For example, older players were more likely to play on grass than AstroTurf, and so had more exposure to fertilizer and pesticides, Podell said.
Lehman said turf might have been a factor, but only as it relates to concussion. "AstroTurf in the '70s and '80s was a much harder surface, and I think almost all the teams have gone away from that now," he said.
"What my study looked at were players in the past," Lehman said. "I think studies need to be done, and are being done, on current players: professional, college and high school players." He and his colleagues acknowledge that the study was limited by the small number of deaths.
While the findings concerning brain disorders were grim, another study result was not. The researchers found that pro football players had a lower overall death rate than others.
"I can't really extrapolate to the rest of football -- current players or any other sports -- but they had definitely a better mortality experience than we would see in the general population," Lehman said.
The U.S. Centers for Disease Control and Prevention has more on concussion in sports.
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