016, Effectiveness Of N95
Respirators Versus Surgical Masks In Protecting
Health Care Workers From Acute Respiratory
Infection: A Systematic Review And Meta-Analysis.
They review some of the same studies we looked at
earlier, but then investigate 23 “surrogate exposure
studies”, ie throwing virus-shaped particles at
different masks in a lab and seeing if they got
through. You can find the results of each in their
appendix. Typically, about 1 – 5%
of particles make it through the respirator, and
10 – 50% make it through the surgical mask.
They summarize this as:
In general,
compared with surgical masks, N95 respirators
showed less filter penetration, less face-seal
leakage and less total inward leakage under the
laboratory experimental conditions described.
I think in general
the fewer virus particles get through your mask, the
better, so I think this endorses surgical masks as
better than nothing, since their failure rate was
less than 100%.
Booth
et al,
2013
examines surgical masks themselves more closely.
They hook a surgical mask up to “a breathing
simulator” and then squirt real influenza virus at
it, finding that:
Live influenza
virus was measurable from the air behind all
surgical masks tested. The data indicate that a
surgical mask will reduce exposure to aerosolised
infectious influenza virus; reductions ranged from
1.1- to 55-fold (average 6-fold), depending on the
design of the mask…the results demonstrated
limitations of surgical masks in this context,
although they are to some extent protective.
The paper doesn’t
discuss how particle number maps to infection risk.
Does letting a single influenza virus through mean
you will get infected? If so, any reduction short of
100% is useless. I have a vague sense that this
isn’t true; your immune system can fight off most
viruses, and the fewer you get, the better the
chance it will win. Also, even respirators don’t
claim to reduce particle load by more than 99% or
so, and those work, so it can’t be that literally a
single virus will get you. Overall I think modest
reductions in particle number are still pretty good,
but I don’t have a study that proves it.
6. Is it true
that the public won’t be able to use N95
respirators correctly?
Yes.
I remember my
respirator training, the last time I worked in a
hospital. They gave the standard two minute
explanation, made you put the respirator on, and
then made you go underneath a hood where they
squirted some aerosolized sugar solution. If you
could smell the sugar, your respirator was leaky and
you failed. I tried so hard and I failed so many times. It was embarrassing
and I hated it.
I’m naturally
clumsy and always bad at that kind of thing. Some
people were able to listen to the two minute
explanation and then pass right away. Those kinds of
people could probably also listen to a two minute
YouTube explanation and be fine. So I don’t want to
claim it’s impossible or requires lots of
specialized background knowledge. It’s just a
slightly difficult physical skill you have to get
right.
Bunyan
et al, 2013, Respiratory And Facial Protection: A
Critical Review Of Recent Literature, discusses this
in more depth. They review some of the same studies
we reviewed earlier, showing no benefit of N95
respirators over surgical masks for health care
workers in most situations. This doesn’t make much
theoretical sense – the respirators should win hands
down.
The most likely
explanation is: doctors aren’t much better at using
respirators than anyone else. In a
California study of tuberculosis
precautions, 65% of health care workers used their
respirators incorrectly. That’s little better than the
general public, who have a 76% failure rate. Bunyan et al
note:
The
fitting of N95 respirators has been the subject
of many publications. The effective functioning
of N95 respirators requires a seal between the
mask and the face of the wearer. Variation
in face size and shape and different respirator
designs mean that a proper fit is only possible in
a minority of health care workers for any
particular mask. Winter et al. reported that, for
any one of three widely used respirators, a
satisfactory fit could be achieved by fewer than
half of the healthcare workers tested, and for 28%
of the participants none of the masks gave a
satisfactory fit.
Fit-testing is
a laborious task, taking around 30 min to do
properly, and comprises qualitative fit-testing
(testing whether the respirator-wearing healthcare
worker can taste an intensely bitter or sweet
substance sprayed into the ambient air around the
outside of the mask) or quantitative fit testing
(measuring the ratio of particles in the air
inside and outside the breathing zone when wearing
the respirator). Attempts have been made to
circumvent the requirement for fit testing, and it
has been suggested that self-testing for a seal by
the respirator wearer (see
http://youtu.be/pGXiUyAoEd8a for a video
demonstration) is a sufficient substitute for
fit-testing. However, self-checking for a
seal has been demonstrated to be a highly
unreliable technique in two separate
studies so that full fit-testing remains a
necessary preliminary requirement before
respirators can be used in the healthcare setting.
Operationally,
this presents significant challenges to
organizations with many healthcare workers who
require fit-testing. Chakladar et al.
pointed out that, in addition to the routine need
for repeat testing over time to ensure that
changes in weight or facial hair have not
compromised a good fit, movements of healthcare
workers between organizations using different
makes of respirators would necessitate additional
repeat fit-testing. Fit-testing is likely to
remain problematic to health-care organizations
for the foreseeable future. In addition to the
requirement for fit-testing, ‘fit-checking’ is
also required each time the respirator is donned
to ensure there are no air leaks.
Is
a poorly-fitting N95 respirator better than
nothing? The reviewed studies suggest that at that
point it’s just a very fancy and expensive
surgical mask.
7. Were the CDC
recommendations intentionally deceptive?
No, and I owe them
an apology here.
I think the
evidence above suggests masks can be helpful. Masked
health care workers were less likely to catch
disease than unmasked ones. Masked travelers on
planes were less likely to catch disease than
unmasked ones. In per protocol analysis, masked
family members are less likely to catch disease from
an index patient than unmasked ones. Laboratory
studies confirm that masks block most particles. All
of this accords with a commonsense understanding of
droplet and aerosol transmission of disease.
None of these,
except maybe the plane study, tell us exactly what
we want to know. The SARS studies were all done in a
health care setting, so they don’t
prove that regular people can benefit from masks.
But health care workers are closely related to homo sapiens and ought to have
similar anatomy and physiology. Surgical masks
aren’t as complicated as respirators and we can
assume most people get them right. And although
health care workers are in unusually high-risk
situations, that should just affect the magnitude of
the benefit, not the sign; obviously the level of
risk ordinary people encounter is sometimes
relevant, considering they do often catch pandemic
diseases. So our default assumption should be that
these studies carry over, not that they don’t.
Likewise, most of
the community studies were done on family members.
Most guidelines already say to mask up if you have a
sick family member, so talking about subways and
crowds requires a little bit of extrapolation. But
again, being in a family is just one form of close
contact. It would take bizarre convolutions to even
imagine a theory where you can catch diseases from
your family members but not from people you sit next
to on a train. Our default assumption should be in
favor of these results generalizing, not against
them.
But
the CDC has recommended against mask use. I hypothesized that the CDC was
intentionally lying to us, trying to trick us into
not buying masks so there would be enough for
health care workers.
But
that can’t be true, because the CDC and other
experts came up with their no-masks policy years
ago, long before there was any supply shortage.
And during the 2015 MERS epidemic, NPR said
South Koreans were wrong to wear masks:
Masks can be
helpful for protecting health workers from a
variety of infectious diseases, including MERS…
But either type
of mask is less likely to do much good for the
average person on the street…Wearing a mask
might make people feel better. After all, MERS
has killed about a third of the people known to
be infected.
But there are
no good studies looking at how well these masks
prevent MERS transmission out in the community,
says Geeta Sood, an infectious disease
specialist at Johns Hopkins University. “On the
street or the subway, for MERS specifically,
they’re probably not effective,” she says. One
problem is that the masks are loose fitting, and
a lot of tiny airborne particles can get in
around the sides of the masks.
So if studies
generally suggest masks are effective, and the CDC
wasn’t deliberately lying to us, why are they recommending
against mask use?
I’m not sure. I
haven’t been able to track down any documents
where they discuss the reasons behind their
policies. It’s possible they found different
studies than I did, or interpreted the studies
differently, or have some other superior
knowledge.
But I think that
more likely, they’re trying to do something
different with medical communication. Consider
legal communication. If a court declares a suspect
is “not guilty”, that could mean that he is
actually not guilty of the crime. Or it could mean
that he did it but they can’t prove it. Or it
could mean that he did it, they can prove it, but the
police officer who found the proof didn’t have a
warrant at the time so they had to throw it out. A
legal communication like “this man is not guilty”
is intended not just to convey information, but to
formally reflect the output of a sacrosanct
process.
Medicine has been
traumatized by its century-long war with quackery,
and ended up with its jargon also formally
reflecting the output of a sancrosanct process.
Remember, there are dozens
of studies supposedly showing homeopathy works, not to mention even
more studies proving telepathy exists. At some point you
have to redesign all your institutions to operate
in an environment of epistemic
learned helplessness, and the result is
very high standards of proof.
Masks haven’t
quite reached these standards. The case-control
trials look good, and the per-protocol RCTs look
good, but there aren’t really the large-scale
intention-to-treat RCTs that would be absolutely
perfect. Even if these studies work, they only
prove things about the health care setting and the
family setting, not “the community setting” in
general. So masks haven’t been
proven to work beyond a reasonable doubt.
Just like the legal term for “not proven guilty
beyond a reasonable doubt” is “not guilty”, the
medical communication term for “not proven
effective beyond a reasonable doubt” is “not
effective”. This already muddled communication
gets even worse because doctors are
constitutionally incapable of distinguishing “no evidence for”
from “there is evidence against” – I have no
explanation for this one.
There’s an even
more complicated language-use issue. The CDC may
be thinking of its recommendations not just as
conveying an opinion but as taking an action –
performing the medical intervention of
recommending people wear masks. All of those RCTs
listed above show that the medical intervention of
recommending people wear masks is ineffective.
Sure, that’s because people don’t listen. But the
CDC doesn’t care about that. They’ve proven that
giving the advice won’t help, why are you still
asking them to give the advice?
I’m not sure this
is really the CDC’s reasoning. It seems pretty
weird from the point of view of an organization
trying to manage a real-world pandemic with people
dying if they get it wrong. But I’m having trouble
figuring out other possibilities that make sense.
8. So should
you wear a mask?
Please don’t buy
up masks while there is a shortage and healthcare
workers don’t have enough.
If the shortage
ends, and wearing a mask is cost-free, I agree
with the
guidelines from China, Hong Kong, and Japan – consider wearing a
mask in high-risk situations like subways or
crowded buildings. Wearing masks
will not make you invincible, and if you risk
compensate even
a little it might do more harm than good.
Realistically you should be avoiding high-risk
situations like subways and crowded buildings as
much as you possibly can. But if you have to go in
them, yes, most likely a mask will help.
In low-risk
situations, like being at home or taking a walk, I
mean sure, a mask might make you 0.0001% (or
whatever) less likely to get infected. If that’s
worth it to you, consider the possibility that you
might be freaking out a little too much about this
whole pandemic thing. If it’s still worth it, go
for it.
You
are unlikely to be able to figure out how to use
an N95 respirator correctly. I’m not saying it’s
impossible, if you try really hard, but assume
you’re going to fail unless you have some reason
to think otherwise. The most likely outcome is
that you have an overpriced surgical mask that
might make you incorrectly risk-compensate.
If
you are a surgeon performing surgery, bad news.
It turns out surgical masks are not very useful
for you (1, 2)! You should avoid
buying them, since doing so may deplete the
number available for people who want to wear
them on the subway. (read
more)