The American Red Cross today reported dangerously low levels of blood and platelet supplies, with 50,000 fewer blood donations than expected for the month of June. The Red Cross calls upon all those who are eligible to give blood to visit Red Cross or other collection centers to donate.

"Donations are down more than 10 percent across the country," said Stephanie Millian, a spokeswoman for the American Red Cross, told ABCNews.com. "We have almost half the amount of available blood on the shelves compared to last year."

Blood donations usually drop in the summer months, and the early heat wave, along with summer vacations, likely contributed to the severely limited supply. Millian also said that the midweek Independence Day holiday may have resulted in longer vacations, more businesses closing for the holiday and, in turn, fewer overall blood drives.

The Red Cross hasn't made a push for more donated blood since last summer when supply was also low.

"Elective surgeries can get canceled when blood shortage is low," said Dr. Louis Aledort, an internist and hematologist at Mount Sinai School of Medicine in New York. This can have a "big impact" on people who have taken time off of work for a surgery.

In such a case, physicians will bring in family members to donate blood, but rejection rates can be higher than for those who come in voluntarily.

"Sometimes they don't tell you they're on an antibiotic or have a disease that may get rejected as a blood donation," said Aledort. "They still have to go through the same screening."

While Millian said blood inventory is not at the level where doctors would have to delay surgeries, "we want to make sure they don't get to that point," she said.

All types of blood are welcome, but O negative is particularly needed. It is the universal donor blood type, and the type doctors turn to during emergencies, because it can be transfused to anyone in need. Blood types O positive, B negative and A negative are in particularly high demand, too, said the Red Cross.

"We need 38,000 units of blood every day to meet the needs of patients across the country," said Millian.

She said blood is needed across the country, but recent severe weather and flooding in the southern part of the country has put it even more in need.

To find blood donation centers in you area, call 800-RedCross or visit www.redcross.org.