The American Red Cross is urging people to donate blood as national supplies reach crisis levels. 

Blood donations have fallen to a four-year summer low, and the Red Cross now has less than a one-day national supply of Type O positive blood, the most commonly transfused blood type.

The organization is limiting distribution of the type to hospitals, which can delay surgeries, trauma care and other treatments, Axios reported. 

"Every donation has the potential to help save lives, and we urgently need everyone who is eligible to make an appointment to give blood as soon as possible," said Chris Hrouda, president of Red Cross Biomedical Services, said in a statement. 

The current supply is failing to keep up with demand, which increases during the summer. Extreme heat, poor air quality and widespread foodborne illnesses are contributing to lower donor turnout this summer, the Red Cross says. 

This is only the second time the Red Cross has declared a national blood-supply emergency. The last time was January 2022, when COVID and staffing shortages were limiting the supply.