A University of Cincinnati study has found that the hotter the liquid, the faster polycarbonate plastic bottles release toxins. Currently, reusable water bottles, baby bottles, and many other containers are made out of polycarbonate plastic. (For an easy guide to types of plastics and their dangers, click here.)
Researchers found that plastic bottles holding boiling water released bisphenol A, an environmental pollutant, up to 55 times faster than those containing room-temperature water. Baby formula is commonly boiled in preparation, so it's likely that very hot formula could leach high amounts of bisphenol A from baby bottles. However, the researchers do not know how much bisphenol A humans would have to consume before it became harmful.
Bipsphenol A is known to cause cancer and hormone irregularities and is "just one of many estrogen-like chemicals people are exposed to," said lead researcher Scott Belcher, "and scientists are still trying to figure out how these endocrine disruptors—including natural phyto-estrogens from soy which are often considered healthy—collectively impact human health."
While scientists figure out the effects, you might consider switching your plastic travel mug to stainless steel.