Between 1994 and 2009 roughly three high school and college football players per year died in practice. Considering that over a million high schoolers alone play football that may not see like an alarming figure (it really is a truly tiny percentage of all people participating). But it is triple the number of deaths compared to the previous 15 years.
As for why, researchers from the University of Georgia examined a database cataloging the temperature, humidity, time of day, height, weight, and field position of each of the 58 players who died from overheating. They found that in the eastern part of the US, where most of the deaths occurred, the heat index in the morning was consistently higher from 1994-2009 than for the preceding 15 years. More than half the players died when practiced ended before noon, with most of the deaths happening in August. Georgia alone had the most deaths of any single state, at roughly 10%.
Also apparently a factor, football players getting bigger, something which has been ongoing since 1980, the researchers note. 86% of all players who died during practice were linemen, who tend to be larger than other positions. Nationally, in all levels of football participation, both professional and amateur, 40% of linemen are actually obese.
Again, we're not talking huge numbers of deaths here. Indeed there are far more dramatic examples of the type of changes global warming and the unhealthy standard American diet is already bringing about—if not the most overweight and obese nation on Earth, the US is right at the top. But it is all interesting to note, an example of perhaps little-noticed environmental and dietary factors filtering down into unexpected areas of life.
The two images above (from this report) compare the obesity levels throughout the US. Notice that now the thinnest state is as fat at the fattest state 20 years ago.