Mystery of Covid19 Masks
(The author of this article is a retired scientist who has designed and developed aviators’ and passengers’ oxygen masks and also has work experience of the NBC masks. Hence, opinion expressed on the Covid19 mask may have significance despite regulatory bodies being at variance)
Introduction. By the time Corona virus SARS nCoV2 entered India, it was already known that it spreads from Human to Human by contact and aerial route (aerosol and micro-droplets) to reach nasal mucosa and then to the lungs and the other tissues. The recommendation of using Hand sanitisers and masks came along with the virus and the two items vanished from the shelves of the shops almost overnight and sold in black 100 times costlier. In the ensuing crisis, panic was paralyzing the nation. In March 2020 itself, this magazine had published an article putting out some of the facts known at those initial times and putting out some information about using hand sanitisers, the masks and its alternatives which were as relevant 2 yrs back as they are today. The many waves of Covid19 and several mutant variansts rampaging the world, the basic preventive and promotive measures remain the same...mandatory mask, frequent hand washes and physical/social distancing.
The concept of wearing masks have been arbitrary at times in both its approach and selection of a particular type. Even today, there are misconceptions about its usage. Hence, this article will try to examine the various facets of the problems in selection and usage of Covid19 masks.
The Droplets. It is extremely important to understand the airborne spread of infection is through aerosol/droplets mostly of sizes <5 microns that remain suspended in the air for varying times. These micro-droplets are produced during cough, sneeze or talk and range in sizes from 5mm to 0.5 microns. Sizes more than 50 microns fall down to the ground immediately, those of 50-5 microns remain in air for some time before falling/ settling but those <5 microns remain afloat and suspended in air and are more likely to infect others breathing those air.
The Masks. Masks have two roles…of trapping the larger droplets from going to others and secondly, preventing larger droplets sprinkled by others getting in to own inhaled air. Sizes <5 microns floating in air still manage to leak through the sides of ordinary cloth masks. N-95 mask has a nasal clip and tight fit to prevent leakage from the sides to a large extent hence, useful for the health workers working in the vicinity of the infected persons…like Covid ward and shelters. It may also be preferable for the attendants of Covid patients under Home Isolation. For all others incl Covid19 patients, the dual/triple layered cloth masks covering nose & mouth would suffice. We must remember that Corona Virus is not in the atmosphere. Sunlight also kills the virus present in micro-droplets.
Different Types of Masks. Home-made single or double layer cloth masks are effective for the general population without symptoms. A 3-layered commercial mask is useful for all incl Covid19 patients that traps more droplets than the home-made single/double layered ones. All these masks trap most of the larger droplets produced by cough, sneeze or talk. N-95 & equivalent masks are more useful for the health workers incl doctors and nurses working in the Covid wards. N-95 mask with valve is also very useful for doctors & nurses of the Covid wards provided its valve remains fully functional for air filtration of up to 0.5 micron. If the integrity of the valve filter is doubtful or cannot be verified, these masks must not be used by them.
Three-layer mask has inner layer of woven cotton that can effectively absorb the droplets, the middle layer is usually a spun-bond non-woven fabric material of Polypropylene that can trap/filter the droplets and the outer layer is hydrophobic that prevents the droplets from passing through. This type of mask is more useful for symptomatic persons. The masks must be changed the moment it is soiled. Washing of the home-made cloth masks are permissible. However, washing of the 3-layer commercial masks causes damages to the weave and spun of the inner and middle fabrics layers thus reducing its efficacy to large extent. Hence, while asymptomatic people may wear the 1-2 times washed masks, the symptomatic people must not wear it. The hygienic disposal of the used masks must acquire high priority of avoiding the spread of the virus...both among its handlers and the community...esp the soiled ones of the symptomatic patients. Masks of even the asymptomatic individuals must be disposed off carefully. The efficacies of the various types of masks are depicted in the table/chart below. It is worthy noting that most of the masks are effective in preventing the droplets leaking out in the surroundings and that protection against inhaling microdroplets is best afforded by N-95 masks and its equivalent but still there are chances of 2-3 reference droplets leaking in. Total protection could be only through a filter-blower or NBC masks.
Masks and Covid19 Infections. Many people believe that wearing of a mask will prevent them from the infection. Very few know that their masks mostly prevent larger droplets during cough, sneeze and talks from getting out whereas smaller aerosol/droplets of the sizes <5 microns escape from its sides and remain suspended in the air for varying periods. Cloth masks are never airtight, even the much talked about N-95. It always allows some air to leak through its sides and nasal bridge (we can observe it by the fogging of our glasses if we wear them). Multi-layered masks (like 7-layered N-99 mask) or wearing 2-3 masks one over the other is no solution as it causes even more leakages from the sides owing to greater resistance to breathing through the multi-layered fabric barrier. It is a simple law of fluid dynamics that within a contiguous medium, a fluid flow takes the path of least resistance. When negative pressure is created within the mask cavity (during inhalation), air from surrounding flows in...from the leaking sides...mostly from the small gaps on the sides of nose and cheek although a very small volume of air may also come filtered through mask fabric. Similarly, when positive pressure is created in the mask cavity (like exhalation, cough, sneeze or talk), although most of the larger droplets get stuck in the inner absorbable cotton fabric, most of the air leaks out from the nasal & cheek sides of the minor gaps. This air from infected patients carries smaller droplets that will contaminate the surrounding. It is equally important the masks fully cover the nose and mouth up to the chin. There is absolutely no point in keeping the mask hanging below the chin.
If contaminated air (with suspended droplets of <5 micron) leaks-in through the sides of the masks during inhalation, infection is very much possible despite wearing those masks. The fool-proof masks preventing air leak and infection to one-self are the NBC masks (with single or dual filter canister) or the ones with filtered air blower, both shown below. The blower mask has an electrically operated miniature fan that forces air in to the mask cavity creating an outward leak from the face-mask interface; thus preventing entry of micro-droplet contaminated air into the lungs. As mentioned, these are the masks to be worn by very high risk people from infection. In the present Covid19 scenario, blower masks are often used by doctors and nurses working by the bed-sides of the Covid19 patients. However, the NBC masks with canisters are only with the military and not available at public commercial shops.
Summary
The spread of airborne human to human Covid19 infection can be curtailed by wearing an appropriate mask over the nose and mouth. These masks trap the virus contaminated droplets produced by Cough, sneeze or talk by Covid infected persons. These masks also become a barrier should another person not wearing mask sprinkle the droplets. For asymptomatic persons, even home-made single-double layer cloth masks may suffice. For the general population as well as for people having cough/sneeze, a 3-layered commercial mask is desirable that trap the droplets more effectively. These masks contain the droplets within the wearer and thus protects others in the surrounding. However, these masks do not fully prevent air from leaking from the sides. Hence, micro-droplets do leak, either out (during cough/sneeze) or are inhaled in if in the ambient air (in the vicinity of Covid case). Washing of 3-layer mask reduces its efficacy.
In highly contaminated Covid wards or in the vicinity of symptomatic patients, a blower mask is more desirable although masks with adequate face-mask seal (like N-95) are also effective in preventing infection.
Some Questions & Answers about Covid masks
1. Should all persons wear masks in pandemic?
Ans: Yes. Mask cuts down the spread of infection from Covid cases when they cough, sneeze or talk. For 1 Covid case, there are ~3 asymptomatic infected cases (tested or not). Since everyone's Covid status is not known, everyone should wear a mask.
2. Should a non-infected person wear a mask?
Ans: If that non-infected person is in the company of confirmed non-infected people (like members of own family), a mask may not be required...BUT...if the infection status of the others is not sure, it is better to wear a mask. You never know an asymptomatic Covid person may unintentionally sprinkle the droplets on you through cough, sneeze or talk.
3. Which mask should one wear?
Ans: A 3-layered commercial cloth mask is ideal for all whether infected or not. An apparently non-infected person may wear even home made one or two-layer mask or wrap their mouth & nose with any cloth. For health care workers in the vicinity of Covid cases, a proper face-mask sealing mask (like N-95 or equivalent; or a blower mask) is a must.
4. Could the mask be reused after washing?
Ans: If the person is not infected, he may use it. A cloth mask may be washed many times and reused by non-infected persons. The filter performance of a commercial 3-layered mask deteriorates after 1-2 washing. A Covid case must never reuse a washed mask.