As concern grows over the long-term danger of concussions, San Francisco medical researchers have found that the brain injuries are more potentially debilitating than previously thought, doubling the risk of dementia even in people who suffer milder trauma without loss of consciousness.

Dizzying knocks to the head trigger dementia later in life in proportion to the severity of the resulting concussion, according to the study by UCSF’s Weill Institute for Neurosciences and the San Francisco Veterans Affairs Health Care System.

The risk is there, though, whether it was a knockout blow or a staggering shot that only left the victim woozy, said the study of military veterans published Monday in the medical journal JAMA Neurology.

“Even if you don’t lose consciousness, getting a serious bonk on the head seems to increase your risk of dementia.” said Kristine Yaffe, a professor in the UCSF departments of neurology and psychiatry and a co-author of the study. “Nobody has ever shown that before.”

The study comes amid continuing fallout over concussions in sports, particularly football. Some California lawmakers sought this year to make the state the first in the nation to ban organized tackle football for children younger than 12, but had to shelve the legislation last month amid an outcry from youth football enthusiasts.

Other sports have prioritized avoiding concussions with rule changes, such as barring using the head to direct the ball by soccer players below certain ages.

The researchers tracked 357,558 veterans nationwide for an average of 4.2 years. About half of the subjects, whose average age was 49, had been diagnosed with a traumatic brain injury. Of those, 54 percent had a concussion at some point during their lives, either in the military or as a civilian.

Yaffe said her team broke the subjects into three groups — each with about the same distribution of ages, races and general health.

The first group included those who were knocked woozy for as long as a day, but did not lose consciousness. The second group was made up of veterans who lost consciousness for less than 30 minutes as a result of their injuries. The third group consisted of those who were knocked out longer than 30 minutes.

The study found that the people who did not lose consciousness were a little more than two times as likely than those who never had a concussion to develop dementia. The second two groups were approximately three and four times more likely to suffer memory loss and other associated problems in direct proportion to the severity of their injuries.

“All three groups had an increased risk of developing dementia,” Yaffe said.

Deborah Barnes, a professor in the UCSF departments of psychiatry, epidemiology and biostatistics, said there was no difference between combat blast survivors who had served in the wars in Iraq and Afghanistan and the other trauma victims.

“Concussions occurring in combat areas were as likely to be linked to dementia as those concussions affecting the general population,” said Barnes, the lead author of the study.

Yaffe believes head trauma may cause abnormal proteins to accumulate, which hastens the development of neurodegenerative conditions such as Alzheimer’s disease. It also may cause the brain to be more vulnerable to other injuries, she said.

Although new, the results aren’t particularly surprising to researchers familiar with the volumes of research on the subject over the past two decades.

Last month, UCSF researchers reported a link between concussions and Parkinson’s disease. Last year, Danish and U.S. researchers reported that the likelihood of cognitive decline later in life goes up dramatically when people in their 20s suffer severe concussions.

The issue intensified in recent years when more than 4,500 former football players, some of them afflicted with Alzheimer’s disease or depression, accused the NFL of concealing the long-term dangers of concussions and rushing injured players back onto the field.

Pathologists recently found evidence of the brain-wasting disease known as chronic traumatic encephalopathy, or CTE, in the brains of several former players.

The NFL paid $5 million last year to a retired player suffering from amyotrophic lateral sclerosis, or Lou Gehrig’s disease, and $4 million to a former player with CTE. Up to $1 billion in payments is expected over the next few years.

The NFL and many other sports organizations have developed concussion protocols, including rules that require inactivity by players while symptoms persist.

Yaffe said the evidence of a connection between cognitive decline and head trauma has been accumulating for years, and that now there is evidence that even a mild concussion can be dangerous.

“It doesn’t mean for sure it will happen to an individual. It’s just a risk factor, another piece of the puzzle,” she said.

The takeaway, Barnes said, is that “more needs to be done to reduce the likelihood of traumatic brain injuries.”

Peter Fimrite is a San Francisco Chronicle staff writer. Email: pfimrite@sfchronicle.com Twitter: @pfimrite