The American Red Cross is calling all blood donors — and is pledging to email those who give blood or platelets through June 10 a $5 Amazon.com gift card.

“All blood types are urgently needed, but we’re facing a critical shortage of type O blood — the blood group most needed by hospitals,” Cliff Numark, senior vice president of Red Cross Blood Services, said in a news release this week. “Because it’s vitally important for hospitals to have type O blood available when every second counts, type O donors are urged to donate now and make a point to donate often.”

Since people with O negative blood are so-called “universal donors,” their donations represent the “go-to” in medical emergencies that leave no time to determine a patient’s blood type, the release explains.

While only 7 percent of people in the United States have O negative blood, 37 percent are O positive, according to the Red Cross, which says the latter is the type most often used in transfusions.

“Spring break schedules are known to decrease blood donations, and schools, including high schools and colleges, begin to host fewer blood drives in late spring as school years end,” the release said. “Because these drives account for about 20% of blood donations during the school year, less drives can also greatly affect the blood supply.”

In an email Thursday, Mary Brant, a New Hampshire-based spokeswoman for the Red Cross, said the shortage is nationwide. But there are plenty of upcoming local opportunities to help:

Jaffrey: May 30, noon to 5 p.m. at Jaffrey Bible Church, 133 Turnpike Road

Keene: May 31, 1 to 6 p.m. at the Norris Cotton Cancer Center at Kingsbury Pavilion, 580 Court St.

June 3, noon to 5 p.m. at St. Bernard Church, 185 Main St.

Peterborough: June 7, noon to 6 p.m. at town hall, 1 Grove St.

Winchester: May 29, 1 to 6 p.m. at the VFW Auxiliary, 35 South Main St.

People can also find donation opportunities or schedule an appointment by going to RedCrossBlood.org, calling 1-800-RED CROSS (1-800-733-2767) or using the Blood Donor App available for free on iPhone and Android devices.

More information about the Amazon cards, including terms and restrictions, is available at https://rcblood.org/2YxQnv2.