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All individuals aged 2 and over, regardless of their vaccination status, will be required to wear a face mask while indoors in public settings starting Friday.
"With the highly transmissible Delta variant causing case rates to increase, now is the time to re-institute this measure to prevent further spread and save lives," said Dr. Arwady. "We continue to track the data closely and are hopeful this will only be temporary and we can bend the COVID curve, as we've done in the past."
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Masks are required in all indoor public settings, including bars and restaurants, gyms, common areas of condos and multi-residential buildings, and private clubs.
"It's just one of those things, like here we go again," said Luis Centeno, owner of Fit Results South Loop. "It's bad news, but at the same time we want to just make sure that we keep everybody safe."
Similar to previous mask mandates, the mandate allows restaurant patrons to remove their masks while sitting at a table and eating.
But for health clubs, like the South Loop's Fit Results -- where many only just started going back once the previous mask mandate was lifted -- the impact could be significant.
"A lot of people were uncomfortable with it. Some people refused to come back," Centeno said. "I'm hopeful it's just masks and for three weeks or whatever it is. And we're just going to include more outdoor classes."
WATCH: Dr. Arwady discusses Chicago mask mandate
Masks can also be removed for certain activities that require their removal, such as beard shaves or facials. Additionally, masks can be removed by employees in settings that are not open to the public, if employees are static and maintaining at least six feet from all other individuals (office cubicles, for example).
Masks also remain mandatory on public transportation, in healthcare settings, schools, and correctional and congregate settings.
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The new public health order does not include capacity limits at public places and masking remains optional in outdoor settings, where the risk of COVID-19 transmission is lower. Masks are recommended for unvaccinated individuals in crowded outdoor settings, as well.
"There's a lot of restaurants, even myself, will close temporarily if there's a restriction on occupancy," said Sam Sanchez, the chair of the Illinois Restaurant Association.
With more than 70% of Chicago adults receiving at least one dose of the vaccine, health officials said Chicago's COVID hospitalization rates and deaths are much lower than they were in 2020 at the same case rate.
"Our goal is to stay open. I don't expect that this will be an indefinite, forever mask requirement and that's why we've also been very clear that we're on the way up now. I expect that we'll come back down and when we get back under that threshold, is when we would drop it. I really think again it's based on what we've learned from COVID to date and wanting to minimize the risk for everybody, including those not yet eligible for vaccination," Arwady said.
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The latest mandate comes weeks after the CDC issued updated guidance that everyone, even those fully vaccinated, wear a mask in public indoor settings where there is "substantial and high transmission," to help prevent the spread of the Delta variant.
The CDC defined four levels of community transmission: low, moderate, substantial, and high. Based on Chicago's population, the city is in the CDC's "high" category of local transmission when there are more than 400 new COVID-19 cases diagnosed per day. The daily average case rate rose to 419 on Monday, health officials said.
For now, hospitalizations and deaths remain relatively low, but if they rise the city could consider capacity limits.
"If we start seeing those hospital numbers looking problematic, those are things - and especially the hospital capacity - that we would need to take some more specific action on," Arwady said.
That mask requirement goes further than current CDC guidelines, which merely recommend that masks be worn indoors. Officials said the move is necessary, but some business owners say enforcement could be difficult.
"Yes, it's going to be a little hectic enforcing it. People are so used to not wearing a mask," Sachez said. "The small little mom and pop restaurants are going to suffer because if you have a choice to sit outside on a large patio instead of indoor wearing a mask, you're going to go outside."
WATCH: Chicago businesses react to city reinstating indoor mask mandate
The businesses affected are not just bars and restaurants, but gyms, theaters, retailers and indoor venues, too.
"If we do go into an establishment, and we see a patron or two maybe not participating in the mandate and wearing a mask, we may give the business owner a warning," said Ken Meyer, the acting commissioner for the Department of Business Affairs and Consumer Protection.
The city said those warnings could then be followed up with citations, but officials added that the goal is not to punish business owners but rather to keep them operating without capacity limits.
"One thing that COVID did teach us that we can't be comfortable. We have to be ready for whatever is coming," Centeno said.
The Illinois Retail Merchants Association (IRMA) has released a statement regarding the City of Chicago's indoor mask mandate:
"Retailers have led the way on safety throughout the pandemic, pioneering numerous practices designed to protect the health of employees, customers and our communities. We support an indoor mask mandate as it is a measured approach that prioritizes public health while ensuring beleaguered retailers can continue to safely operate without further restrictions that would slow down hiring and interfere with economic recovery efforts," said Rob Karr, president and CEO of IRMA. "However, like previous mandates, this once again misses the mark by failing to place the responsibility on individuals who refuse to comply, especially after employees were threatened or attacked for simply trying to uphold public health orders. We call on the city to place responsibility for abiding by this mandate on individuals, not businesses."
Governor JB Pritzker also recently announced a mandatory mask mandate for all K-12 schools, which has sparked debate in the past few weeks as students prepare to go back for in-person learning.
Arwady added that city officials are not looking to reconsider starting school with in-person learning.
"We want to be careful and keep Chicago open, but we also want to keep schools open," she said.
The mask mandate will remain in effect until the city goes below 400 new cases a day for at least 1-2 weeks. After that, Arwady said we would revert back to a recommendation only. She also added that she hopes the mandate will bend the curve, preventing any further restrictions from becoming necessary.
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Officials also updated Chicago's Travel Advisory Tuesday, adding Minnesota, New Jersey, North Dakota, Pennsylvania, Rhode Island, Utah, Virginia, West Virginia - and the District of Columbia to the advisory after reporting 15 daily cases per 100k residents.
COVID-19 Travel Advisory Update: Minnesota, New Jersey, North Dakota, Pennsylvania, Rhode Island, Utah, Virginia, West Virginia – and the District of Columbia are being added to the advisory after reporting 15 daily cases per 100k residents. pic.twitter.com/YK34cvL09g— ChiPublicHealth (@ChiPublicHealth) August 17, 2021
The states and territories already on the list are: Alabama, Alaska, Arkansas, Arizona, California, Delaware, Florida, Georgia, Hawaii, Idaho, Indiana, Iowa, Kansas, Kentucky, Louisiana, Mississippi, Missouri, Montana, Nevada, New Mexico, New York, North Carolina, Oklahoma, Oregon, South Carolina, Tennessee, Texas, Utah, Wyoming, Puerto Rico, the Virgin Islands, Washington and Wisconsin.