The N95 respirator has been considered to be the Cadillac of face masks, an object of desire for all of those wanting full protection from the coronavirus. While officials stressed that these masks should be reserved for health care workers because of their proficiency, many Americans have been able to procure them despite the scarcity, and wear them with pride.

But there’s a catch. All N95 masks are not created equal, and one type can actually be hazardous to the health of those around you.

They come in two flavors: N95’s with valve openings, and those without.

New evidence reveals that the N95 masks that have valves may actually do more harm than good. According to Fox News, the San Francisco Department of Public Health took to Twitter on Monday to warn residents that the N95 masks with valves or opening in the front aren’t safe.

“Still seeing a lot of these masks out there, it is confusing, because they are called N95 — but the ones with the valves or openings on the front are NOT safe, and may actually propel your germs further!” the tweet reads, according to Fox News.

In fact, in California’s Bay area, there is a new stipulation that you can wear any style mask you want, as long as it doesn’t have a valve in it.

Valves are there to make breathing easier, but while the N95s with valves do protect the wearer from exposure to airborne particles, if the wearer coughs or sneezes, infectious droplets could travel through the one-way valve. San Francisco city officials followed up with advice on how to make these masks safer. They suggested wearing them with a surgical mask or other cloth covering over them or covering the valve with tape.

According to Fast Company, even the Centers for Disease Control and Prevention does not recommend that hospitals use the N95s that have valves because “the exhalation valve allows unfiltered air to escape within a sterile field.”