Scientific tests have found that simple pollution masks are incredibly effective, but how long do masks last? Most tests are done with brand new masks, so there’s very little data on used ones.
And in that data vacuum, people have wildly different ideas about how masks remain effective. I’ve seen several people say they only last a day or two, such as this person who emailed me about the question.
The 30-Day Mask Longevity Test
It’s not a complete data vacuum. We tested a new 3M mask and an 11-day-old 3M one in Beijing and found effectiveness dropped just 1.4%. But what happens after 11 days? And is this true of all masks or just that particular 3M model?
To get some good data on this question, Smart Air India ran fit tests on the same type of Venus V-430 SLV N99 mask under three conditions:
- New
- Used for 20 days
- Used for 30 days
This was heavy usage. Smart Air India head Dhariyash wore these masks in Delhi, which is a tough environment for masks because it’s hot, sweaty, and one of India’s most-polluted cities.
Test Method
The fit test works by using the white tube to sample air inside the mask while the tester is wearing it. Meanwhile, the blue tube is sampling air just outside the mask.
The wearer completes a number of movements during the test, such as bending over and moving the head side to side. Thus, the results represent how well the mask seals while people are moving—not just while sitting still.
The TSI PortaCount detects particles down to .01 microns—far smaller than the oft-mentioned 2.5 microns.
Then the machine uses this data to estimate the percentage of particles going through the mask (as opposed to leaking in around the sides).
Results: Masks Last Far Longer Than One Day
The data showed the mask was far more resilient than the “just a day or two” story.
Even after 30 days of use, the mask still scored over 99%. Masks last far longer than just a day or two.
Is That Just A Fluke?
It can seem hard to believe that masks retain their effectiveness so well. Yet this surprising result is similar to earlier test results with a 3M mask. The 9332 mask effectiveness decreased just 1.4% over 11 days of heavy use.
Masks “Age” by Getting Loose, Not by Letting in Particles
A simple understanding of how masks “age” can help explain this counter-intuitive finding. Before I saw the data, my intuition about masks was that, over time, they become worse at filtering particles.
It turns out, I was wrong. Masks actually get better at capturing particles over time.
What does get worse is air resistance. As masks get more particles stuck in them, it gets harder for air to get through, and harder to breathe. It works just like strainers in the sink when they are full of gunk.
Do All Masks Last This Long?
One reason the Venus mask aged so well is that its straps are made out of elastic fabric. This fabric retains its tightness even after heavy use.
In contrast, many 3M masks use rubber bands, which loosen and snap after heavy use. Thus, those with rubber straps will probably have shorter lifespans.
N95 masks last far longer than a single day. This data suggests masks can retain fit effectiveness up to the point masks become so gross and hard to breathe in that most people would want to throw them out anyway.
A Practical Guide to Mask Longevity
Based on the data, I throw my masks out when:
- The straps break or become so loose that I can’t get a tight fit.
- The mask is so full of particles that it becomes noticeably hard to breathe in.
- The mask gets so sweaty and gross that I want to throw it out anyway.
Breathe safe!
Free Guide to Breathing Safe
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Does washing the outer side or the inner side (that touches your face) of the n95 diminish the mask’s ability to filter small particles out…. i.e. by somehow condensing or compressig the “mesh” matrix that the mask is made of? I’m thinking washing with soap and hot water and rinsing with hot water and drying them in the sun.
Starting in March, I ordered 1000 N95 masks which I donated to our local Sheriff’s department. After speaking with the group responsible for distribution, the Sheriff’s department was the most needy as they run the local jail and have medical, corrections, inmates and support staff that along with regular policing come in direct contact with infected individuals. My advice (put in writing) in each 2 mask set was to spray the outside of the “used” mask with a solution of 1/3 bleach to 1 gallon water and let air dry (as per CDC website) while using the second mask. Reading… Read more »
Don’t throw away you mask when the straps are loose/broken, replace it with a fabric one. You can staple it or even sew it: https://imgur.com/2BlRCoc To avoid breaking them at the wrong time (e.g. at the checkout of a store during a covid pandemic…), it is better to add the new fabric straps even before the first use. Then you let the newly added elastic hang behind your neck, until one of the red elastic breaks. You don’t need special staples, those of a mini paper stapler are sufficient. Remember to put 2 staples on each side, lengthwise.
Bravo! I’ve been looking for this information for a while. I thought the masks would get better at filtering as they became more full (like a HEPA filter) but I couldn’t find any information, except for the old, “that’s what they told us at the hospital I worked at”.
Thanks for posting and… Stay Healthy
I am in the USA; I am a critical care registered nurse sine 1991; I cannot find the Venus V-430 SLV anywhere; where can i find a reputable place to purchase? since they are reusable, I am very interested in finding a good reusable mask that is durable as I am taking care of covid-19 patients in the ICU in ‘not ideal’ situations to say the least; I need a very good reusable mask; please help?