Philadelphia’s return to indoor masking on Monday has drawn as many different reactions as there are new variants of the coronavirus.

Relief among those concerned about the recent rise in cases of COVID-19. Grudging acceptance from others. And resentment among those who see the move as overcautious and arbitrary. Over the weekend, several businesses and residents filed a lawsuit in Commonwealth Court challenging the city’s right to impose the mandate.

Though the city is an outlier among major metropolitan areas in bringing back masks, the decision did not come out of nowhere. It is based on benchmarks set by city health officials in February — with levels of caution triggered by the number of cases, how fast they are rising or falling, and hospitalizations.

In this case, masks are back because the city is averaging more than 100 new cases each day, and because the total, as of April 11, had risen by more than 50% over the previous 10 days.

Will Philadelphia get it right, where other health agencies may regret not taking action sooner? Or will the move turn out to be a false alarm?

We asked the city to explain the evidence driving its decision. And we spoke to three outside experts about what is appropriate at this stage of the pandemic, when most people in Philadelphia are vaccinated (and exhausted) yet the potential for risky new variants remains unknown.

Where do the numbers come from?

Very simply, from the past.

By looking at the data from the weeks leading up to each previous surge, health department analysts identified a mix of warning signs that could predict future surges.

Case levels, hospitalizations, and the positivity rate — what percent of COVID tests come back positive — all seemed to be useful indicators. (Positivity was later dropped from the equation, as the rise in unreported, at-home test results made it harder to get an accurate reading.)

The department then devised four response levels, ranging from extreme caution (masks required in indoor public places, plus a vaccine mandate for indoor restaurants and bars) to all clear (no restrictions).

For each indicator, the thresholds that trigger a change in precautions are set at round numbers such as 50, 100, and 500.

“We wanted numbers that were easy to understand, and were seen during previous waves and troughs of the pandemic,” James Garrow, a department spokesperson, said in an email.

While some may quibble with where the lines were drawn, the city gets high marks for clarity and transparency, said Julia Raifman, an assistant professor of health law, policy, and management at Boston University.

“This policy is a reasonable mitigation measure,” she said. “It’s linked to data, and it lets everyone know what to expect.”

The mask precaution is the second of four possible response levels. If cases and hospitalizations were to rise higher, the city would reimpose even more stringent measures, such as requiring proof of vaccination.

The CDC says the city’s risk level is ‘low.’ Why not go with that?

Current guidelines from the Centers for Disease Control and Prevention gauge Philadelphia’s risk level, as of Friday, to be “low” — no need for universal masking.

That’s because the CDC’s approach relies more heavily on the number of hospitalizations, whereas the city gives more weight to case numbers.

Here’s why: It can take more than a week from the onset of illness for someone with COVID to become sick enough to go to the hospital. By the time hospitalizations rise, it may be too late to stem an outbreak, Garrow said. That’s especially true in Philadelphia, with a large population of the poor and vulnerable.

“We’re trying to keep people out of the hospital,” he said.

No one would argue with that goal. Yet the pandemic may have reached a stage where universal masking is not warranted, other experts said.

Mohamed Yassin, the chief of infectious diseases at UPMC Mercy in Pittsburgh, said recent trends look similar to what happened at the same time last year: a wintertime peak in cases, followed by a slight bump in the spring, then a decline by the summer.

The patterns are starting to look seasonal and predictable, he said. And compared with last year, many more people are vaccinated, and thus largely protected against severe disease.

“It makes me think we’re almost headed to this being a disease like the flu,” he said.

Requiring masks at a time of relatively low risk could alienate some people, making them less willing to pay attention when risk levels are higher, said Yassin, an associate professor at the University of Pittsburgh’s medical and public health schools.

Leana Wen, a former Baltimore health commissioner and professor of health policy and management at George Washington University, agreed that a blanket mask requirement won’t get the job done. Too many people opt for face coverings made of cloth, which are of limited use against the highly transmissible delta and omicron COVID strains.

“It seems that there’s a better way to do this than to require everyone to wear flimsy masks,” said Wen, who stressed that she was not familiar with Philadelphia’s trends. “Instead, make N95 masks freely available to everyone who wants them.”

Why wait a week?

Philadelphia’s renewed mask requirement was announced April 11, based on the rise in cases over the previous 10 days. Yet the rule does not take effect until Monday, April 18 — a week later.

If the numbers warranted the wearing of masks, why was it safe to wait?

Garrow’s response:

“We are not in crisis right now; we’re taking action to try to head off a crisis.”

Raifman, who tracks the impact of state and local COVID policies at statepolicies.com, called the approach sound.

The perennial challenge in implementing public health measures is that when they work, it is easy for naysayers to look back and claim they were unnecessary, she said.

“The harms of letting it get more out of control,” she said, “are far greater than the harms of wearing a mask a few weeks longer than needed.”

How much can masks help if the suburbs don’t go along?

Philadelphia is surrounded by eight counties in Pennsylvania and New Jersey, none of which is currently requiring masks. Yet thousands of commuters and other visitors from these counties enter the city each day.

With requirements differing widely from place to place, can a mask requirement still help?

The city health department says yes.

“When those commuters come into Philadelphia, they will be required to wear masks when they go into public indoor spaces,” Garrow said. “Masks are not only good at protecting you, but are an excellent means of source protection and diminishing the spread of COVID-19 from people who are infected.”

The same thing happened in the fall. Philadelphia had a mask mandate, yet none of the surrounding counties did. And the city’s case rate was generally lower, Garrow said.

“We are hopeful that the same thing will happen now,” he said.