A blood supply shortage in the US has reached a "crisis" level, according to the American Red Cross, which says it is the second time in its 150-year history that levels have been this low. Factors including wildfires and extreme heat have combined to create the current situation, Dr Courtney Lawrence, who works with the American Red Cross, told the BBC. The crisis has struck during summer, when the amount of blood needed by hospitals typically increases. "We don't have the ability to stockpile blood super far in advance because blood does have a finite shelf life, it does expire," she said. Donated blood has to be used within 42 days because blood cells degrade over time.