The U.S. Centers for Disease Control and Prevention urged retail and service employees not to attempt to engage with customers who refuse to wear masks.

The CDC issued the guidance about workplace violence associated with the coronavirus pandemic on Monday and included tips about what to do and not do when confronted by someone refusing to cover his or her face.

“Don’t argue with a customer if they make threats or become violent. If needed go to a safe area (ideally, a room that locks from the inside, has a second exit route, and has a phone or silent alarm),” the guidance reads.

“Don’t attempt to force anyone who appears upset or violent to follow COVID-19 prevention policies or other policies or practices related to COVID-19,” it added.

The CDC also provided “warning signs” that should be spoken about during employee training.

“As part of training, employees often learn verbal and non-verbal cues that may be warning signs of possible violence. Verbal cues can include speaking loudly or swearing. Non-verbal cues can include clenched fists, heavy breathing, fixed stare, and pacing, among other behaviors,” the CDC said, adding that the more cues a customer is showing means the customer is more likely to become violent.

The guidance follows incidents of violence across the country stemming from confrontations over mask usage. Earlier in August, a New York couple was arrested after they allegedly beat a teenage employee of a Pennsylvania theme park in the face after he reminded them of the park’s mask policy. The victim had to receive surgery because of the magnitude of his injuries.

The United States has reported some 5.7 million confirmed cases of the coronavirus and more than 178,000 deaths since the pandemic began.