The blood supply is critically low -- less than a one-day supply for a key blood type, the New York Blood Center said in calling for donors.
Hospital demand for blood has been high but donor turnout lower than normal, because of the elections, holidays and the later opening of schools, which contribute 25 percent of the blood supply, officials at the nonprofit center said.
Lately, the center has only about 2,000 units of blood on any given day, far from the week's supply of 10,000-plus units needed to handle the normal load of emergencies, said Harvey Schaffler, the nonprofit's marketing director.
"If there's a mass trauma situation," he said, "we're on the edge."
Critically, the stock of O-negative, considered the "universal donor" blood, has fallen to less than 100 units on a given day -- or less than a day's supply, officials said.
It's the blood type used in many cases because it can be given to most people with other blood types without causing serious reactions. A-negative and B-negative types are also low, the New York Blood Center said.
The center has contacted past O-negative donors for help, but this type of blood is shipped out almost as soon as it comes in, after a two-day period to test and package the blood, Schaffler said.
"It's too tight for comfort," he said.
The nonprofit center will have donor sites open this weekend, with locations listed at nybloodcenter.org and operators setting up appointments at 800-933-BLOOD (2566). Officials have also urged companies, houses of worship and others to host blood drives.
Suffolk County Executive Steven Bellone added his voice Friday to the cause by urging residents to donate.
"It is imperative that we do all we can to increase the supply to help save lives," Bellone said. "Suffolk County employees are very active in donating blood, and I am asking for all Suffolk County residents to consider this life saving opportunity."
A human has eight to 12 units of blood, but it can take many more units to save a person from bleeding to death, Schaffler said:
"You can have one accident victim come in and require 40 units of O negative. As quickly as you're giving the person O negative, it's draining out."
Blood donors receive free mini-medical exams on site, including getting their temperature, blood pressure and hematocrit level checked. Donors must be at least 16 years old, weigh at least 110 pounds and be in good health. People older than 75 may donate with a doctor's note.
The entire process takes about an hour.