The city of Philadelphia on Wednesday ordered all unvaccinated city employees to double-mask at all times.

City employees are required to get vaccinated against COVID-19, but those who aren't by Sept. 1 will have to wear a cloth mask over a disposable or surgical mask at all times while working on-site, according to a press release from the Philadelphia Department of Public Health.

All incoming city employees must be vaccinated as a condition of their employment.

The city noted in its press release that residents have a "25-fold reduction" of their "risk of being hospitalized or dying from COVID" if they receive the COVID-19 vaccine.

A lab experiment conducted by the Centers for Disease Control and Prevention earlier this year using simulated respiratory breaths found that placing a cloth mask over a medical procedure mask or using a medical procedure mask with knotted ear loops and tucked in sides decreased exposure to potentially infectious aerosols by about 95%.

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News of the double-mask mandate came the same day Philadelphia Mayor Jim Kenney implemented a citywide mask mandate for customers at indoor businesses and restaurants that do not require proof of vaccination, as other major cities have done in recent weeks, including New York City and San Francisco.

"The science is clear: these measures will protect Philadelphians and save lives," Kenney tweeted on Aug. 11. "We also strongly urge everyone who has not yet received the vaccine to get it immediately."

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The city is also requiring residents to wear masks at non-seated outdoor events with more than 1,000 attendees

"We know that you’re probably tired of wearing a mask, but more and more people are getting infected with COVID and that’s dangerous – and even deadly," a Thursday press release reads.

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The Philadelphia Department of Public Health on Friday recorded 336 new cases of COVID-19 and a 6% positivity rate. More than 60% of the city's population is fully vaccinated, and nearly 80% are partially vaccinated.

Fox News' Alexandria Hein contributed to this report.