Red Cross declares emergency blood shortage after national inventory falls by 25% in July

The American Red Cross said Monday that it is experiencing an emergency blood shortage, and extreme heat may be partly to blame.

In a press release, the organization said its national blood inventory fell by more than 25% since July 1.

Rolling heat waves and record temperatures have impacted more than 100 drives in the last month in every state where the Red Cross collects blood. This has led to drives ending early or being canceled and fewer people donating and has contributed to a shortfall of more than 19,000 donations in July, based on projected hospital need, according to the Red Cross.

This is in addition to other factors that typically limit blood donations during the summer, such as travel and seasonal activities, the group said.

The Red Cross also suspects that August also will threaten replenishment of the available blood supply due to the start of peak Atlantic hurricane season, which often leads to flooding and power outages that can cancel blood drives and otherwise lead to low donor turnout.

The Red Cross said it supplies about 40% of the nation's blood donations.

"When we start to have a shortfall in donations that has a really significant impact," Dr. Baia Lasky, division chief medical officer for the Red Cross, told ABC News. " This time of year is challenging for us. The demand for blood is constant throughout the year whether its trauma patients, cancer patients."

Donors with Type O blood are the most needed right now, according to the Red Cross, for two reasons. Type O is the most common blood type, meaning that most people in need of blood will need type O blood.

Secondly, those with an O negative blood type are so-called "universal donors," meaning their donations can be used in transfusions for any patient with any blood type.

Because the supply of O blood is currently very low, the Red Cross said it had had to reduce distributions of that type to hospitals.

"We will welcome anyone of any blood type, but O negative is considered universal and O positive is nearly universal," Lasky said. "It's the most transfused blood type and for patients with unknown blood type for emergency transfusion."

She also said that the Red Cross is looking for platelet donations, which have a shelf life of five days, and are often critical for trauma and cancer patients.

This is not the first time the American Red Cross has warned of the risk of a blood supply shortage. Earlier this year, the organization it was seeing the lowest number of people giving blood in 20 years, something it is still experiencing.

According to the Red Cross, only 3% of age-eligible people, or about 6.8 million Americans, donate blood every year.

In an attempt to increase donations, the Red Cross is offering anyone who donates blood by August 31 a $20 Amazon gift card by email.

"As you go about your summer plans, remember there are those who are in hospitals," Lasky said. "Let's save lives and we really need people to come out and donate."