NEW YORK (AP) — Experts say how often you can safely wear an N95 or KN95 mask will vary depending on how it’s used.

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Using a mask to run to the grocery store, for example, is very different than wearing it all day at work.

Richard Flagan, who studies masks and aerosols at the California Institute of Technology, says the amount of time a mask is worn is more important than how frequently it’s worn.

But in general, he recommends limiting the use of an N95 mask to about two or three days.

N95 masks can’t be washed and should be thrown away once you can no longer use them.

Answers to 25 common COVID-19 vaccine questions

Vaccinations for COVID-19 began being administered in the U.S. on Dec. 14, 2020. The quick rollout came a little more than a year after the virus was first identified in November 2019. The impressive speed with which vaccines were developed has also left a lot of people with a lot of questions. The questions range from the practical—how will I get vaccinated?—to the scientific—how do these vaccines even work?

Keep reading to discover answers to 25 common COVID-19 vaccine questions.