When it comes to kidney donation, deciding you want to go through with it is actually the easy part. Most Americans couldn’t donate a kidney even if they wanted to, finds a new study presented at the American Society of Nephrology’s Kidney Week conference in Philadelphia.

Dr. Anthony Bleyer, professor of internal medicine at Wake Forest Baptist Medical Center, and his son Anthony Bleyer, Jr., an economics major at Wake Forest University, looked at data from a representative sample of 7,000 U.S. adults from the National Health and Nutrition Examination Survey survey. They discovered that a full 55% of the U.S. population would be ineligible to donate a kidney because of medical conditions—most of them preventable. Based on the criteria the Bleyers used, 15% of adults would be excluded due to obesity, 19% to hypertension, 12% to excessive alcohol use and 12% to diabetes.

That’s not necessarily because a medical condition has rendered the organs damaged. “Our number one thing is we want to preserve the health of the donor,” says Dr. Bleyer. “The donors have to be in really pristine condition.”

The more you weigh, the more strain you’ll put on your remaining kidney, and obese people also have a higher risk of complications after surgery and wounds that heal more slowly, he says.

The Bleyer team also looked at how financial concerns might prevent donation. Because kidney donors don’t receive compensation for lost work time in the U.S., 36% of healthy, medically eligible people make less than $35,000 per year, so they probably couldn’t afford to donate, the study found.

“Only 6% of patients who need a transplant get a living-donor kidney transplant,” Dr. Bleyer says.

That might not be because people are getting more stingy about their organs—but poorer and sicker instead.

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Write to Mandy Oaklander at mandy.oaklander@time.com.