Who is that masked woman?

Masked woman
Who is that masked woman? It is the author's wife wearing a home-made mask with Cottonwood Marsh in the background. We did not wear masks during our 1-hour or so walk around Walden Ponds in Boulder, but people were extremely careful to stay 2 or more meters apart.

The White House has scooped me: White House to urge Americans to wear face coverings in public to slow spread of coronavirus, but at least they have relieved me of the responsibility to think of a good lead. At any rate, I think I will post what I was about to post anyway. So: I have been doing a little research on the value of wearing a mask, particularly a cloth mask, using reputable technical journals such as The Washington Post and The New York Times, not to mention Science and Nature.

What, then, is the value of wearing a mask?

• It may protect me.

• It may protect you.

• It may keep me from touching my face with my hands.

What is the value of wearing a home-made cloth mask?

• All of the above, as well as

• It will not deprive health-care workers of N95 masks.

To brass tacks! The following is an annotated list of some of the material I have read during the last week or so.

Should I make my own mask?, by Tara Parker-Pope, New York Times, March 31, 2020. Ms. Parker-Pope, a founding editor of the Times’s Well section, comes down, perhaps a little unenthusiastically, on the side of wearing cloth masks when in close quarters in public.

Would everyone wearing face masks help us slow the pandemic?, by Kelly Servick, Science, March 28, 2020. Ms. Servick, a staff writer at Science, notes that the authorities in Asia have encouraged people to wear masks, whether they are ill or not. Mostly, she gives what I would call informed opinions from health-care experts, and she notes that some studies have had very small sample sizes and incomplete compliance by the volunteers. She draws no conclusion but notes that one expert expects masks to become important when social distancing is relaxed and people crowd together again.

Simple DIY masks could help flatten the curve. We should all wear them in public, by Jeremy Howard, University of San Francisco, The Washington Post, March 28, 2020. Dr. Howard, a distinguished research scientist at USF, has found 34 scientific papers that support the contention that simple masks can reduce the transmission of viruses from person to person and none that claim otherwise. He provides additional evidence, some of it anecdotal, as well as a study (which I did not read) showing that people who wear masks on airplanes neither transmit nor receive infections.

Rational use of face masks in the COVID-19 pandemic, Shuo Feng et al., Oxford Vaccine Group, University of Oxford, The Lancet, March 20, 2020. Short, somewhat equivocal survey of mask use in Asian vs. Western countries. They suggest, however, that people in quarantine and vulnerable populations should wear face masks if they have to leave home. They call for research on the use of face masks and on measures to prolong the life of disposable masks.

Untitled hour-long talk, by David Price, Weill Cornell Medical Center, March 22, 2020. Dr. Price is a specialist in pulmonary disease and critical care medicine. He discusses wearing masks from approximately 9:30 to 15:30 in the recording. In that segment, he insists that (excluding a hospital setting) Covid-19 is transmitted entirely by contact and claims that, if you apply hand sanitizer every time you touch an elevator button or a door, you will not get Covid-19. He nevertheless recommends wearing a mask to keep yourself from touching your face and stresses that you do not need an N95 mask. If he is correct, you need not read any of the following references.

Testing the efficacy of homemade masks: would they protect in an influenza pandemic?, by Anna Davies, Department of Veterinary Medicine, University of Cambridge, et al., May 22, 2013. This group compared home-made masks made of cotton T-shirt material with surgical masks (they found that vacuum-cleaner bags and tea towels were not porous enough to be used in masks). Depending on how you look at it, the surgical masks were 3 times better than a home-made mask, or the home-made masks provided 1/3 as much protection as a surgical mask. The authors recommend that home-made masks be considered only as a last resort for protection against aerosol exposure from infected individuals.

A cluster randomised trial of cloth masks compared with medical masks in healthcare workers, C. Raina MacIntyre, School of Public Health and Community Medicine, University of New South Wales, Sydney, Australia, et al., April, 2015. This study compared cloth masks with medical masks and with “standard practice” in hospital wards. The cloth masks provided the least protection, but the comparison was not entirely fair in that “standard practice” means sometimes using medical masks and sometimes not. Further, the mask wearers were always in close contact with sick people. I therefore do not consider this study to be relevant, though it has been mentioned as evidence that cloth masks are not useful. (See also the authors’ comment COVID-19, shortages of masks and the use of cloth masks as a last resort, March 30, 2020, in which they mostly opt against health workers’ using cloth masks. They also dispense with my criticism above.)

Professional and home-made face masks reduce exposure to respiratory infections among the general population, Marianne van der Sande, National Institute for Public Health and the Environment, Netherlands, et al., Plos1, July 9, 2008. The authors studied N95 masks, surgical masks, and home-made masks, using both a mechanical head and human volunteers. They found that the home-made masks conferred what they called “a significant degree of protection,” though not as much as surgical masks or N95 masks. Oddly, by comparing the mechanical head with human volunteers, they found that the mask afforded better protection to the wearer than to others (which, I think, is contrary to what I have read elsewhere).

Covid-19: Why we should all wear masks — there is new scientific rationale, by Sui Huang, Institute for Systems Biology, Medium, Mar 26, 2020. The author argues that coughing or sneezing can project large droplets up to 6 m. He makes the case that large droplets are important and therefore that cloth masks may be effective.

Which DIY mask pattern should you use? Even experts can’t pick one to recommend, by Sindya Bhanoo, The Washington Post, April 1, 2020. The title tells it all, but the author provides three links to instructions for making your own mask. My wife prefers the pattern given in How to make a face mask by Deaconess Hospital, Evansville, Indiana; and in How to sew a face mask from The New York Times. The mask in the photograph is the Deaconess mask. I think I prefer the mask with the elastic, because men do not know how to tie bows behind their backs.

Conclusion.When I finally emerge from hibernation (or, possibly, estivation) and go out in public (other than outdoors, such as the bike paths), I will wear a home-made mask (and probably not an N95 mask, though I have several that I use when operating an orbital sander). I suspect that Dr. Price underestimates how much the virus is transmitted by aerosols, and I think that the mask will give some protection and, as several authors have noted, will keep my hands away from my face. If I may be so presumptuous, I would also recommend some kind of goggle for those who do not wear spectacles.

That said, I feel to some extent on shaky ground, and I would welcome comments from people who may have more-informed opinions.


Aerosol emission and superemission during human speech increase with voice loudness, Sima Asadi, Department of Chemical Engineering, University of California Davis, et al., Nature, February 20, 2019. Perhaps somewhat irrelevantly to our discussion, the authors observe in the Introduction that normal speech emits particles (droplets) around 1 μm in diameter. The number of particles increases with volume and is independent of language. Over time, speaking may release more particles than coughing. Some people are super-emitters and may infect a disproportionate number of people. I read only the excellent figure captions in the rest of the article.