Why You Shouldn't Drink Water From a Bottle Left in a Hot Car

You may have heard someone warn you not to drink water from a plastic bottle that sat in a hot car. But have you ever wondered why? Scientists say that heat can cause plastic bottles to release harmful chemicals and tiny pieces of plastic into the water inside. Knowing the facts can help you make smarter choices about what you drink.

Most disposable water bottles are made from a type of plastic called PET. PET is considered safe under normal conditions, but heat changes things. When these bottles get very warm — like sitting in a car on a sunny day — certain chemicals inside the plastic can move into the water. This process is called 'leaching,' and it happens faster when temperatures rise.

One group of chemicals found in some plastics is called phthalates. Phthalates are used to make plastic soft and bendable, but they are also known as endocrine disruptors, meaning they can interfere with the natural hormones in your body. Emily Barrett, a professor at Rutgers School of Public Health, explained that phthalates can quickly leach out of plastic packaging when it is exposed to heat. Hormones control many important things your body does, like growth and mood, so disrupting them is a serious concern.

Another chemical called BPA is used to make hard plastics and is also an endocrine disruptor. BPA has been linked to serious health problems, including issues with fertility, the thyroid, and even some types of cancer. Experts say it is best to limit how much of these chemicals you are exposed to. The government considers current levels in food packaging to be safe, but many scientists say reducing extra exposure is still a smart idea.

You may have seen posts online claiming that hot plastic bottles release dangerous chemicals called dioxins. However, scientists have debunked, or disproved, this claim. Dr. Nicole Deziel, an environmental scientist at Yale School of Public Health, confirmed that heated plastic bottles do not release dioxins. So while that particular worry is not real, there are still other concerns that are backed by science.

One real concern is microplastics — tiny, often invisible pieces of plastic that can break off from a bottle and end up in the water you drink. Heat speeds up this process, causing bottles to shed billions of microscopic fragments. Bottled water already contains some microplastics, but a hot bottle makes the problem worse. Scientists are still learning about how microplastics affect the human body, but early studies suggest they may cause damage to cells and DNA.

Research done on animals showed that microplastics can harm reproductive organs and cause other health problems. A professor at Harvard noted that scientists are seeing more data suggesting microplastics affect human biology. While researchers do not yet have all the answers, many experts agree that reducing your exposure to microplastics is a wise choice. This is especially true for young people whose bodies are still growing and developing.

Heat also creates a perfect environment for bacteria to grow inside water bottles. Dr. Bruno Silvester Lopes, a microbiology expert at Teesside University in England, explained that bacteria thrive in warm, moist places — especially when saliva or drink residue is left in the bottle. These bacteria can start multiplying in as little as 20 minutes, and within 24 hours there could be billions or even trillions of bacterial cells inside. Drinking that water can give you an upset stomach or make you seriously ill.

Many people have started switching to reusable water bottles, which is great for both health and the environment. Fewer disposable plastic bottles means less plastic waste in landfills. Reusable bottles are also less likely to break down and release chemicals when exposed to heat. The best choices are insulated bottles made from stainless steel, like a Stanley or Hydro Flask, or glass bottles.

Stainless steel bottles do not contain BPA and are much less likely to release microplastics. They are also more hygienic because they are less likely to develop tiny scratches where bacteria can hide. If you prefer a lighter plastic bottle, options like the Nalgene are still safer than single-use disposable bottles. However, experts remind us that any plastic can break down over time — and no matter what bottle you choose, washing it regularly is the most important rule of all.