Starting June 12, when someone in Chicago, New York City, San Francisco, Baltimore, or Washington, DC indicates on Facebook that they would like to be a blood donor, nearby blood centers will be able to send them notifications for local blood drives. The feature is intended to help blood centers address chronic and seasonal shortages in their blood supplies.

While everyone in the US is now able to sign up on Facebook to be a blood donor, only people living in those five cities will get notifications for upcoming blood drives. The rest of the country will have to wait a few months to start getting Facebook blood requests. Signing up on Facebook doesn’t guarantee that a user can donate; it’s simply an indication that the person would like to. People still need to check with their local blood donation centers to make an appointment and verify that they’re eligible to give blood.

Blood is always in high demand around the world. The Centers for Disease Control and Prevention estimates that about 17.2 million units of blood are transfused every year from 13.2 million donors in the United States. That’s a lot. But blood has a limited shelf life, and hospitals and health care centers are constantly in need of a fresh supply. “Recruiting donors is a universal problem,” Hema Budaraju, the product director of health at Facebook, says. She notes that shortages are especially acute in the summer and winter. “Blood banks used to recruit using traditional media, TV, radio, flyers, email, but we have to meet people where they are,” Budaraju says. In the US, “where they are” is on Facebook: about 69 percent of adults in the United States use the site, making it attractive to blood banks that are in need of donors.

Today marks the feature’s United States debut, but Facebook has had a similar system in place in India since 2017 and in Brazil, Pakistan, and Bangladesh since last year. Since then, “we’ve had more than 35 million people who put their hand up and said they want to help their community by donating blood.” Budaraju says.

Business Insider first reported on the new Facebook feature’s planned US rollout back in February, noting that there were existing concerns about the feature in India. There, individuals can directly request blood donations through Facebook, raising fears that the blood donation feature could exacerbate the existing illicit blood trade in that country.

That isn’t a problem in the United States. The only groups that can put out calls for blood using Facebook’s service are recognized blood donation centers like the American Red Cross, Vitalant, and the New York Blood Center.

Through Facebook, those organizations can send out Facebook notifications about blood drives and indicate what types of blood are most needed. The notifications will go out to people who have told Facebook they’re willing to donate and who are nearby. To figure out where your closest blood center is, Facebook will use your location (if you have location services turned on) or other markers on your profile (like your hometown) to determine which blood center is most convenient for you.