The number of people donating blood is lower than normal for this time of year, say American Red Cross officials. And that translates into a low blood supply.

Red Cross officials say O negative blood -- the blood type often transfused to trauma patients and premature babies -- is at just a 16-hour supply. Nationally, the blood inventory is at a less than a two-day supply, instead of the preferred seven-day supply.

"We've been low throughout the summer, and we can't determine a specific reason," said Erin Behrens, marketing coordinator for the American Red Cross Mid-America Division in Omaha. "Usually, we see an increase in donors in September and October, and we're not seeing that this year."

Officials stress that blood is always needed.

"People see a national disaster as motivation to donate," Behrens said. "Last year after Hurricane Katrina, we saw an increase of donors. But every day, there is an ongoing need for blood."

Of the 60 percent of the U.S. population eligible to give blood, only 5 percent choose to do so, Behrens said.

A blood donor card or driver's license are required at check-in. Donors must be at least 17 years old (16 in Iowa and Kansas with signed Red Cross parental consent form) and must weigh at least 110 pounds.