Recently, we’ve been hearing a lot of questions about washing and reusing fabric masks. There are a wide range of “best practices” emerging from government organizations, sewist groups, and journalists. This post discusses cleaning of masks containing spunbond non-woven polypropylene (NWPP) layers, and clarifies some distinctions between washing and disinfection.
First, a caveat — there are more questions in this space than good answers. Those looking for cut and dry certainty may wish for more. We sure do. The good news is that we are actually at the cusp of an exciting time for this topic, especially with the introduction of the new ASTM F3502 standards. But more on that below. In this post, we’ll discuss:
- Is NWPP difficult to care for?
- Why do we wash masks?
- How does washing impact mask effectiveness?
- How does washing affect the water resistance of NWPP in masks?
- Our recommendations for mask reuse
Is NWPP Difficult to Care For?
There is a misconception that NWPP is finicky and hard to care for. NWPP can withstand hot wash, machine drying on low, and boiling just like cotton masks (NWPP’s melting point of 160-166°C / 320-331°F is well above the boiling point of water). However, to prolong the life of any type of fabric masks, MakerMask recommends a gentle approach to washing, disinfecting, and drying.
The main difference between the care for NWPP and cotton is that NWPP should not be ironed.
Why Do We Wash Masks?
Reuse is a major benefit of cloth masks for those looking to minimize waste and cost, or to make their own masks for daily community use. As detailed at great length in our earlier post, “Fabric Masks: Cleaning and Disinfection”, while the community colloquially talks about cleaning reusable masks between uses, it may be useful to distinguish between washing to remove dirt and washing to kill SARS-CoV-2.
Washing to Remove Dirt
Over time and with use, masks get dirty both inside and out. Dust, oils, spit, sweat, and makeup can all build up on the fabric. Washing masks like we wash our hands or clothes can help keep them clean and more comfortable to wear.
Washing to Kill SARS-CoV-2
Killing germs is a related consideration. A widely reported recent study from the UK showed that a coronavirus similar to SARS-CoV-2 can survive for three days on some unwashed fabrics. The study looked at washing methods for cotton hospital garments, and demonstrated that washing in hot water alone (67°C, 153°F) was sufficient for killing SARS-CoV-2. This is consistent with previous research suggesting exposure temperatures above 60°C for at least 30 minutes may be sufficient for washing with the goal of killing COVID [Abraham et al, 2020; Seifer et al, 2020].
However, in settings within the home, most hot water heaters (at least in the U.S. and Canada) are set to limit the temperatures at the tap (as well as at washing machines and dishwashers) to be less than 49°C (120°F) to reduce the risk of scalding [Levesque et al, 2004]. Thus temperature alone may not be sufficient to kill COVID when machine or handwashing masks.
As a result, it is important to use detergent when washing masks. Detergent helps ensure that Sars-CoV-2 is killed when washing with water temperatures commonly used when machine or hand washing masks. The results reported from Owen et al, suggest that warm water with detergent (40°C, 104°F) can eliminate the virus from tested fabric.
For those looking for a higher degree of disinfection, MakerMask recommends immersing masks in boiling water, which has been shown to kill SARS-CoV-2 and other harder to kill pathogens.
How Does Washing Impact Mask Effectiveness?
The short answer is that it depends on the mask and the types of materials it is constructed from. However, key factors to consider is how washing impacts the fit of the mask as well as the The Big Four: Breathability, Filtration, Water Resistance, and Reusability.
Breathability, Filtration, and Water Resistance change with time, care, and use for ALL mask types.
Cleaning techniques have a tendency to break down fabrics over time, as anyone who has a favorite old, soft t-shirt can attest. This mechanical breakdown takes multiple forms. These include, piling of fibers, changing of pore sizes, and degradation of the microstructure within a fabric. As breakdown occurs, breathability generally gets better but filtration worsens. Both phenomena occur as the pathways for air to travel between the fibers of the fabric become simpler. This is true regardless of whether or not masks are constructed from cotton or NWPP.
To Extend the Life Span of Your Reusable Masks:
- Minimize Mechanical Agitation: Minimizing mechanical agitation can lengthen the lifetime of a mask by reducing fabric breakdown. Options include hand washing, lingerie bags, gentler cleaning cycles, and hanging to dry.
- Minimize Exposure to Harsh Chemicals: do not use bleach or alcohol on masks.
For Masks With Meltblown NWPP Layers (e.g., N95s and many disposable masks)
While N95 respirators and other masks with meltdown NWPP materials can provide great single-use performance, they are very challenging to clean or disinfect at home. For example, Rogak, et al. found that H400 surgical wrap went from filtering ~70% to ~40% of 1.5 micron particles after laundering. The UBC Energy and Aerosols Lab hypothesizes that this is due to moisture degrading electrostatic treatments in the thin meltblown layer. Even N95 respirators lose effectiveness with reuse, for similar reasons.
How many times can you reuse a mask?
The short answer is that it depends what type of mask you are using, and your metric for mask effectiveness.
Manufacturers’ recommendations for reuse vary from 5 to 50 washes. Unfortunately, until recently, the standards for mask lifetime have been uneven. The WHO specifies that reusable masks must be good for at least 5 cycles, but few manufacturers have provided test data on the effects of their recommended care and use.
Good data on both breathability and filtration are difficult to come by, and require expensive test apparatus to quantify. Thankfully this is an area that is ripe for change. The new ASTM F3502 standards for barrier face coverings offer guidance on reusability. In other words, a reusable mask at the store labeled as being compliant with ASTM F3502 will have to show that it meets specific breathability and filtration standards when new, and at its recommended end of life. The package will have to detail recommended care instructions and a maximum number of cycles the mask should be used.
The team at MakerMask is hopeful that this will lead to a wealth of new test data on different mask materials and care recommendations. We look forward to future rounds of testing in which we may be able to contribute our own mask data to his growing public dialogue!
How Does Washing Affect the Water Resistance of NWPP in Masks?
Water resistance or hydrophobicity of a material can change with care and use. Importantly, this change is NOT at the level of the individual fiber. NWPP fibers do not absorb water like cotton does even as they age, and an NWPP mask should not weigh appreciably more after being exposed to a humid environment like cotton masks do. But at the aggregate fabric level, NWPP may become less good at rejecting fluids over time. The breakdown of fibers of time can provide more nooks and crannies for small droplets to migrate through via surface tension. Build up of detergents can have a surfactant effect, reducing the natural hydrophobicity of the material as they prevent water from beading up on the surface. Rinse thoroughly to minimize soap residue.
While water resistance changes are not part of the ASTM F3502 reusability standards, they are easier to qualitatively assess at home. An easy preliminary test is as simple as flicking water at the surface of a mask to see if water beads on the surface. A more quantitative test can be run by stretching mask fabric across the top of a jar, measuring a known quantity of water, and measuring transmission through the mask over 60 seconds. See Water Resistance Test.
For example, Mask Makers UK has performed a simple home experiment on washed NWPP to demonstrate how soap residue disrupts water resistance (Figure 1, left), and how subsequent thorough rinsing restores water resistance (Figure 1, right).
What We Recommend
There are a lot of different types of masks out there. When available, the washing instructions provided by the manufacturer are the best place to start.
In general, MakerMask recommends a simple strategy that we have found works well for spunbond NWPP or spunbond NWPP/cotton masks:
- Boil or steam to disinfect: After each use, throw masks into a pot, boil for 10 minutes, allow to cool and hang to dry. (Alternatively, see here for pressure cooker/Instapot instructions.) Disinfection destroys harmful organisms, both SARS-CoV-2 and more robust microbes. Do not disinfect by microwaving, baking in the oven, or using chemicals.
- When dirty, hand wash or launder in a lingerie bag, then either hang to dry or machine dry low: Clean masks as needed with warm or hot water and mild detergents. AVOID bleaches and fabric softeners. Do not iron.
- Replace when visibly worn or fabric begins to fray/pile.
Can You Launder NWPP Masks in Hot Water and Machine Dry?
Absolutely. Laundering with soap and hot or warm water should kill the relatively fragile SARS-CoV-2. But just like with cotton masks, machine laundering will shorten the lifetime of the mask. How much? We can’t currently predict this with certainty, but ASTM F3502 testing should start to answer these questions soon!
How Often Do I Need to Clean and/or Disinfect My Masks?
It depends on usage, exposure, and personal hygiene preference. In general, the CDC recommends washing your fabric mask “whenever it gets dirty or at least daily.” The WHO also recommends that fabric masks be washed daily.
However, if the mask is very dirty, or if you have been exposed to high risk environments, we recommend either discarding the mask, or disinfecting plus cleaning it. Otherwise, decide on the level of cleaning and/or disinfection as desired by usage, exposure, and personal hygiene preference. Some questions to consider:
- Usage: How dirty is your mask? Have you been wearing it for a short time? Have you been wearing it all day? Is it soiled with makeup, spit, sweat?
- Exposure: Were you exposed to high risk, medium risk, or low risk environments?
- Personal Hygiene Preference: Are you sensitive to any dirt/germs on the mask? Are you developing acne from wearing masks? How clean do you want your mask to be?
Is Boiling the Only Way to Disinfect Masks?
No, but is an easy, accessible, and repeatable way for home users to safely disinfect masks. Boiling for 10 minutes is Maker Mask’s recommended method of disinfection, with the goal of killing/inactivating up to 99.9999% pathogenic microorganisms.
- For additional details, click here for our earlier post on Cleaning and Disinfection.
As mentioned above, the new ASTM F3502 standards offer a resource to community members seeking more robust data on mask performance and reusability. As standards-compliant testing becomes more common, we expect to see more head-to-head comparisons in the literature and the marketplace.
MakerMask will also continue to seek resources and partnerships to expand our own test data on this important topic.
Thank you to all our volunteers and collaborators who shared your experience and research for this post! A special thanks goes out to Hope Metzler and the participants of the NWPP Washing Group including:
- Ania Mitros
- Devon Ostrom
- Eugene Desayatnik
- Hope Metzler
- Iris Luckhaus
- Joan Fearnley
- Kc Yennor
- Kim Leo
- Natalie Garrison
- Rebecca Lau
- Sophie Passmore