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.

Supply levels dropped by a quarter just last month. The crisis has struck during summer, when the amount of blood needed by hospitals typically increases.

That is because of an increased number of car accidents and more people spending time outdoors, resulting in more injuries that require blood transfusions, Lawrence said.

The American Red Cross – which is the largest single blood supplier in the US, providing about 40% – is currently distributing about 3,500 more units than normal during this time of year, according to Lawrence, who added that the organisation is having to dip “very, very low into our blood inventory”.

“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.