As the days heat up, the bottled water comes out. It’s in cars, trunks, sheds, at soccer practice and parties. It’s convenient to have a quick source of water wherever you are, but in recent years, microplastics have been discovered in bottled water. What’s more, chemists who study food packaging have found that these bottles are not inert in the heat and sunlight. Substances in the plastic—including the metal antimony, which is used as a catalyst in plastic manufacturing—can migrate into liquids stored within.

“There is a direct association between the migration rate and the temperature,” says Birgit Geueke of the nonprofit Food Packaging Forum, citing a 2008 study. Other substances, including bisphenol A, or BPA, have also been found leaking into some bottled waters as temperatures go up. “It shouldn’t be there—industry keeps telling us it shouldn't be there—but it’s being measured," says Olwenn Martin, an associate professor in health and environment at University College London. For these and other reasons, experts caution against exposing bottled water to sun and high temperatures.

What are plastic water bottles made of?

Many single-use plastic water bottles are made with polyethylene terephthalate, or PET, a mainstay of packaged drinks and one of the only plastics that can be recycled into food containers. But aside from the immense environmental footprint of single-use plastics, there are other reasons to be sparing in your consumption of bottled water. 

The bottles are made of more than just the plastic polymer, says Eleni Iacovidou, a senior lecturer of environmental management at Brunel University of London. Additives and other substances give the plastic particular qualities. In a 2022 paper, she and her colleagues reviewed the available evidence about what could be leaching into the drinks housed in PET bottles. “We started looking into the additives and how the additives behave,” she says. 

What they found was troubling. A number of research groups had observed substances migrating into bottled liquids, although the amounts varied and the health implications are unclear. Many groups specifically examined the effects of temperature on migration; in many warmer parts of the world, tap water isn’t safe, and paper authors were responding to safety concerns shared by many who rely on bottled water.

“The reality is that we shouldn’t expose the PET bottles” to high temperatures and sunlight, Iacovidou says. In such conditions, research suggests, they become less stable. 

What are the health effects of drinking water from hot plastic bottles?

Over the years, studies have found that a wide variety of chemicals, in a wide range of quantities, migrate out of single-use plastic containers into the food or drink inside. Some of these chemicals, such as phthalate esters that act as endocrine disruptors, are known to be unhealthy, while others are more of a black box still. “We lack evidence on the cumulative effect of these chemicals in human bodies,” Iacovidou says. “There is also the synergistic relationship of this with other chemicals that we are exposed to, so linking it to specific health implications—we're not there yet.”

As science catches up, experts say they are limiting food and drink in plastics where possible—and especially avoiding heating them up. Different foods interact differently with plastics, says Martin, who worked on the review with Iacovidou. “Tomato or fruit juices will favor certain compounds and increase leaching; heat will increase leaching,” she says. “But you also have fatty contents that may increase the leaching of other compounds.” 

What’s the best way to store water bottles? 

Bottled water should be kept somewhere cool and dark, Iacovidou says. But even in a cupboard in your home, temperatures could climb to levels that encourage migration, she notes. 

In a fact-sheet for consumers, the International Bottled Water Association “advises consumers to store bottled water at room temperature or cooler, out of direct sunlight.”

What temperature should worry you? In tests with a variety of liquids, some substances start to migrate around 20°C, which is 68°F, while others are more mobile above 30-40°C (86-104°F), Iacovidou says. “At this range…we start having migration of chemicals.” Iacovidou herself uses a metal bottle for carrying water, as does Martin. 

“If people can avoid putting anything in plastic in full sun, particularly with the kind of weather we're having at the moment,” says Martin, that’s a good move.