Masks and other protections | COVID-19 Aerosol Transmission FAQs (7/9)

7.1. Do masks work to reduce the aerosol spread of COVID-19?

Yes! The physics are well understood. If a porous obstacle is put in the path of air that contains aerosols, some of the aerosols will end up in the obstacle. See the figure to the right from Science.

This video presents a good overview of filtration mechanisms of masks. The electrostatic mechanism discussed in the video is specific to N95 and surgical masks, but everything else is applicable to all masks.

Masks reduce airborne transmission: Infectious particles can be released during breathing and speaking by asymptomatic infected individuals. No masking maximizes exposure, wheras universal masking results in the least exposure.

Unfortunately there are many many misconceptions about masks, including:

  • “Masks don’t work because the virus is much smaller than the pores of the mask.” First, the virus is not thought to be “naked” in the air, this is a misconception. Masks are just filters that we wear, so see this discussion below of filtering efficiency vs. size and why much of what you may hear online is wrong. Second, aerosol filtration does not work in the same way of a sieve that we may be more familiar with. Filters can capture particles that are much smaller than the pore size, see the Minute Physics video for a good explanation.
  • “Masks only protect against ballistic droplets, not aerosols.” Again, this is another misconception. Masks always provide some partial protection against exhaled and inhaled aerosols, with the protection depending on the quality of the mask material, how well they fit (no gaps between mask and face), and the size of the aerosols that matter.
  • Some videos are circulating showing someone exhaling vaping aerosols, and concluding that masks don’t work based on that. This is wrong, as we discussed in this previous FAQ.

7.2. What is the best type of mask?

The best type of mask is one that fits well with no gaps and that is made of multiple layers of densely woven fabric. Masks with pockets that can hold HEPA filters should be especially effective. It should be worn over the nose and mouth at all times when you are indoors with other people, except in your own residence or vehicle.

Prof. John Volckens at Colorado State Univ. has made public a database with the results of their testing. here.

7.3. How effective are different types of masks for the wearer and for others?

The effectiveness of a mask depends on two main factors: how well it fits and how well the material filters out aerosol particles of different size. The mask should completely cover the nose and mouth and should be pulled tightly against the face around the edges. There should be no gaps. A properly-fitted N95 respirator filters out at least 95% of aerosols of all sizes for the wearer. It is expected to work well to protect others, too. N95 respirators and other types of masks with valves allow aerosols to escape and do not protect others, so you should not use these; or if you do, put a piece of tape over the valve. Surgical masks can offer protection in the range of 70-80%, although there is wide variability among different brands. Studies have found that homemade masks made out of tea cloth or cotton t-shirt material offer about 50% protection. Unpublished studies by Linsey Marr, John Volckens, and Carl Wang indicate that common fabrics have low filtration efficiencies for 0.3-μm particles but block about 50% of 2-μm particles and 80% or more of 5-μm and larger particles. Virus is found in aerosols of all sizes, but those larger than 1 μm are probably most important for transmission. Materials such as vacuum bags, HEPA filters, and MERV 13 filters perform better. Furthermore, results from Linsey Marr’s lab indicate that homemade masks, tested on a manikin, offer similar or slightly lower protection for the wearer (inward flow) than for others (outward flow).

7.4. Do I need to wear a mask outside?

When you regularly are passing by others at close distance (urban sidewalk) you should wear a mask outside. In situations where others are encountered outdoors infrequently it is good etiquette to either ensure adequate space between you and the other person, or wear a mask during the encounter.

7.5. Is it OK to just wear the mask over my mouth and leave my nose out?

No, the mask should cover your nose to block aerosols coming out of it and block any that you might breathe in from the air around you.

7.6. Is the fit of a mask important?

Yes, mask fit is very important to protect against aerosols. If there are gaps, a substantial fraction of the air will flow through there. See examples in the figure below (stills from this great video). Some of the masks do not fit well, on purpose to illustrate what to avoid. The best fitting mask is the cloth mask on the right, and it shows the least leakage. If your mask fits well, you should feel the material suck up against your nose and mouth when you are breathing in. Many people wear poorly fitting masks, which significantly reduces their filtering efficiency. Perhaps this is partially left over from the initial introduction of masks to protect against ballistic droplets, which only need a “parapet” in between the two people, and where fit is much less important.


Figure: stills from mask visualization video showing leaks around gaps, which are major for the mask on the left, small for the mask in the middle, and not visible for the mask on the right.

7.7. Where should I stand around someone with a poorly fitting mask?

In this case you should avoid being behind and near the person. As the figure above shows, the curvature of the masks direct any exhaled aerosols in that direction.

7.8. Is it ok to remove my mask to talk?

No, unless you are outdoors at a sufficient distance. Speaking results in about x10 times more respiratory aerosol emission than just breathing, and singing (at high volume) or yelling can be about x50 times more. Ballistic droplets are not exhaled at all when breathing. You should also not allow others to talk to you without a mask from a close distance.

7.9. But I have seen some video online that shows vaping aerosols going through a mask. Doesn’t this show that masks don’t work?

In a word, no. Those videos make several errors and misconceptions, as discussed at the smoke FAQ above.

7.10. Are transparent masks safe?

It depends on the specific mask. The most important feature is that it must seal well around the face. If there are gaps, then it is not a useful mask. Then of course it has to filter aerosols well through the material through which the air will pass, and it should not fog, which are specific to each mask. Below are examples of masks that appear (from the pictures) to have gaps (left) or not (right). We haven’t tested these masks and do not endorse any in particular, just apply these principles to any transparent masks that you are considering.


Figure: examples of clear masks that appear to have visible gaps (left) vs not (right). We do not endorse any specific mask as we have not tested them, see text for criteria to identify potentially useful transparent masks.

7.11. Is there an easy way to assess my mask at home?

Bill Nye the Science Guy suggests the candle test in this video starting at 1:30. This is a good indicator of how well a mask works, although it isn’t perfect. If you want to test your mask like they do at hospitals and you have a medical nebulizer, you could set up a fit test, like the one described by OSHA using saccharin (Sweet’N Low) aerosols. You’ll need a cardboard box that your head can fit inside, a packet of Sweet’N Low, and a medical nebulizer. Make a small hole in the box right in front of your nose and mouth for the nebulizer tubing. Dissolve 1 packet of Sweet’N Low in ½ cup of warm water. Put this solution into the nebulizer, run the nebulizer into the box, put your head in the box and breathe with your mouth slightly open with tongue extended. It should taste sweet. Turn off the nebulizer and wait for 15 minutes. Repeat the test with your mask on. You should not taste the saccharin.

7.12. Do I need eye protection?

The eyes are thought to be a possible, but not major route of transmission for aerosols, because much more air is drawn into our lungs than may pass by our eyes. If someone was to cough or sneeze towards you, the risk from ballistic droplets impacting the eyes is significant. Some kind of eye protection can be useful to protect your eyes from ballistic droplets and aerosols, and also to keep you from sticking your fingers in your eyes. Some experts think this is useful, others think that it may be overkill, except in more crowded or risky environments. Regular glasses will be protective against ballistic droplets, while some kind of safety glasses that are more closed around the eyes, as in the figure below, is better for aerosols.

Figure: example of closed safety glasses to provide some protection against aerosol deposition on the eyes when needed. This particular model costs $8.

| --- | --- | | | Figure: example of closed safety glasses to provide some protection against aerosol deposition on the eyes when needed. This particular model costs $8. |

7.13. Are face shields and masks interchangeable?

No, face shields do not offer much protection against aerosols (also see this video), while masks do. Face shields are good for blocking ballistic droplets released by the wearer or that might fly into the wearer's face when close to others. Face shields are considered a supplement to masks for partial eye protection (but less useful than closed glasses, as discussed above), but not a substitute for them.

7.14. Are plexiglass barriers helpful?

Plexiglass barriers can be helpful when people are close, such as at the checkout of a supermarket, to block ballistic droplets and break the flow of aerosols in close proximity. In other situations, however, they may interfere with proper ventilation and their effectiveness may be affected by air flow patterns caused by HVAC systems.

Next: 8. Ventilation

Previous: 6. Music

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Published with authorization to use copy "as you see fit" with attribution as specified., Version: 1.60, 14-Sep-2020

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