Cook County's new mask mandate, which states that everyone over 2 years old must wear a mask indoors, regardless of vaccination status, begins Monday.
The county first issued the mandate Friday, which was also the day Chicago's new indoor mask mandate took effect.
Under the new county requirement, masks must be worn inside multi-unit residential buildings and public places, including restaurants, movie theaters, retail establishments, fitness clubs and on public transportation, the Cook County Department of Public Health said.
The mandate applies to everyone who can medically tolerate a mask, regardless of vaccination status.
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"We are in a dangerous period, with the Delta variant surging, during which we must return to previous remediation measures," said Dr. Rachel Rubin, co-lead and senior medical officer of Cook County Department of Public Health, in a statement. "We have no choice but to mandate that people wear masks indoors to help contain this spread of the virus."
Businesses will be required to post signage, such as this CCDPH-created version, stating that masks are required.
"As the virus changes, we need to adapt our guidance and recommendations to keep residents safe," Dr. Kiran Joshi, co-lead and senior medical officer of Cook County Department of Public Health, said in a statement. "This variant is highly contagious and represents over 90 percent of our recent cases. We must act now."
Chicago unveiled its new mask mandate on Tuesday, one day after the city surpassed the metric of 400 average new cases of COVID-19 diagnosed per day - roughly 12 times the low of 34 that the city saw in late June before cases began to rise again.
Chicago, Cook County and all 101 other counties in Illinois were seeing "high" levels of transmission of COVID-19 as of Wednesday, according to the Centers for Disease Control and Prevention, which updated its guidance late last month to recommend that fully vaccinated people wear masks in indoor settings again in areas of the U.S. that are seeing "substantial" or "high" transmission - a reversal from its previous guidance.
Illinois Gov. J.B. Pritzker on Friday declined to give a specific metric at which the state might impose another indoor masking mandate, leaving the door open for further mitigations but deferring to local authorities to take action, even as he called the current COVID-19 surge fueled by the delta variant a "very dangerous moment."