The American Red Cross says blood donations have dropped significantly and is warning an "emergency situation" might develop within just weeks.
The organization has put out an urgent appeal for donors of every blood type.
"Donations through the Red Cross are down approximately 8 percent over the last 11 weeks, resulting in about 80,000 fewer donations than expected," it said in a release. "The number of donors continues to decline."
Part of the impending shortage is a result of this year's calendar, the agency said, explaining that because the Fourth of July fell on a Friday, numbers of volunteers and donors going on vacation reduced the quantity of blood drives the Red Cross was able to schedule.
"In an average summer week, about 4,400 Red Cross blood drives are scheduled, compared to Independence Day week when only 3,450 drives occurred," the agency's statement said.
Summer has always a problem period for blood donations, it said, as regular donors are off on vacations or off of school.
While urging "all eligible donors" to respond to its appeal, the Red Cross said type O negative, A negative and B negative were most needed.
The "universal" Type O can be administered to anyone requiring a blood transfusion, while Types A and B negative can safely be given to to Rh positive or negative patients.
Blood platelets, the vital blood clotting factor often needed by burn victims, cancer patients and bone marrow transplant patients, are also in short supply, officials said.
A constant supply of platelets is needed because they must be transfused within 5 days following donation, unlike blood donations that, if refrigerated, can be kept for 42 days under Food and Drug Administration rules.
Blood donation take about 45 minutes to an hour, a people can safely donate every 56 days, the Red Cross said.
Individuals who are 17 years of age (16 with parental consent in most states), weigh at least 110 pounds and are in overall good health may be eligible to donate blood, it said.
July 13 was the halfway point in the agency's "100 Days of Summer. 100 Days of Hope" campaign.