CHICAGO (CBS) — The combination of an extended cold snap, the recent holidays, and the ongoing flu season have led to a 30 percent drop in blood donations for LifeSource, prompting hospitals to keep a close eye on their supplies.

“The most unsafe unit of blood is the unit of blood I don’t have,” said Dr. James Perkins, director of the blood bank for NorthShore University HealthSystem.

He said the frigid weather is making the annual lull in donations around the holidays and flu season even worse.

“Everybody’s huddling around their radiators. It can become critical,” he said.

LifeSource said people have been putting off donating – even regular donors – until the weather improves.

School closures also can add to the problem, as one in five donations to LifeSource come from high school and college students, and during school blood drives.

That means concerns about blood inventory.

“A woman, let’s say, begins bleeding after delivering her baby, and then within a few minutes a car slides on the ice out on the highway, and we have three injured family members in the emergency department,” Perkins said.

When donations drop, hospitals must manage blood supplies carefully, and have to be ready for a sudden spike in need.

You can help, by donating blood as soon as possible.

“It’s the most direct thing that you can do for another human being.,” Perkins said.