The New York Blood Center has declared a blood emergency for the metropolitan area, including Long Island, after supplies of some blood types have dwindled to just two days.

Harvey Schaffler, the agency’s executive director of donor marketing, said blood agencies in the metro area normally have at least a seven-day supply of blood from donations totaling 10,000 weekly.

But right now, he said, there is less than a two-day supply of O-negative and B-negative blood. O-negative donors are considered “universal,” according to a news release Monday from the blood center, and it is the blood type needed most in trauma and emergency situations.

The last time supplies were so short was during the Feb. 9 snowstorm, Schaffler said.

Supplies usually become “fragile” beginning around the July 4 holiday through Labor Day, he said, in large part because blood drives at local colleges — about 20 percent of the overall supply — languish over the summer break.

In Suffolk, County Executive Steve Bellone and Health Commissioner Dr. James Tomarken pleaded for new blood donors to help combat the holiday shortage. They also announced a schedule of blood drives in county buildings beginning this week.

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“School’s let out, and people are preoccupied with vacations,” Bellone said at an event in Hauppauge. “But we need people to step forward. And if they do, they in a real and significant way will become a hero in someone else’s life.”

Cassidy McCarthy, 5, of West Babylon, who has received 13 transfusions in her battle against stage four kidney cancer and is scheduled to begin kindergarten in September, appeared with Bellone.

Her father, Daniel McCarthy, said, “The blood she gets during chemotherapy is like a super power and it helps make her the hero she is.”

According to the blood center, the donation process takes less than an hour and a single donation can be used to save multiple lives. It adds that nearly 2,000 donations are needed each day in New York and New Jersey alone. About one in seven hospital admissions requires a blood transfusion, and with a limited shelf life, blood supplies must be continually replenished.

To donate blood or for information on how to organize a blood drive, visit nybloodcenter.org or call 800-933-2566.

With Rick Brand