Brief answer: Please wear a cloth mask. You do not need an N95 or surgical mask to prevent spreading your germs to others. Also, a cloth mask will protect you from touching your face.

Not so brief answer: The COVID 19 pandemic has brought this simple item into the limelight. In this newsletter, I will describe some basic principles of respiratory virus transmission and mask design, hopefully, laying the groundwork for understanding what to wear and when.

Respiratory viruses, such as COVID 19, propagate from an infected individual via germ laden particles released by coughing, sneezing or even breathing. The larger particles are called droplets and tend to be larger and heavier, settling out of the air relatively quickly. Smaller particles can be formed by evaporation of droplets creating lighter, smaller particles that can float in the air for longer time periods leading to airborne transmission. Our current understanding is that most COVID 19 transmission occurs from inhaling the larger droplets or by transferring droplets that have landed on a surface by touching them and bringing them to one’s eyes, nose or mouth.

Let’s define “small” and “large” particles. The unit of measurement here is “micron” or 1/1,000,000 of a meter. For reference a human hair is 50 microns in diameter. The actual COVID 19 virus is about 0.1 micron in size, but is usually encased in a mixture or protein, salts, secretions creating particles ranging in size from 0.2 to several hundred microns.

How do masks protect us from this array of viral particle sizes? Masks are made from a variety of substances creating different pore sizes depending on the tightness of the meshwork weave and number of layers of material used. Everyone has heard the term “N95” which refers to a rating of the level of that mask’s filtration ability. Defined as the ability to filter out at least 95% of particles as small as 0.3 microns. For example, a medical grade surgical mask would have a rating of 60-80% filtration and a homemade cloth mask may filter 20-40% of those tiny particles.  Preventing large particles from entering the air requires less sophisticated masks than those preventing small particles from being inhaled.

We are now ready to address:  who needs what mask and when? Medical providers treating COVID 19 patients are best protected by wearing N95 masks. These providers are in close proximity, for longer durations of time, to patients who are putting a large volume of droplets into the air which can be evaporating into smaller airborne particles. Unfortunately, there is a dire shortage of these life saving masks and health care providers are being infected as a result. N95 masks should only be used in the appropriate medical context. These masks provide protection against infection in high risk situations.

Last week, the CDC recommended that people wear cloth face coverings when going out in public. At first glance, this would appear to contradict the Surgeon General’s earlier admonishment via Tweet to “Seriously people-STOP BUYING MASKS! They are NOT effective in preventing general public form catching Coronavirus.” To understand the apparent contradiction, we need to revisit our discussion of particle size. When a symptomatic or asymptomatic person coughs, sneezes or breaths out COVID 19 he or she does so in large particles, amenable to being stopped by even the large pore sizes in homemade cloth masks being recommended. The masks can reduce the spread of germs TO others as apposed to the N95 mask which reduces the exposure to germs FROM others. Given the high prevalence of asymptomatic COVID 19 a policy of universal mask wearing is a good way to reduce the amount of germ transmission in the community, by preventing respiratory droplets from being put into the air. Additionally, wearing a mask reduces the risk of touching one’s face.

The policy specifically asks for homemade cloth masks to ensure that surgical and N95 masks are reserved for health care workers, a compromise to reduce disease transmission without furthering the shortage of protection equipment.  I will not address the obvious question of why this recommendation was not made months ago when there were hundreds of thousands less infected Americans.

Here are my practical recommendations. (1) If you have any unused N95 masks, you don’t need them. Donate them to a healthcare facility. (2) If you have any surgical masks, you can use them. Please don’t buy more. They are in short supply. (3) Please buy or make your own cloth masks. The CDC has good guidance on the topic https://www.cdc.gov/coronavirus/2019-ncov/prevent-getting-sick/diy-cloth-face-coverings.html

Please wear a mask when you are out in public especially in circumstances when you cannot maintain 6ft spacing for social distancing, such as waiting in line or grocery shopping. However, the masks are difficult to wear when exercising. My personal sense is that when outdoors, a few seconds or less than 6ft separation, such as when passing someone on a hiking trail is not very risky due to dilution, wind and limited time exposed.

Now is the time to be proactive in our collective effort to beat back the COVID 19 virus. Please wear a cloth mask when going out in public, wash your hands obsessively and continue to maintain social distancing.

I welcome your comments and suggestions.

Thank you,

Barry Rotman MD