The American Red Cross (search) is issuing an urgent call for blood donations, saying parts of the country have less than a day's supply and some hospitals are postponing elective surgeries to conserve.

People with Type O-negative blood, the universal blood type, are particularly needed, but the Red Cross urged anyone who's eligible to donate: people in good health age 17 or older, who weigh at least 110 pounds.

Summertime blood shortfalls are common, as regular donors go on vacation. The Red Cross calls this year's shortage particularly bad, reporting less than 46,000 units of blood on shelves Thursday. Anything below 60,000 units is considered a critical shortage; an optimal inventory is 100,000.

Two-thirds of Red Cross regions report having less than a day's supply of O-negative blood, said spokeswoman Stephanie Millian.

The Northeast seems particularly hard-hit, and the Red Cross had advised hospitals in some states to postpone non-emergency Type O transfusions as a result, she said.

The Red Cross supplies about half the nation's blood.

America's Blood Centers (search), which supplies the other half, says 20 percent of its reporting centers have a day's supply of blood or less. It hasn't yet issued a nationwide donor appeal, saying its supplies are creeping up slowly since the July 4th holiday.

To donate: Contact the Red Cross at 1-800-GIVE-LIFE or http://www.givelife.org, or America's Blood Centers at 1-888-USBLOOD or http://www.americasblood.org.