Raleigh, N.C. ? Heat-related football deaths at all levels have steadily increased, replacing direct fatal injuries as the sport’s biggest on-field safety concern.

Eight football players died nationwide last year due to injuries, while another three died from heatstroke, according to a study released Thursday by the University of North Carolina. Twelve more deaths were due to natural causes aggravated by exercise, such as a heart attack.

The number of injury deaths reflected a substantial drop since stricter rules about tackling and blocking were enacted in the mid-1970s, when fatalities regularly reached double digits.

Heatstroke deaths nearly doubled in the 1990s over the previous decade, the study showed.

“We are concerned about all these deaths, of course, but especially those that resulted from heatstroke, which are almost always preventable,” said Frederick Mueller, a professor and chair of physical education, exercise and sport science at North Carolina.

All three heat-related deaths occurred within one week at the end of July and start of August last year.

Minnesota Vikings tackle Korey Stringer collapsed during practice July 31 and died the next day, as did Travis Stowers, a high school player near Michigantown, Ind.

Less than a week earlier, Eraste Autin, an incoming freshman at the University of Florida, died of complications of heatstroke. He collapsed at the end of a voluntary summer conditioning session and was in a coma for six days.

Mueller is also the chairman of the American Football Coaches’ Committee of Football Injuries and directs the National Center for Catastrophic Sports Injuries, based at North Carolina.

He said 20 football players suffered heat-related deaths since 1995. In the previous five years, only two football players’ deaths were attributed to heat. From 1980 to 1989, 13 players died of heatstroke.

“The trend is really up for those, and it’s a real concern,” he said Wednesday. “Coaches, players and even parents need to remember how to prevent these tragedies, and that’s not hard to do.”

Mueller said the number of “direct” injury-related football deaths has generally held level in recent years. Forty players died of “direct” causes from 1995 to 2001; 50 died in the 1990s overall. In the 1980s, 71 “direct” deaths occurred.

Seven of the eight players who died from injuries in 2001 played for high school teams, and one was in a Pop Warner league. Six of those deaths resulted from brain injuries, one from a fractured neck, and the other from a ruptured spleen.

Injury-related deaths have dropped considerably since rule changes, adopted in 1976, prohibited use of the head as a first point of contact while blocking and tackling.

From 1976-2001, 162 players died from injuries; During 1966-75, 213 players died.

Mueller said team officials should keep close track of the temperature, and schedule regular water and cooling-off breaks during hot weather. He also suggested shorter practices and non-contact drills without helmets, as well as holding practices at cooler times of day.

At Florida, where this season’s conditioning sessions have already begun, flyers showing the warning signs of heatstroke were posted in each player’s locker as well as around the locker room, team spokesman Zack Higbee said.

Gators strength and conditioning coordinator Rob Glass said all of Florida’s new athletes, not just football players, meet with the school’s nutritionist to discuss how each athlete reacts to heat.

“We spend a lot of time with them one on one, trying to educate them,” Glass said.

Before Autin’s death, the football team was already holding its voluntary workouts early in the day. The start time was moved up gradually to match the hour of regular practices that start in August, acclimatizing players to the weather.

Officials with the Indiana High School Athletic Association are urging coaches to pay special attention to heat-related problems during preseason conditioning. Health and safety data being sent to schools includes extra material on how to measure heat and humidity.

“Hopefully, we can increase the awareness and the attention given to outdoor conditions at practices and contests,” IHSAA assistant commissioner Robby Cox said Wednesday.

The annual survey of football deaths, started at Yale University in 1931, looks at amateur and professional football players across the nation. It is based on news reports and on data from the NCAA and the National Federation of State High School Associations.