(NewsNation) — Philadelphia is reinstating its mask mandate for indoor public spaces after a recent rise in new COVID-19 cases, the city’s health department announced Monday.

Health Commissioner Dr. Cheryl Bettigole said confirmed coronavirus cases in the city increased by more than 50% in a 10-day span, the threshold at which the city’s guidelines call for people to wear masks indoors.

Philadelphia only dropped its mask mandate about a month ago, but it’s the first major American city to bring the precaution back as case numbers are steadily rising.

Health inspectors will start to enforce the mask mandate at city businesses starting Monday, April 18, the city’s health department tweeted.

Philadelphia is now averaging 142 new cases per day. Bettigole said the recent increase in infections indicates the city might be at the beginning of a new wave, and city officials are seeking to stay ahead of it by requiring indoor masking.

“We want to protect our most vulnerable residents. Wearing a mask around others is an easy way to do that. The sooner that we can stop this wave, the sooner we can get back to Level 1,” the health department tweeted.

The city’s criteria for re-instituting the mask mandate, however, does not line up with guidelines from the Centers for Disease Control and Prevention.

The CDC has not recommended an indoor mask mandate in Philadelphia. In fact, Philadelphia County is still listed in the low category for COVID-19 transmission levels. But health leaders say they just don’t want to take any chances.

“We don’t know if the BA-2 variant in Philadelphia will have the kind of impact on hospitalizations and deaths that we saw with the original omicron variant this winter,” Bettigole said. “I suspect that this wave will be smaller than the one we saw in January, but if we wait to find out and put our masks back on, we’ll have lost the chance to stop the wave.”

It’s unclear if other major cities will follow Philadelphia’s lead on masking. Both New York and Washington, D.C. have seen cases spike in recent days.

The Associated Press contributed to this report.