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ORCHARD, J. W., and J. W. POWELL. Risk of Knee and Ankle Sprains under Various Weather Conditions in American Football. Med. Sci. Sports Exerc., Vol. 35, No. 7, pp. 1118–1123, 2003.Previous studies have found conflicting relationships between type of playing surface and injury in American football but have not taken into account possible variations in the surface conditions of outdoor stadiums due to changing weather.A total of 5910 National Football League team games between 1989 and 1998 inclusive were studied to determine associations between knee and ankle sprains, playing surface, and the weather conditions on the day of the game.There was reduced risk of significant ankle sprains (at least 7-d time loss) for games in natural grass stadiums compared with domes (indoor stadiums using AstroTurf) (RR 0.69, 95% CI 0.58–0.83). There was also reduced risk of significant knee sprains on grass compared with domes (RR 0.77, 95% CI 0.66–0.91), although most of this reduction was related to cold and wet weather on grass (RR 0.66, 95% CI 0.47–0.93 compared with hot and dry weather on grass). In open (outdoor) AstroTurf stadiums, cold weather was associated with a lower risk of significant ankle sprains (RR 0.68, 95% CI 0.51–0.91), significant knee sprains (RR 0.60, 95% CI 0.47–0.77) and ACL injuries (RR 0.50, 95% CI 0.31-0.81) compared with hot weather in the same stadiums. Weather did not have any significant effects on the injury risk in domes. The ACL incidence rate was lower during the later (cooler) months of the season in open stadiums (both AstroTurf and natural grass) but not in domes.Cold weather is associated with lower knee and ankle injury risk in outdoor stadiums (both natural grass and AstroTurf), probably because of reduced shoe-surface traction.