How Effective Are 口罩?

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Richard Klasco, MD, Ask Well: How effective are antimicrobial 'courtesy mask' at preventing the spread of contagious airborne illnesses?  New York Times, Mar 27, 2018.

Note:
(a) There is no need to read the essay. As you can imagine, there have been few published research concerning efficacy of a mask. I attributed photos of East Asians wearing masks to herd mentality: could it help alleviate harm by pollution, for example?.
(b) "Well" is the name of a half-page section (about human health) in the Weekly Science section.
(c) Dealing with patients (not to spread germs) and apparently healthy people (who do not want to get germs), the essay points out, "Courtesy masks, or what we doctors refer to as surgical masks."  The essay then quotes CDC (the second sentence within quotation marks next), without mentioning that CDC is talking about the influenza epidemic since last fall. See Interim Guidance for the Use of Masks to Control Seasonal Influenza Virus Transmission. CDC, last updated Jan 25, 2018
https://www.cdc.gov/flu/professi ... ol/maskguidance.htm
("Masks are not usually recommended in non-healthcare settings * * * No recommendation can be made at this time for mask use in the community [ie, outside healthcare settings, such as hospitals] by asymptomatic [ie, healthy-looking] persons, including those at high risk for complications, to prevent exposure to influenza viruses")
(d) We are not talking about Ebola virus. Here is CDC on Severe Acute Respiratory Syndrome.

Fact Sheet for SARS Patients and Their Close Contacts. CDC, last updated: July 2, 2012
https://www.cdc.gov/sars/about/fs-closecontact.html

four consecutive recommendations:

"• Limit your activities outside the home except as necessary for medical care. For example, do not go to work, school, or public areas. If you must leave the home, wear a mask, if tolerated. Do not use public transportation.

"• Wash your hands often and well, especially after you blow your nose [secretions are the worry].

"• Cover your mouth and nose with a tissue when you sneeze or cough.

"• If possible, wear a surgical mask when around other people in your home. If you can't wear a mask, the members of your household should wear one when they are around you.