We posted info and data on our findings on whether masks really can stop pollution. My judgment after reading up on mask tests and testing them myself is that the science is pretty well settled. Masks do work.
But there are still mysteries that need answers. Perhaps the biggest mystery out there is how long do these masks last?
Really Just One Day?
I frequently run into people who suspect that masks only last one day, such as this person on Zhihu (and the 115 people who upvoted him):
Or this person who sent me an email:
What Does 3M Say?
Last year I had the chance to visit the 3M lab:
While I was there, I asked them how long masks last. They wouldn’t give an answer.
Why? I think they’re worried about giving a specific timeframe because many users are in industrial environments with super high or very toxic pollutants. So giving a general “it lasts X days” answer could mislead people who work in completely different environments. But for folks like us in China who see a fairly “normal” range of pollution, we should be able to find a rough guideline.
Test
Fortunately, when I went to do fit tests, I had a 9332 mask that I had been using for 11 days. And when I say “used,” I mean “used intensely.” It was summer in Beijing, and I wore the mask while biking around town and sweating heavily. I estimate I used the mask at least four times every day for those 11 days.
It’d be a lot better if we had lots of test data with more masks over more durations. But this data can at least test the popular conception that masks are single use or one-day use only.
So what did the data say? Here are the results for a completely new 9332 mask (left) and my 11-day-old mask (right).
98% after 11 days is pretty darn good! This contradicts the idea that you can only use these for a day or two.
So, How Long Do Masks Last?
The truth is, we need more data. With this limited data, my guess is masks last at least two weeks. I use mine for 2-4 weeks, at which point the rubber bands break or they get so gross I want to throw them out anyway!
Keep breathing clean air, both inside and out!
If changing the straps on this model, to a far more durable fabric-elastic material (used on other 3M models), they can be used in excess of a month. Try re-testing after a month or two. In conversation with 3M reps here in Taiwan, they’ve suggested that the filtration effectiveness increases over time, though the breatheability declines. There’s a logic there, as the membrane becomes infiltrated with particles, blocking transparency in either direction. Please do test again after 4-6 weeks.
You’re spot on David, just like HEPA filters, the filtration effectiveness of masks increases over time but at the same time the air resistance builds up which makes breathing more difficult.
I’d still say the number one most important factor when considering when to change your mask is the breathability and dirtiness of the mask. After wearing a mask for 1-2 weeks, it often gets to the point where it’s smelly, sweaty and so not pleasant to wear. Again, just like HEPA filters, washing them will decrease their effectiveness, so there’s no easy way to clean them after they get dirty. Of course, if you only use your mask whilst walking, and it doesn’t get too sweaty, then there’s definitely a chance it can last much longer!