She told me I should be concerned about plasticisers leaching out of the plastic due to heat build-up in the car.
"Bottled water" bottles are typical made of polyethylene (does it have a #1 on the bottom of the bottle?). Polyethylene can leach phthalates, which are known hormone disruptors. Polyethylene does not contain bisphenol-A.
Or are you talking about "water bottles" like a Nalgene?