Nothing will slow you down or stop you like heat and humidity. As the temperatures rise in June and July, so does the number of heat-related problems experienced by triathletes. Most experts agree that your body will acclimatize to heat and humidity - mostly in the first two to three weeks of exposure, and maximally after about two months. However there is a genetic limit on how much you can acclimatize.
We can't all have the genetic gifts and ability that Dave Scott and Mark Allen had to tolerate the scorching heat and stifling humidity in the lava fields of Kona! But there are ways to improve the ability of the body to tolerate extreme temperatures and to move the process along by cautiously increasing your exposure to heat and humidity. The payoff will be safer and more tolerable running, swimming and cycling when training and racing. (click from link for full article)