Update: On Apr. 21, the city announced that it’s ending its mask mandate.

Philadelphia revived its indoor mask requirement Monday to combat the spread of the latest COVID-19 variant and minimize the risk of overwhelming hospitals.

However, the local mandate came just as a federal judge in Florida struck down a national mask requirement on planes and other forms of mass transit. Hours later on Monday, SEPTA lifted its mask mandate for riders.

The change has set off a fair amount of confusion for some Philly-area residents, who are grappling with an odd dichotomy: They still have to wear masks at places like the Wells Fargo Center and Philadelphia International Airport, but they don’t need a mask on the train to the game or on the airplane they board.

» READ MORE: COVID-19 masks still required at Philadelphia airport, but SEPTA and PATCO drop their mandates

Health Commissioner Cheryl Bettigole last week called for the mandatory masking as a precaution. She said it’s not clear yet whether the latest rise in cases will have the same impact on hospitals as the surge this past winter. Recently rising numbers met the city’s threshold to bump up from its lowest COVID response level to its second, which includes a mask requirement indoors.

The seven-day average of new cases for every 100,000 residents, although still far below this winter’s peak, has more than doubled over the last month in New Jersey and has risen by about a third in Pennsylvania.

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

Philly dropped its mask mandate less than two months ago. While some residents told The Inquirer last week that they support the return of the mandate, others are reluctant to mask up yet again. Even some City Council members have expressed concerns. And on Saturday, a group of businesses and residents filed a lawsuit in Commonwealth Court, claiming the city’s mandate is “a renegade standard unfound anywhere else in the world” and contrary to CDC recommendations.

» READ MORE: Philly’s return of masks gets both eyerolls and support from residents. Can health officials bridge this divide?

Here’s what to know about Philly’s mask mandate:

Where will you be required to wear a mask?

Masks are required in all indoor public places, including schools, businesses, and restaurants.

The exceptions: Any business can require proof of vaccination instead of requiring guests and employees to mask up. And masks are now optional on public transit.

» READ MORE: Here’s where you need to wear a mask in Philly

Are other cities bringing back mask mandates, too?

Philly appeared to be the first big major U.S. city to reinstate a mask mandate against the latest COVID variant, dubbed BA.2.

The decision has sparked conversation in the last week about whether other cities or states should follow suit, but most leaders so far are holding back. New Jersey Gov. Phil Murphy said he was surprised by Philly’s decision and would be “shocked” if New Jersey put such a mandate back in place based on the current data, according to NJ.com.

» READ MORE: Philadelphia’s revived mask mandate is the talk of the nation, but no other cities are following suit

How long will Philly’s mask mandate be in effect?

We don’t know. It all depends on the COVID data.

Health officials moved the city into response level 2 last week, a tier called “mask precautions,” after Philly surpassed two out of three self-imposed benchmarks. The average number of cases had risen above 100 per day and cases had increased by more than 50% in the previous 10 days.

If either of those metrics falls below the benchmarks — and hospitalizations stay below 50 — health officials could revert to response level 1 and drop the mask mandate again.

However, if the numbers spike, the city could also end up in response level 3, called “caution.” In addition to mandatory masking, the city would require you to present proof of vaccination, a vaccine exemption, or a negative COVID test taken within 24 hours to eat out.

» READ MORE: Philly's new COVID system makes it hard to stay unmasked. We crunched the numbers.

What kind of mask should I wear?

The Centers for Disease Control and Prevention notes that “any mask is better than no mask” but has also pointed out that cloth masks generally provide the least amount of protection.

When possible, you should aim to wear a more protective KN95 or N95 mask. You can find them at local pharmacies, home improvement stores, and hardware stores.

You can find them online, too — including countless retailers on Amazon — but be sure to check the CDC’s list of approved makers to ensure your mask isn’t fake.

This article has been updated since it was first published. Graphics editor John Duchneskie contributed to this article, which also contains information from the Associated Press.