Philadelphia's case levels have risen quickly in recent days.
CNN  — 

Philadelphia will reinstate indoor mask mandates Monday, April 18, as its Covid-19 cases quickly rise, the city’s Public Health Commissioner Dr. Cheryl Bettigole announced Monday.

The city is the first major US metropolitan area to bring back masking requirements after a slew of cities and states dropped restrictions as cases began trending downward in January.

The city will move to its second level of Covid-19 restrictions, which requires Philadelphians to wear masks in all indoor public places, including schools, businesses, restaurants and government buildings.

“Starting today, I’m asking all businesses and institutions in Philadelphia to dig up those old masks-required signs and start hanging them in your windows,” Bettigole said in a news conference.

Monday’s case count is more than 50% higher than it was 10 days ago, Bettigole said, noting the number of people hospitalized is still low. The city is now averaging 142 cases per day.

“By wearing masks consistently, we can continue to go about our daily lives and continue to take part in the life of our city without contributing to increasing transmission of Covid-19,” Bettigole said.

Last week, city health officials announced they were considering a mask mandate and recommended people start wearing masks in indoor public spaces even though the city was at the “Level 1: All Clear” restriction level. In order to remain at Level 1, the city’s Covid-19 metrics must meet two or more requirements: new daily cases must stay below 100 average new daily cases, hospitalizations must be below 50 and cases must have “increased by less than 50% in the previous 10 days.”

The city has now surpassed Level 1 requirements, though Bettigole believes masking precautions could prevent cases from increasing to previous levels.

“I suspect that this wave will be smaller than the one we saw in January,” she said. “But if we wait to find out and to put our masks back on, we’ll have lost our chance to stop the wave.”

On March 2, the city moved to its lowest “All Clear” level, which dropped the indoor mask mandate. At the time, major cities and states across the US were dropping their mandates.

Many states see cases rise

Nationally, cases are trending upward in more than half of states, though case numbers are still relatively low. Averaging around 34,000 new cases per day, the daily case ratein the US remains one of the lowest since mid-July.

It’s expected that Covid-19 case numbers will rise and fall over the course of the pandemic, White House Covid-19 Response Coordinator Dr. Ashish Jha told CNN’s New Day Monday, and said the Centers for Disease Control and Prevention’s guidance about what actions to take is in the right place.

“In terms of what to do about it, we really want to be guided by the CDC on this,” Jha said. “The CDC policy is right on this and that’s what I’ve been following for months, and that’s what we should be doing.”

According to the CDC, the Covid-19 community level for Philadelphia County – where the city of Philadelphia is located – is low. In such communities, CDC urges people to stay up-to-date on Covid-19 vaccinations and get tested if they have Covid-19 symptoms, but it does not suggest wearing masks.

Jha also said the CDC will share a scientific framework this week for the federal transportation mask mandate set to expire April 18. The federal mandate requires masks on public transportation such as planes, trains, buses and hubs like airports and bus terminals.

On NBC’s Today Show Monday, Jha said extending the federal transportation mask mandate is “absolutely on the table,” adding the CDC will make a decision based on the scientific framework it is creating.

CNN’s Raja Razek, Naomi Thomas and Amanda Watt contributed to this report.