The American Red Cross announced that it is experiencing the lowest number of people giving blood in the last 20 years, in what the organization says is an emergency shortage.

In all, the Red Cross says that the number of people donating blood has dropped by 40% over the last two decades, and that the shortage could worsen in coming months if winter weather or seasonal respiratory illnesses like the flu or COVID-19 cause people to cancel their donation appointments.

"The potential for severe winter weather and seasonal illness may compound the dire blood supply situation," said Dr. Eric Gehrie, executive physician director for the Red Cross, in a statement on Monday. "Donors of all types – especially those with type O blood and those giving platelets – are urged to give now."

Type O blood products are among the most transfused blood types.

The Red Cross said there was a nearly 7,000 unit shortfall in blood donations between Christmas and New Year's Day alone, which can have drastic consequences for people who are in need of life saving transfusions.

The average red blood cell transfusion is approximately three units, and a single car accident victim can require as many as 100 units of blood. About one unit of blood — roughly a pint — is collected during a typical donation.

According to the Red Cross, several converging factors over the past two decades have made it a challenge to keep and grow a motivated donor base, and meet patient needs.

Since the pandemic, some locations that used to host blood drives — like campuses or employers — have not hosted since, as many businesses have gone remote. Other factors include certain eligibility changes and changes in blood transfusion protocols at hospitals.

"One of the most distressing situations for a doctor is to have a hospital full of patients and an empty refrigerator without any blood products," said Dr. Pampee Young, chief medical officer of the Red Cross, in a statement on Sunday.

A person needs lifesaving blood every two seconds in the U.S., according to Young.

"Its availability can be the difference between life and death, however, blood is only available thanks to the generosity of those who roll up a sleeve to donate," she said.

The Red Cross announcement coincides with the start of National Blood Donor Month. The organization is urging people to schedule an appointment by downloading the Red Cross Blood Donor app, visiting RedCrossBlood.org or calling 1-800-RED CROSS (1-800-733-2767).

Individuals who give blood, platelets, or plasma this January have the chance to win a trip for two to the Super Bowl.