The cold weather is keeping some blood donors away.

Memorial Blood Centers said its supply of O-negative blood is dangerously low across its 11 donor sites in Minnesota and Wisconsin.

Spokesman Larry Silber says the organization has been able to keep up with the demand for O-negative blood so far, but more donors are urgently needed. A combination of cold weather, vacations and the flu season has led to the decrease in donations.

The shortage is particularly critical because O-negative is a universal blood type that can be used in emergencies.

"We support the helicopter air ambulances with O-negative blood, because when they come across a person who needs blood in a critical fashion, they know they can safely give that blood," Silber said.

As important as O-negative donors are, they are also rare: According to the Red Cross, only 7 percent of people in the U.S. have O-negative blood.