With the N.F.L. regular season just two weeks away, and with thousands of youth, high school and college teams on the practice field, everyone involved in the game faced alarming new knowledge of its risks on Tuesday.

A new study found that one in four former N.F.L. players who died over a recent six-year period — and likely many more — had the incurable, progressive brain disease known as C.T.E.

Much remains unknown, including why some athletes get severe chronic traumatic encephalopathy, while others show no signs at all. But the study, the first to establish just how widespread the disease is in professional players, raises new questions for athletes and their families about whether the risk is enough to stop playing.

Here are five takeaways from the study and reaction to it.

Researchers were conservative in reporting the study results. It’s probably far worse.

Because its symptoms are similar to those of dementia or Alzheimer’s disease,
C.T.E. can be definitively diagnosed only by examining the brain after death.

To try to figure out how common it is among professional football players, researchers from Harvard Medical School, Boston University and elsewhere sought to obtain the brains of every former N.F.L. player who died from 2016 to 2021.

The study’s main finding was simple math: At least 215 of the 878 former players who died in that time period, or 24.5 percent, had C.T.E. But because researchers could obtain only 235 brains, the actual prevalence could be far higher.

Of the 643 brains that were not examined, some of those, maybe many of them, likely had C.T.E. too.

Read more about the study and how it was conducted.

What do the N.F.L., its players and their union have to say?

The league and its players’ union each released a statement when contacted before the study’s publication, but they did not make public statements after The New York Times reported it.

The league mentioned its support of research and funding for player health, and said it had “significantly driven down the number of concussions that occur in N.F.L. games” in recent years.

Those numbers, and what the N.F.L. counts as a concussion, are hard to verify.

The N.F.L. Players Association said the findings “should serve as a call to action across the entire football ecosystem.” It vaguely recommended more research, improved access to care, better treatment options and support of those affected “by the long-term impacts of the game.”

C.T.E. is a workplace injury, but it’s a workplace where regulators are loath to tread.

Former officials at the Occupational Safety and Health Administration, the federal agency tasked with enforcing workplace safety, called the number of players who likely have C.T.E. astoundingly high — “apocalyptically high,” as a former chief scientist for the agency, Adam Finkel, put it.

Yet many of those same officials said the league remains too powerful and popular to regulate. “The public would not welcome OSHA telling the N.F.L. how the game has to be played,” said David Michaels, who led the agency from 2009 to 2017.

Before he joined the Supreme Court, Justice Brett M. Kavanaugh argued that inherently dangerous entertainment can’t be regulated with the same approach as what governs other industries. And the agency has largely followed that tack.

“In most cases,” officials said in a statement, “OSHA does not take enforcement action with regard to professional athletes.”

Read more about regulating N.F.L. player safety.

Is anything likely to change as a result of the study?

Not in the N.F.L. Change in football may come from the bottom up, as parents and youth leagues adapt to medical advice that children delay playing tackle football or other contact sports that threaten the brain.

The less exposure to head impacts over the course of a lifetime, the less chance of C.T.E., researchers say. This might boost other sports that do not involve collisions, or lower-contact forms of football, like flag football.

The N.F.L. and its union are certain to talk about some of those recommendations from researchers, and how they might be applied to the professional game, when negotiating the next collective-bargaining agreement.

There are some ways to reduce the risk if your child plays tackle football.

The simplest way to protect against C.T.E. is to avoid impacts to the head. Though C.T.E. does not manifest in everyone (scientists are unsure why), the accumulated force of head impacts over a lifetime is the key factor.

Those impacts can cause concussions, but they also include the countless bumps that happen on every play. They all add up.

Researchers warn parents to be wary of coaches who push young players to hit and tackle, as well as those who push the latest helmets, mouth guards or other pieces of equipment as a panacea.

C.T.E. researchers have found no evidence yet that those effectively reduce brain injuries.

Read more about how to minimize the possibility of C.T.E.