Thursday, June 18, 2020

The Effectiveness of Homemade Masks

Since there is a worldwide shortage of mass=produced masks, various governments have been encouraging their citizenry to make their own masks as part of the mandatory public masking to save us all from a COVID fate worse than death.  It seems to be a viable option given the current pandemic and the lack of readily available masks, however, a study from 2010 would suggest otherwise.  

A rather prescient study from June 2010 entitled "Simple Respiratory Protection - Evaluation of the Filtration Performance of Cloth Masks and Common Fabric Materials Against 20 - 1000nm Size Particles" by Samy Rengasamy et al looks at how readily available fabrics can be used to block aerosols at two different velocities and then compares the results to the penetration levels of N95 respirator filter media. The authors note that, in 2006, the CDC predicted that the demand for disposable N95 respirators by healthcare workers could exceed 90 million for a 42 day-long outbreak of influenza.  This would result in a significant shortfall of N95 respirators for non-healthcare workers, necessitating the use of alternatives.  


The authors tested the 3 samples of the various materials for NaCl polydisperse aerosols (75 nm +/- 20 nm median diameter and used both 5.5 cm per second and 16.5 cm per second face velocities.  This is a common testing protocol for filtration performance testing and allows comparisons to standard filters including N95 respirators.  Initial penetration levels of the NaCl particles were measured for one minute.  Percentage penetration was calculated as the ratio of particle concentration downstream to upstream multiplied by 100.


Using the polydisperse method, the authors found the following penetration levels at the 5.5 cm per second face velocity:


N95 respirator - 0.12 percent


Hanes 100 percent cotton sweatshirt - 40 percent


Norma Kamali Tunic 85 percent cotton/15 percent polyester - 70 percent


Hanes 100 percent cotton T-shirt - 86 percent


Pem America 100 percent cotton towel - 63 percent


Today's Gentleman 100 percent cotton pocket square scarf - 90 percent


The authors found the following penetration levels using the 16.5 cm per second face velocity:


N95 respirator - less than 5 percent


Hanes 100 percent cotton sweatshirt - 57 percent


Norma Kamali Tunic 85 percent cotton/15 percent polyester - 82 percent


Hanes 100 percent cotton T-shirt: 88 percent


Pem America 100 percent cotton towel - 64 percent


Today's Gentleman 100 percent cotton pocket square scarf - 89 percent


The results of the study show that cloth masks created from various fabric materials had far higher penetration levels when challenged with polydisperse NaCl aerosols at both 5.5 cm per second and 16.5 cm per second face velocities.  The authors note that the use of fabric materials "...may provide only minimal levels of respiratory protection to a wearer against virus-size submicron aerosol particles (i.e. droplet nuclei)".  This is because fabric materials show marginal filtration performance against virus-sized particles even when they are sealed around the edges.  As well, face seal leakage will result in decreased respiratory protection, a problem with homemade masks since they are generally loosely held around the margins of the mask.


Here is the authors' conclusion:


"Common fabric materials and cloth masks showed a wide variation in penetration values for polydisperse (40–90%) at 5.5 cm per second face velocity…. The penetration values obtained for common fabric materials indicate that only marginal respiratory protection can be expected for submicron particles taking into consideration face seal leakage." (my bold)

But, on the upside, the Centers for Disease Control and Prevention does have a "simple respiratory mask" made from a Hane's Heavyweight 100% cotton preshrunk T-shirt that it claims can "...provide a good fit and a measurable level of protection..." as shown on this diagram:

Given that the 2010 Regasamy et al study was completed well before the current pandemic thus removing any bias that government narratives currently employ to cajole us into masking up, it would appear that homemade cloth filters are providing Main Street with a false sense of protection against the SARS-CoV-2 virus unless people are willing to go to the lengths necessary to fabricate a CDC-approved design.


Thanks to the commenter who posted the missive on face masks.  Unfortunately, for some reason, Google Blogger is not allowing me to comment in return.

The idea of an insertable and removable paper filter in a homemade mask is an excellent one, however, such a solution is not recommended by at least some governments.  For example, here is the pattern for a sewable homemade face mask by the Government of Canada:

Here is the solution to the problem from the United States Surgeon General/Centres for Disease Control and Prevention which also does not mention anything about the use of a replaceable paper filter:

Please don't misunderstand the purpose of this posting; my intention was not to dismiss the use of homemade masks, rather, it was to alert my readers that homemade masks may be providing the illusion of protection that could result in unexpected consequences (i.e. a COVID infection).


  1. Not necessarily. A double layered cloth mask, made out of non elongating fibres, that also allows for the insertion of a paper filter will likely reduce even more the filtering capacity.

    Also, you are using the N95 respirators, which are to be used by professional health care workers that come in direct and prolonged contact with sick patients. On a day to day basis, for carrying on with normal activities, and with some care for physical distancing, these home made masks, as well as the single use surgical masks will do just fine.

    Add to that the compound probabilities when you have everyone is wearing masks, and even if you have 30% reduction in filtration, the actual infection might fall to more than 90%.

    Plus, even if you get some viruses, the viral load is likely reduced and that has a very great impact on the trajectory of the disease.

    I appreciate your postings, but dismissing so casually the wearing of cloth masks just based on an article that has not considered the ability to enhance the filtering capability of home made masks, as well as the regular use of such masks (as opposed to N95s), and the reduction in the viral load, which is tremendously important, makes such postings a danger for population and public health. Sutor, ne ultra crepidam!

    1. Yes I totally agree. As a health worker we were instructed that the droplet size of Covid 19 was large enough to deem it safe to wear just a surgical mask. Clearly I did not work in a Covid ward where that would change. We were instructed to wear the N95 if the patient was on O2 therapy. Only then the aerosol value increased. So this article could make it clear in its evaluation at what speed their experiment compared to someone on O2 therapy or normal breathing. In my opinion the cloth mask made from sweatshirt fabric would be quite sufficient in normal everyday circumstances, and certainly better than none. With surgical and N95 masks we have always been told they have a certain life. 4 hours for surgical from my memory and I am not sure of the N95 lifespan. Of course this varies from 3 minutes to 20 minutes according to social media "experts".. lol. In hospitals we generally don't use them for long periods however in a Covid ward this would be very different. I think the public could do well with the cloth mask with more knowledge on how to fit a cloth mask and how long to wear it. The alternative cloth mask above is very good.