This post comes courtesy of Smart Air, a social enterprise and registered B-Corp dedicated to helping people in China breathe clean air without paying out of the nose for it on expensive purifiers.

Scientific tests have found that simple pollution masks are incredibly effective, but how long do masks last? Most tests are performed using brand new masks, so there’s very little data on the efficacy of used masks.

Therefore, in that data vacuum, people have wildly different ideas about how masks remain effective, with some people going as far as to say that they only last a day or two.

The 30-day mask longevity test

It’s not a complete data vacuum, however. A couple of years ago, we tested a new 3M mask and an 11-day-old 3M mask in Beijing and found effectiveness dropped just 1.4 percent. But what happens after 11 days? And is this true of all masks or just that particular 3M mask?

To get some good data on this question, Smart Air India recently ran fit tests on the same type of Venus V-430 SLV N99 mask under three different conditions:

1. New
2. Used for 20 days
3. 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.

Two tubes are used to test the air in and outside of 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.

Meanwhile, the TSI PortaCount is able to detect particles down to .01 microns – far smaller than the oft-mentioned 2.5 microns.

The machine then uses this data to estimate the percentage of particles entering through the mask (as opposed to leaking in around the sides).

This graphic shows how small particles are in relation to the thickness of a strand of human hair

Results

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 percent.

Is this a fluke result?

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 by just 1.4 percent over 11 days of heavy use.

After 11 days the 3M mask’s efficacy only dropped 1.4 percent

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 we saw the data, our intuition told us that masks, over time, would become worse at filtering particles. It turns out, we were wrong. Masks actually get better at capturing particles over time.

Air resistance becomes worse, not the mask’s ability to capture particles

What does get worse is air resistance. As masks get more particles stuck in them, it gets harder for air to get through, and therefore harder to breathe. It works just like strainers in the sink when they are full of gunk.

More gunk equals more resistance

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.

Elastic fabric tends to age better than rubber bands

In contrast, many 3M masks use rubber bands, which loosen and snap after heavy use. Thus, masks with rubber straps will probably have shorter lifespans.

Rubber straps tend to loose their elasticity quicker, creating more gaps for unfiltered are to enter

Bottom line: Masks last far longer than a single day. This data suggests masks can retain fit effectiveness up to the point they 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 this data, we’d recommend throwing your mask out when:

1. The straps break or become so loose that you can’t get a tight fit.
2. The mask is so full of particles that it becomes noticeably hard to breathe in.
3. The mask gets so sweaty and gross that you’re considering throwing it out anyway.

Breathe safe!

READ: The Best Pollution Masks of 2019

This post originally appeared on the Smart Air website.

Images: USA Today, courtesy of Smart Air