Double Masking & Over Masking

Double Masking: Cloth Mask Over Medical or Procedural Mask

We recommend this as the preferred approach to double masking.

Over masking, wearing a cloth mask over a medical mask, has been shown to improve both the inward and outward filtration of masks. Experiments were conducted on manikins using a disposable mask underneath and a three-layer cloth mask as an outer layer. A 2022 study by Blachere, testing on human volunteers, confirmed these findings.

The filtration properties of non-certified disposable masks are not known and are likely highly variable. Over masking with a well-fitting cloth mask will likely improve the fitted filtration and overall protection from these masks, in the same way that it has been shown to improve fitted filtration for medical masks.

​Sickbert-Bennet 2021 found that a cloth mask on earloops, a bandana and a gaiter resulted in fitted filtration of around 44%, and procedure masks averaged 55%. Wearing the cloth mask over a procedure mask resulted in 66 to 81% filtration; wearing the procedure mask over the cloth mask resulted in 55 to 60% filtration. This is in keeping with the theoretical idea that the cloth mask worn over the procedure mask contributes by improving the edge seal and decreasing leaks around the mask, while the procedure or medical mask underneath contributes by improving filtration through the mask. When worn the other way around, the cloth mask doesn’t seal the edges of the procedure or medical mask and the improvement in fitted filtration is not as great.

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Double Masking: Two Cloth Masks

There is extensive previous literature on layering, summarized here. Double masking, wearing two cloth masks, is essentially a method of layering and improving fit. Watch out for breathability, be sure you are comfortable in the double mask, and that the extra thickness is not making the leak around the edge actually worse. The aim is for most of each breath, in or out, to go through the material.

If your mask subjectively fits well, is breathable, and has at least one layer of non-woven industry-grade spunbond polypropylene​ as recommended by Health Canada, at is at least 3-layered, we think it is likely that you already have this advantage and do not need to double mask.

​If you are unsure of the composition of your mask, it may improve the overall filtration to wear two cloth masks or to overmask, using a medical or non-certified disposable under your cloth mask.

​Wash cloth masks after every 8 hours of use, or when soiled, replace and allow to dry when damp. Industry-grade spunbond polypropylene can be hot-washed but must be hung to dry - it will melt in a tumble dryer or if ironed. More information on washing can be found here.

​The CDC, Health Canada and the WHO recommend layering medical and cloth masks. Here is a link to a longer article from our group, "Are two masks better than one?"

Double Masking: Procedure Mask over Cloth Mask

We think this is likely not as good as wearing the cloth mask on the outside, and the medical or procedure mask underneath.​

Sickbert-Bennet 2021 found that a cloth mask on earloops, a bandana and a gaiter resulted in fitted filtration of around 44%, and procedure masks averaged 55%. Wearing the cloth mask over a procedure mask resulted in 66 to 81% filtration; wearing the procedure mask over the cloth mask resulted in 55 to 60% filtration. This is in keeping with the theoretical idea that the cloth mask worn over the procedure mask contributes by improving the edge seal and decreasing leaks around the mask, while the procedure or medical mask underneath contributes by improving filtration through the mask. When worn the other way around, the cloth mask doesn’t seal the edges of the procedure or medical mask and the improvement in fitted filtration is not as great.

Double Masking: Two Procedure or Medical Masks

​Sickbert-Bennet 2021 tested double masking with ear-loop procedure masks on 3 participants (2 men, 1 woman). Wearing one mask resulted in average 55% fitted filtration; wearing 2 of the same mask in average 66% filtration. This difference was not statistically significant (our calculations), meaning it could have arisen by chance.

We do not recommend wearing two medical masks. The filtration of the material in medical masks is excellent: particles reaching the wearer are likely the result of edge leak. We don’t think this will be improved by wearing a second medical mask. Instead, consider wearing a well-fitting cloth mask (overmasking, above) or using a hack to improve the fit and the fitted filtration efficiency of the medical mask.

Medical Mask Over Respirator: Not Usually Recommended

Wearing a medical or procedure mask over a respirator is a different issue. Fitted filtration may actually be decreased, presumably because the additional mask interferes with fit. Wearing a medical mask over an FFP2 led to decreased fitted filtration of 1 mcm particles from about 94% to about 90%. Wearing a procedure mask over an N95 led to failed fit tests for 13% of participants, all of whom passed fit testing when the N95 was worn alone. Fit testing is a pass-fail test: >= 99% filtration of 0.02 to 0.1 mcm is a pass. The only reason to wear a medical or procedure mask over a respirator (N95 or FFP2/3) is to facilitate extended use of the respirator in an environment where there is a high chance of external splashing or soiling. It’s important to recognize that adding a mask over the respirator may compromise the protection of the wearer, and that the combination of masks may not meet fit testing standards.