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Double-Masking and the Science of Fabric Layering

In the spring of 2021 the CDC published advice on double-masking which was based on the results of a study published in February of this year. Although some people that have been fully vaccinated are wearing masks less frequently (per the CDC guidance published in May), there are  many instances where mask use is still recommended and/or mandated, and many individuals continue mask up. Double-masking may be a masking option to consider where masking is warranted. 

Understanding the principles of fabric layering for masks is key to figuring out if double-masking is the right solution for you. As always, there is more than one right answer. Our goal is to help guide you through the science and help you choose the “right mask for the task”. 

Table of contents

THE GOAL OF THIS POST

In this post we will explain the scientific principles at play when layering fabrics for masks. In particular we will review the CDC study on double-masking, their recommendations, and the study limitations. We will also provide tools for evaluating whether double-masking is appropriate for your circumstances. 

WHY DOUBLE MASK? 

The purpose of double-masking is two-fold: 1) to improve fit and 2) to improve filtration. The CDC study, detailed later in this post, suggests improvements for both fit and filtration. However, they did not measure the breathability associated with these options.  

The main consideration for good masks and for double-masking is the delicate balancing act between breathability and filtration, and the impact of layering on these two considerations.

Illustration of a see-saw balancing fit, filtration, and breathability 

How does layering affect breathability?

After a year of mask wearing most of us have gained a real-life understanding of how breathability varies with different masks. The more layers you add the more breathing feels restricted. In fact the relationship between breathability and the number of layers is proportional. Two layers of a given fabric results in double the breathability loss (double the pressure drop). Despite the decrease in breathability, there are potential benefits to filtration with layering. 

How does layering affect filtration? 

Additional layers increase filtration performance. However, research has shown that the relationship between the number of layers and the filtration gains is neither proportional nor linear [Zhao et al, 2020; Zangmeister et al, 2020; Rogak et al, 2020]. In other words, two layers of fabric, while better than one, will not necessarily result in doubled filtration. Although each additional layer continues to improve filtration the relationship between layering and filtration is non-linear and still under investigation in labs across the country and the world. 

¿Sabías que?

Illustration showing droplet atomizationEver wonder why single layer masks are not recommended? You might think that it is because two layers are simply better than one. While that is in fact true, the reason is not simply because of increased filtration. Some scientists are concerned about a phenomenon called atomization, which may be an issue for single layer masks. Although  a single layer mask contains some of the emitted particles and is better than no mask, this single layer can also act as a screen that breaks apart large droplets into several smaller aerosolized droplets in a process referred to as atomization. So while part of the large droplet is contained, a number of smaller atomized droplets can escape as aerosols of multiple sizes. Atomization is generally more of a concern for when masks are used as source control because its greatest effects are seen with higher force exhalations such as those seen with coughs and sneezes. Luckily, research has shown that masks containing two or more layers  neutralize this effect. [Sharma et al.

How do fabrics affect layering? 

The basic scientific principles concerning layering and how it affects filtration and breathability are consistent across all fabrics. However, different fabrics have different filtration characteristics, which impact how effectively they can be layered together. For example spunbond non-woven polypropylene (NWPP) is much more breathable than quilters cotton and could withstand more layers while remaining breathable. MakerMask recently tested two fabric combinations (cotton+NWPP+cotton and 3 layers of NWPP) and found that while filtration results were similar, breathability was quickly lost for the cotton combination while the NWPP combination remained within acceptable standards. Consequently when selecting masks for use in double-masking, consider more breathable masks such as masks constructed from NWPP. 

Current three-layer mask advice

It is no surprise then that the current advice (WHO, Health Canada) for three-layer community masks has remained unchanged. However, some gains on fit or better filtration may still be made by double-masking. 

Key Findings of the CDC Study 

The CDC conducted two experiments to evaluate double-masking. The first analysed source control (my mask protects you) and the second analysed protection (my mask protects me). In each experiment different masking options were examined: A) procedural mask without fit adjustments, B) a double-mask option (cloth on top of procedure mask), and C) a medical mask adjusted for better fit using a “knot & tuck” hack. See image.

Headforms showing double-masking from the CDC
Image source: CDC

Take-aways: 

  • Double-masking outperformed or performed equally to the other masking options tested.
  • The experiment demonstrates the power of universal masking even with poorer fitting masks.

Study limitations

  • Filtration. The cloth mask option was not described in detail. Given the wide range of cloth mask options it is likely that a well designed multi-layer mask would perform much better. 
  • Breathability was not measured and it is not possible to determine the comfort level associated with the double-mask option.
  • Fit. The study was conducted using a headform and could not take into account the loss of fit due to normal, real-world, head and body movements.

Experiment #1. The cotton mask alone and the procedure mask alone without fit adjustment had similar results (51%, 56% with overlapping standard deviations). The fit adjustments on the procedure mask (knot & tuck) resulted in improved filtration (77%) and the double-mask strategy even more so (85%). 

CDC Bar graph showing double-masking

Experiment #2. Overall, double-masking produced superior results. In partial masking scenarios, where only the receiver was masked or only the source was masked, filtration was significantly greater (83% and 82% respectively). When universal masking was adopted (both the source and receiver were masked), the filtration results of double-masking were identical to the Knot &Tuck option of the medical mask (96%).

HOW TO DOUBLE MASK

Consider Fit

The CDC study demonstrates the importance of fit and reinforces already existing public health messaging about the effectiveness of masking (See CDC guidance, MakerMask post). When double-masking, fit will only be improved if the outer mask is a mask with superior fit to the inner mask. The outer mask essentially presses the inner mask closer to the face sealing all the edges of the inner mask. However, residual fit issues may still occur if the nose wire is not properly adjusted or there is gapping at the sides of the mask.

Another consideration with fit is the stability of the mask configuration over the range of motions typical in day-to-day life. The CDC study was conducted using immobile headforms. Consequently, there is none of the normal jaw and head movements that could alter the fit over time. When considering fit, Section 14 of the OSHA fit test protocol can serve as inspiration for test movements that you can try at home:

  • Normal breathing
  • Deep breathing
  • Turning head side to side
  • Moving head up and down
  • Talking (read a paragraph)
  • Grimace (smiling or frowning)
  • Bending over or jogging in place

If you find yourself constantly re-adjusting your mask while doing these movements you may need to consider a different masking strategy or a different combination of masks. Also, if you are in a situation where you are required to don and doff your mask frequently (eating/drinking, public speaking) you will want to consider a fit strategy that can be accomplished quickly and efficiently. 

Consider the Breathability and Filtration Tradeoff

The CDC study suggests there may be added filtration provided by the additional mask. However, the CDC study did not measure the pressure drop increase (breathability decrease) caused by the additional mask. In other words, breathability may be altered beyond acceptable levels with some mask combinations. This could result in an uncomfortable mask and leakage around the sides of the mask making it less effective. If double-masking, ensure that your mask combination is breathable without leakage around the sides of the mask.

How/what to choose if double-masking

If you already have a good 3-layer mask that fits well and is breathable, double-masking may not provide additional benefit. However, should you wish to double-mask, consider the following:

Mask Combination Recommendation Comment
Cloth mask over medical mask Good option
  • The outer cloth mask provides fit and will likely be breathable provided the cloth mask is a breathable mask.
  • A good option if you have a two layer cloth mask and are looking for extra filtration. 
  • A good strategy when visiting medical clinics where medical masks might be mandatory (provided by the clinic). Your own cloth mask worn on top of the surgical mask can provide that extra fit. 
2x Cloth masks Questionable
  • The effectiveness of layering two cloth masks depends on the materials used in these masks and the fit of the masks. 
  • Layering two cotton masks may lead to poor breathability. 
Medical mask  over cloth mask Not recommended
  • Medical masks are loose fitting and will not improve the fit of the cloth mask. The filtration benefit of double-masking will be lost through leakage.
  • Best to opt for the “cloth mask over medical mask” combination.
2x Medical mask Not recommended
  • Medical masks are loose fitting. Layering two provides no additional fit benefit. 
  • If using a medical mask simply adopt the fit hacks suggested by the CDC (e.g. knot and tuck)
Cloth mask over respirator* Not recommended
  • The CDC does not recommend this combination because fit-tested respirators alone should provide sufficient filtration, fit, and breathability. 
  • The addition of a cloth layer would compromise breathability and may be counterproductive by compromising the fit. 
*This double-masking strategy is not to be confused with single layer N95 covers that were made in the early days of the pandemic to prolong the life of N95’s when they were very scarce. Maker Mask’s Cover design was tested for this purpose by ATOR Labs. 

CONCLUSION

How does double-masking stack up?

The CDC study demonstrated that double-masking can be an effective strategy for improving fit and filtration. It also showed that universal masking is a powerful public health tool for mitigating the spread of infectious disease. While the study did have some limitations, these did not deter from the core importance of properly fitting masks with sufficient layering. 

Following the current guidelines of properly adjusted three-layer masks with at least one layer of 70gsm NWPP is still appropriate for low risk settings (e.g. shopping). For those considering double-masking, the key is keeping in mind the basic principles of layering approaches that balance breathability, filtration and fit. 

Double-Masking and the Science of Fabric Layering

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