The blood supply maintained by the US Red Cross has fallen to "emergency levels" as donations have ebbed due in part to an early warm weather spell, a spokeswoman said on Tuesday.
"The current blood supply is at extremely low levels," Red Cross spokeswoman Stephanie Millian told AFP. "The weeks ahead are most concerning."
For the month of June, the Red Cross has collected 50,000 fewer donations than expected, meaning the blood supply has reached a precarious level that is slightly worse than last year, she said.
The group on Monday sent out a call for donations to ward off the shortage, saying the supply has reached "emergency levels."
"There is always a chance that a physician could have to postpone an elective surgery, or worst-case scenario, a physician might have to forego a more serious procedure because of shortage of blood," Millian said.
The Red Cross welcomes all blood types but is in particular need of types O negative, O positive, B negative and A negative.
Each day the Red Cross has to collect about 17,000 pints of blood to meet the current US needs.
Reasons for the current shortage may include an unseasonable warm start to the summer, which prompted some Americans to depart on vacation earlier than usual, Millian said.
Also, fewer donations are anticipated next month due to the Independence Day holiday on July 4, which will mean an extended holiday for some workers since it comes this year on a Wednesday.
Donations typically drop off in the summer. About 20 percent of donations are collected during the school year at high school and college blood drives, Millian said.
Earlier this month, the World Health Organization appealed for more people to donate blood regularly because the need for blood and blood products was on the rise worldwide.
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