Every two seconds, someone in the United States needs a lifesaving blood transfusion.

Burn and accident victims, patients undergoing heart surgery or organ transplants, and those receiving treatment for leukemia, cancer, and blood disorders, all depend on the availability of blood and platelets.

But the American Red Cross and the American Association of Blood Banks (AABB) are warning that blood supplies across the country are critically low, with less than a five-day supply of blood products to spare.

“Maintaining a safe and adequate blood supply is critical to the nation’s public health and a priority for the medical community ... it is indispensable and required in the treatment of millions of patients,” Miriam Markowitz, CEO of AABB, said in a press release.

Around 36 thousand units of red blood cells are needed in the United States every day, as well as seven thousand units of platelets and 10 thousand units of plasma.

To meet this demand, the Red Cross has to collect, on average, 14 thousand blood and platelet donations every day to meet the needs of 2,600 hospitals and blood transfusion centers across the country.

Over the past few months, the amount of blood donations has failed to match hospital requirements. Overall, they’re around 39,000 donations short.

Learn more: Where your blood donation goes »

Summer shortage coming earlier

The Red Cross says summer is one of the most challenging seasons to collect enough blood.

“Blood donations at high school and college blood drives account for as much as 20 percent of donations during the school year. During the summer months, these donors are not as accessible,” Kara Lusk Dudley of the American Red Cross told Healthline. “This year’s summer shortage is several weeks earlier than we’ve experienced in the past couple of years.”

Ideally, the Red Cross aims to have a five-day supply of blood available at all times to meet the needs of hospital patients. This also includes having a safety net to be prepared for emergencies when large volumes of blood products could be needed.

In a situation when blood products aren’t readily available, elective surgeries may need to be postponed. In a worst-case scenario, when a hospital doesn’t have enough blood supply, even serious procedures may need to be delayed.

“The Red Cross is working hard across the country to restock the blood supply,” Lusk Dudley said. “Beginning last month, the Red Cross began contacting donors via phone, email, text, app, and mail, alerting them to our increased need for blood and platelet donations this summer.”

Read more: FDA moves to end ban on gay blood donors »

How donating blood actually works

Although 38 percent of the U.S. population is eligible to donate blood, less than 10 per cent of that group actually does.

Blood donors must be in good general health and be feeling well, weigh at least 110 pounds, and in most states, be at least 17 years of age. They must also satisfy other eligibility requirements concerning medical conditions, lifestyle factors, and travel outside the United States.

The process of donating blood takes about an hour and includes undertaking a miniphysical, giving a complete medical history, the blood draw, and spending 10 minutes resting in the refreshment area.

The blood donation itself usually takes around 10 minutes depending on a donor’s level of hydration. The average adult has around 10 pints of blood in their body, and roughly one pint is collected during a donation.

Four types of transfusable products can be taken from blood: red cells, platelets, plasma, and cryoprecipitate (an extract rich in a blood clotting factor). Typically, two or three of these products are taken from one pint of donated blood. As such, a single donation can potentially help more than one patient.

This is important because a single car accident victim can require as much as 100 pints of blood.

Around 6.8 million people in the United States donate blood every year. To encourage donors to give blood over the summer, the Red Cross is offering $5 Amazon gift cards through August 31 to those who donate.

“It is important to remember that it is the blood products already on the shelves today that help save lives in an emergency or disaster situation,” Lusk Dudley said. “That’s why it’s so crucial for eligible donors to give blood or platelets regularly.”

Read more: Should we incentivize people to donate blood? »