SEATTLE — Bloodworks Northwest has issued an urgent appeal for blood donors, saying collections have dipped sharply because of mounting heat and summer vacations.

There is a special need for O-positive, O-negative and A-negative blood types, the center said Thursday.

Formerly Puget Sound Blood Center, nonprofit Bloodworks Northwest serves patients in 90 hospitals in the Washington, Oregon and Alaska.

“Even though summer just started, we’re already at the point where inventories of the most-needed blood types are approaching critical level — meaning we have only a two- or three-day supply,” said Clint Kendrick, senior director of collections and recruitment.

“With record-breaking heat forecast over the next two weeks, we face serious shortages if donors do not come in to donate.”

Blood collections usually go down by 15 percent to 20 percent during the summer with schools and colleges on break and donors on vacation.

“But extremely hot weather disrupts our mobile drives at locations without climate-control because we suspend collections when temperatures go over 80 degrees to ensure the comfort and well-being of donors,” Kendrick said.

“All our donor centers are air-conditioned, but attendance still falls off as people stay home to avoid the heat.”

The need for blood is often higher in the summer from patients undergoing surgeries, emergency rooms treating traumatic injuries, people receiving cancer treatment and surgeons performing organ transplants, Kendrick said.

“To avoid a crisis, we’re asking donors who have not donated recently to schedule an appointment at one of its 12 centers or to find a mobile drive in their community close to where they live or work,” Kendrick said.

Information about locations and times can be found at www.bloodworksnw.org.

Appointments can be made online at http://schedule.bloodworksnw.org or by calling 800-398-7888.