Do Masks Only Last One Day?

There’s good data showing that masks (including surgical masks) really can stop pollution and even have demonstrated physiological benefits. Masks work. But one place data is lacking is for air pollution mask lifespan. Just how long do air pollution masks really last?

Really Just One Day?

I frequently run into people who suspect that the lifespan of masks is only one day. After all, these masks are disposable so surely they’re only designed to last one day? This person on Zhihu (and the 115 people who upvoted him) suspects you need to change your mask every day:

As does this person who sent me an email convinced that the lifespan of masks is no more than one day:

What Does 3M Say on mask lifespan?

Last year I had the chance to visit the 3M lab to test out the effectiveness of a bunch of different masks.

While I was there, I asked them how often I should change my mask for a new one. They wouldn’t give an answer.

Why? I think they’re worried about giving a specific mask lifespan or timeframe to replace your mask, because many users are in industrial environments with super high or very toxic pollutants. So giving a general “you should change your mask after X days” answer could mislead people who work in completely different environments. But for folks like us in polluted cities like Beijing or Delhi who see a fairly “normal” range of pollution, we should be able to find a rough guideline.

Air Pollution Mask Lifespan: The Test

Fortunately, when I went to do fit tests, I had a 9332 mask that I had been reusing 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. That’s a minimum 1.5 hours a day for 11 days, for a minimum total mask usage of 16.5 hours.

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 (disposable) 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) showing how well it lasted.

98% after 11 days is pretty darn good! This contradicts the idea that you can only use these for a day or two.

Bottom Line: How Long Do Masks Last?

This limited data suggests that N95 masks retain their effectiveness for at least two weeks. Washing masks with water or alcohol decreases filtration effectiveness, so the only thing to do when they get this gross is to throw them out.

Breathe safe–both inside and outside!