My wife has always had a fear of drinking water bottles that have been in the car for any amount of time.  If we take our son to the park, and the bottle of water we brought from the house sits in the car while we play - she thinks that the heat releases harmful chemicals from the plastic bottle into the water.  It's not crazy, a lot of folks believe this theory.  This website claims that heat can release 16 dangerous substances into your drinking water, and it's only one of hundreds that claim this or something similar.

Dr. Lance Williams, Professor of Biology at the University of Texas at Tyler recently went on record with CBS 19 to say that this is not true.  He cited a 2014 study conducted at the University of Florida.  20 bottles of water were left for several weeks in temperatures up to 158 degrees Fahrenheit.  Out of the 20 bottles tested, only one had trace amounts of BPA (bisphenol A) in the water.  According to the good doctor:

if you’re leaving a bottle of water in a car for a day, there's really no risk of any chemicals leaching into the water,"