"We usually don't see these types of record heats," Numark said. "I'm concerned that given that this is the beginning of the summer it may bode ill for the rest of the summer."
National blood supplies for the Red Cross — one of the nation's largest blood-collection agencies — sit at about two days' worth of blood, which is low but not critically so, according to Stephanie Millian, the Red Cross' director of communications for biomedical services.
There is no specific temperature at which blood centers must shut down a drive, but Deininger said that the Red Cross' policy is to judge situations on ad hoc basis, taking into account the comfort of both donors and staff, who wear protective clothing when they withdraw blood.
According to the most recent blood-collection and -utilization survey by the U.S. Department of Health and Human Services, a total of 135 hospitals — 8.4 percent of all hospitals — had postponed elective surgeries at least once in 2004 because of blood-inventory shortages. Nationwide that year did see a net blood surplus: Approximately 4.7 percent more blood was collected than was transfused into patients, the report said.
This year's heat wave has exacerbated an annual decline of blood supplies each summer, as office workers go on vacation and students are off from school, limiting blood banks' access to some of their largest populations of donors. Furthermore, the rising mercury often dissuades potential donors from leaving their homes to donate blood.
"The summertime is a protracted time where it isn't business as usual," said Harold Kaplan, the director of transfusion medicine at Columbia University Medical Center. "We rely heavily on volunteer donations, and as you might imagine summertime is a time when school's out, families take vacations, people travel and the routines change."
Not all blood centers are facing the shortages of the Red Cross' Arizona and Southern California regions this summer.
The Associated Regional and University Pathologists, which procures blood for the University of Utah, currently has a two-day supply of blood — relatively large for this time of year — thanks to promotional activities including the raffling of a 42-inch plasma television, an iPod and $50 gas cards.
Because of these efforts, "[summer] reserves are the highest they've ever been," group spokesman Lance Bandley said.
And after a Fourth of July drive and an event at US Airway's Center, the home of the Phoenix Suns, the United Blood Services of Arizona also has a relatively strong two-day supply of blood, according to Sue Thew, a spokeswoman for the organization.