U.S. blood collection centers are facing a problem. As the number of blood donors has decreased, they rely more and more on teenagers as a source of donations. But according to a study, young people of 16 and 17 years old are more likely to bruise, faint or have other complications when they donate. As an obvious result, there is a low chance that these donors donate a second time.

The study was led by Dr. Anne Eder, executive medical officer of biomedical services at American Red Cross National Headquarters in Washington, D.C. and published in the May 21 issue of the Journal of the American Medical Association. It analyzed data from 1,8 million people from nine American regions.

Complications such as lightheadedness, loss of consciousness or bruising occurred in 10.7 percent of donations by 16- and 17-year-olds, 8.3 percent of donations by 18- and 19-year-olds and 2.8 percent by donors age 20 years or older, as Reuters informs.

The level of complication ranged from mild to moderately severe. In some cases, fainting caused a concussion, a cut requiring stitches, dental injury or a broken jaw.

It is not yet known why teenagers react in this way when they donate blood. Seemingly, that’s how their bodies respond to stress.

"Our study did not explore the underlying reasons for the increased tendency for reactions in young donors. But others have shown that teens respond to stress differently, related to age-related physiological and psychological differences," Dr. Eder said in a statement.