The American Red Cross declared an emergency blood shortage on July 9, calling for donations of all blood types—but especially type O—as the agency scrambles to make up for the Fourth of July holiday week.

An adequate supply of blood in the summer is already challenging, considering high school and college students account for up to 20 percent of donations during the school year, officials explained. And with travel and vacations on top of that, 550 fewer blood drives were held during the holiday week than an average week, the Red Cross said.

“Blood donations are currently being distributed to hospitals faster than donations are coming in—we need both new and current blood donors to make an appointment as soon as possible to help patients battling illness and injury,” Chris Hrouda, president of Red Cross Blood Services, said in a statement. “Unlike many other lifesaving medical treatments, blood donations cannot be manufactured and stockpiled. Red blood cells have a shelf-life of only 42 days and platelets just five days. Each donation, each day, makes a difference.”