Bloodworks Northwest has issued an urgent call for blood donors. High temperatures and the start of summer vacations are behind some critical shortages.

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Rising temperatures and the start of summer vacations are driving down blood donations, resulting in critical shortages, officials with Bloodworks Northwest said Thursday.

The agency, which supplies blood to 90 regional hospitals, issued an urgent appeal for donors after collections dropped about 20 percent — from 900 donations a day in the Puget Sound area to about 720 donations.

Although dips are typical in the summer months, the sharp drop so soon in the season is alarming, said Dave Larsen, communications director for Bloodworks Northwest, formerly Puget Sound Blood Center.

“If we have a four-day supply, we’re fine. No matter what happens, we’re prepared to cope with it,” he said. “When the supply goes down to a three-day supply in a certain blood type, it pops up as an alert.”

As of Thursday, the agency had a two-day supply for three of the most-needed blood types: O positive, O negative and A negative, Larsen said. “It’s in the critical area now,” he said. “So we start to pay attention.”

The shortage has not reached an emergency level, when rationing of blood products might be necessary, he said. The need for blood is often higher in the summer because of an increase in surgeries, traumatic injuries, people requiring cancer treatment and organ transplants, Larsen added.

But officials are also concerned about Seattle-area temperatures expected to hover in the high 80s and low 90s the next week.

Blood-donation centers and buses are air-conditioned, but some centers that host mobile drives are not. When temperatures inside reach 80 degrees, blood drives are canceled out of concern for keeping donors comfortable and safe, Larsen said.

In addition, summer vacations cut down the number of regular blood donors who show up, he added.

To help avert a crisis, Bloodworks is urging people who have not donated recently to schedule appointments. Information about blood-donation locations and times is available at or by calling 1-800-398-7888.