Im not sure about this but it might be the type of plastic the bottles are made of. Look at the bottom of the bottle and see what number is inside the 3 arrows ( recycle symbol ) then look into that type of plastic.
Not all plastic bottles are the same, Some are suspected to release carcinogens ( cancer causing ), as well as cause shit like heart disease, obesity, something with hormones, probably other shit too I but I never dug toooo deep on it so Im not exactly well informed on this.
Some are also said to be very dangerous to a fetus. Notice how most baby bottles these days say "BPA free"? Oddly enough in the past these polycarbonate's were and still are used in things like baby bottles and large water jugs. If you have a water cooler in your shop look at the bottom, chances are it says "7" in the triangle, Im not saying they contain these "BPA's" but if I remember right that would be the type that would.
I cant help but to wonder if this plays at least a small part in the high percentage of cancers in both men and woman these days. Among other things of course.