- Studies show that leaving plastic water bottles in hot environments, like a car, causes them to release billions of microplastic particles and harmful chemicals into the water.
- Heat and UV exposure accelerate the breakdown of plastics, while bottles stored in cool, dark places shed little to no microplastics.
- To reduce exposure, experts recommend avoiding long-term storage of bottled water in hot conditions and opting for reusable glass or metal bottles instead.
Worried about your exposure to microplastics? Then you’d better think twice before drinking that water that’s been sitting in your car for days.
Microplastic research has gone into overdrive in recent years as more people recognize how much they ingest and its potential health risks. Studies show it could cause everything from liver damage to inflammation and oxidative stress. And none of that is good. The problem is that avoiding microplastics feels like a game of whack-a-mole: as soon as we find and eliminate one source from our routine, another pops up. However, there’s a new source that seems fairly easy to avoid: that old, hot plastic water bottle sitting in your cup holder for who knows how long.
In 2023, researchers examined how different temperatures and UV exposure affect four common plastics, the same types used in packaging and bottles that could easily be left in a hot car. They discovered that although each type of plastic degraded slightly differently, all of them released tiny microplastic fragments, and not just a few. During testing, the plastics released billions of microscopic particles into the water. In contrast, the bottles kept in a refrigerated environment as the control group released almost none.
And this was far from a one-off conclusion. In 2014, researchers from the University of Florida reached similar results with their study, which examined how chemicals were released from 16 brands of bottled water stored at 158°F for four weeks. They described their findings as a “worst-case scenario” for human consumption.
“If you store the water long enough, there may be a concern,” soil and water science professor Lena Ma, and lead researcher on the study, noted at the time. The team noted that they would advise people against leaving bottled water in a “hot garage for weeks on end or in your car all day during the summer.”
Generally, heat and plastics don’t mix well. As Food & Wine previously reported, a 2025 study published in the journal Science of the Total Environment examined the varying levels of microplastics found in 31 popular drinks in the United Kingdom. The study found that hot beverages are the primary source of microplastic exposure, with hot tea containing about 60 microplastic particles per liter and hot coffee around 43. However, iced versions of these drinks had significantly fewer microplastics, with iced tea carrying 31 particles per liter and iced coffee 37.
This doesn’t mean you need to go thirsty if that day-old water bottle is your only option. As Jaime Ross, an assistant professor of neuroscience at the University of Rhode Island, shared with the Washington Post, it’s more about avoiding repeated exposure, because not only could you be ingesting microplastics, but heat “could contribute to the growth of pathogens.”
As for what can be done, the researchers of the 2022 study noted that “Product labels, incl. bottled waters, should contain information regarding the composition of its packaging and inform about the possibility of releasing bisphenol A into water.” However, if you don’t want to wait for companies to act, you can do so yourself by storing your water in cool, dark places or switching to reusable glass and metal bottles whenever possible. Then, follow this advice to reduce your exposure to microplastics at home, so you can stop playing microplastic wack-a-mole, too.
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