As schools across the U.S. decide whether to reopen this fall, many are left wondering how to know if it’s safe. Public health experts say virus rates in the community should be low, but there’s little agreement on a specific threshold or even a measurement. The federal government has largely left it to state and local governments to decide when it’s safe to bring students back to the classroom. The result is a patchwork of policies that vary widely by state and county. Minnesota, for example, suggests fully in-person classes if a county’s two-week case rate is no higher than 10 per 10,000 people. In Pennsylvania, it’s considered safe if a county’s positive virus tests average lower than 5% for a week.