If your talking about the thin plastic bottles that store-purchased water comes in, then yes.. if you leave the water in your car for a week, under hot conditions, there will be trace evidence of that plastic breaking down into the water.

Those chemicals have been linked to a couple types of cancer, but primarily to breast cancer.

Now.. what they don't say, is that the concentrations of that chemical would need to be like 12,0000 times greater than what they found (in 1 week old water) to really hurt you.

To be honest, I keep some Datrex water (3 year lifespan I think?) in the car for emergency, and just carry a Nalgene bottle around with me every day that I empty out and re-use.

I think you should probably be much more concerned with the quality of the water, than the container in my opinion.

Rod