How long can you wear face masks? How often you should replace your surgical or N95 mask depends on who you ask. The WHO recommends changing surgical masks every 4 hours. If that’s true, that means a lot of masks and a lot of waste.
To find out how long surgical masks stay effective, Smart Air put it to the test for 40 days.
How Surgical Mask Effectiveness Varies Over Time
Smart Air team members Keer and Vivian wore surgical masks for 40 days as they commuted to and from the Smart Air office in Beijing. In total, they wore the masks for about 40 hours each.
These were not fancy masks. They were 3-layered, non-woven masks from a supermarket in Beijing. They claimed to meet the Chinese YY-0469 standard, at filtering out 95% of bacteria (3.0 microns), and at least 30% of all particles (0.1 microns).
Both Keer and Vivian wore their surgical masks every day for 43 days as they commuted to the Smart Air office.
They commuted by bike, subway, and bus.
How We Tested Surgical Mask Effectiveness
We mimicked a setup commonly used by researchers called a Henderson apparatus. In our setup, a fan on the left end blows air and particles through the mask material on the right.
On the other side of the DIY mask material, a Met One GT-521 laser particle counter measures the number of particles that penetrate the mask material.
Results – Surgical Masks Remain Effective 40 Days
Data showed that both masks retained high effectiveness in blocking both 0.3 micron and 1.0 micron particles even after 40 days.
After 30 hours, the surgical masks still met NIOSH standards. NIOSH standards require masks to have at least 95% protection against 0.3-micron particles.
This data may be surprising. But the findings fit with our two previous tests of N95 longevity, even using different test methods (2). In one test, an N99 mask retained its fit effectiveness unchanged after 30 days in Delhi.
Breathability Increases With Use
When we set out with this experiment, we expected the masks to get more and more difficult to breathe through as they were worn. The thinking here is that as the masks get more and more clogged with particles, they become harder to breathe through.
But we were wrong. The masks pressure resistance actually dropped 8% over 40 days.
This probably happens as the fabric material gets pulled over time and loosens the fibers from each other. That allows air to pass through more easily.
Don’t Masks Grow Bacteria?
Skeptical readers of our past mask longevity tests have worried about the masks getting gross. Don’t they grow bacteria?
Keer and Vivian made sure to let the masks dry out. They stored them on a desk or hanging on a knob.
At the end of the 40 days, the masks were not smelly or visibly dirty (other than a bit of makeup and lipstick). This fits with a peer-reviewed study of masks, which found that masks did not grow bacteria. The only bacteria on the masks was what was already on people’s faces.