For the past 31 years, the Food and Drug Administration has prohibited blood donations from men who say they have had oral or penetrative sex with men. But that policy could change. The FDA began a two-day meeting Tuesday with an advisory panel that will review data and consider lifting the restriction.

The FDA questionnaire used to screen donors asks, “From 1977 to the present, have you had sexual contact with another male, even once?”

If a male respondent answers yes, his donation is deferred without a time limit, meaning that he is not allowed to give blood indefinitely. The FDA says the regulation is based on a concern that, as a group, men who have had sex with men (“MSM” in policy documents) have an increased risk of certain infectious diseases that can be transmitted via blood transfusion, such as HIV and hepatitis B.

Several key groups have criticized the policy. After the American Medical Association voted against the FDA’s lifetime ban last year, board member William Kobler said the policy “is discriminatory and not based on sound science.” In a statement released last month, the American Association of Blood Banks, America’s Blood Centers and the American Red Cross said the FDA’s indefinite deferral was “medically and scientifically unwarranted,” recommending instead a 12-month deferral.

Currently 9.2 million people donate blood in the United States each year, according to the American Red Cross. How might that number change if men who have had sex with men were no longer prohibited from giving blood?

The Centers for Disease Control and Prevention estimates that 1.8 percent of U.S. men are gay and 0.4 percent are bisexual based on the responses of more than 34,000 adults in a 2013 survey. According to the U.S. census, there are 120.2 million males in the U.S. age 17 and over (that’s the minimum age to donate blood without parental consent). So, based on the CDC’s numbers on sexual orientation (and setting aside any other possible restrictions on those individuals giving blood), one could estimate that about 2.6 million U.S. men are currently prohibited from giving blood. But that’s a misleading calculation.

The sexual orientation that a person states in a survey is not a perfect indicator of his sexual behavior. Some men who identify as heterosexual may have had sex with a man. And some men who identify as gay or bisexual might not have had sex with a man.

There’s evidence that the discrepancy is a considerable one. Although less than 3 percent of U.S. men say they are gay or bisexual, in a separate survey almost 9 percent said they have had a male sexual partner. Researchers at the Williams Institute, a think tank on sexual orientation and gender identity, used data from the 2008, 2010 and 2012 General Social Survey (GSS) to estimate that 8.5 percent of men say they have had at least one male sexual partner since age 18.

Based on that figure, there would be about 4.2 million more eligible blood donors in the U.S if the FDA were to lift the ban entirely.

Rather than allowing all men who have had sex with men to donate blood, the FDA might (like some other countries’ regulatory bodies) change the deferral period from “indefinite” to “five years” or “12 months.” (That means if you’re a man who wants to give blood, you have to abstain from sex with men.) The GSS also has relevant data here — 4.1 percent of U.S. men say they have had a male sexual partner in the past five years, and 3.8 percent say they have in the past 12 months.

But eligibility isn’t a guarantee that someone will donate. The American Red Cross estimates that 38 percent of the U.S. population is eligible to donate blood, but that less than 10 percent do so each year. Using that donation rate, the researchers at the Williams Institute estimate that there would be about 172,000 additional blood donors if the FDA changed its policy to a five-year deferral.

Overall, that’s a relatively small increase in the total blood supply — somewhere between 2 and 4 percent. But blood is important. The American Red Cross suggests that each blood donation could be used in lifesaving procedures on three individuals — so the Williams Institute estimates that a complete removal of the FDA restrictions on MSM has the potential to help save the lives of 1.8 million people annually.