Several types of plastics release chemical substances when they are heated, creating a high risk of contamination in the solid, liquid or gas that is near the material. Solid the food, liquid the water and air the gas for being more specific about health. When you buy mineral water that comes in a plastic format, it has the possibility of including phthalates as they increase the flexibility of the material. These chemical substances are considered a risk in several circumstances, with different intensity toward males or females and in different states of growth.
It's pretty usual to read in the plastic package for water or similar drink something like “do not place near direct sunlight”. Some sources cite that this has relationship with BPA, a chemical present in the PET bottles (transparent plastic, with the number one in the symbol of the bottle or simply stating the name) that could be released in the water with temperature.
If this is correct not only sunlight as the house has many sources of heat and it's very common to have PET bottles in several locations of it, also in vehicles or whatever place. And thinking a bit more, it looks like the reason is no other than the possible release of chemicals from the plastic as the mineral water could be constantly exposed to sunlight in its natural location without possible problems but an interesting decontamination of bacteria.
When the labels do not include the appropriate information has usually two explanations. First the government do not consider it mandatory and second who sells it pretends to not care about potential health issues as it could lead to less sales.
A good personal therapy to claim for the use of other materials without those potential issues, or to buy with the knowledge of who's hiding and why. Several locations go further banning the use of plastics and promoting reusable containers. This has the benefit of guaranteeing less risks for health while the environment do not receive massive quantities of plastics for recycling, with the worst scenario of not being recycled at all.
Another concern is the topic of dioxins, when the plastic is heated to high temperatures and they are released to the public environment with a good chance of ending in any organism. Burning plastics, for instance, continues as a common practice when the recycling industry despises plastics and they are still produced in massive quantities by who should have more responsibility and information of the hazards.