Bottled water contains a surprisingly long list of potentially harmful compounds, from microscopic plastic particles and industrial chemicals leaching from the bottle itself to heavy metals and disinfection byproducts. Some are present at trace levels unlikely to cause harm on their own, while others accumulate over time or spike dramatically depending on how the water is stored. Here’s what’s actually in there and why it matters.
Micro- and Nanoplastics
The most striking finding in recent bottled water research is the sheer volume of plastic particles suspended in the water. A study published in collaboration with the NIH found that a single liter of bottled water contains roughly 240,000 tiny pieces of plastic. About 90% of those are nanoplastics, particles so small they can pass through biological barriers that larger microplastics cannot.
The water contained all seven major types of plastic. The most common was polyamide, a form of nylon used in water filtration systems. PET (polyethylene terephthalate), the same material most water bottles are made from, was also abundant. Other plastics detected included polyvinyl chloride, polymethyl methacrylate, and polystyrene. Some of these come from the bottle walls, while others are introduced during the purification process itself.
At the cellular level, nano-sized plastic particles can cross into cells and trigger a cascade of problems: oxidative stress, immune activation, mitochondrial dysfunction, and DNA damage. In intestinal cells specifically, these particles damage the gut lining by weakening the tight junctions between cells, potentially allowing substances that should stay in the digestive tract to leak into the bloodstream. This isn’t theoretical speculation. These effects have been observed in cell studies, though the long-term consequences of daily exposure at the levels found in bottled water are still being quantified.
Bisphenol A (BPA)
BPA is one of the most well-known endocrine disruptors, meaning it mimics hormones in the body and can interfere with reproductive, metabolic, and developmental processes. It leaches from polycarbonate plastic, which is commonly used in large 5-gallon water cooler bottles.
What makes BPA particularly concerning is how dramatically its concentration rises with time and heat. A study on 5-gallon bottles found BPA levels of about 0.36 micrograms per liter in freshly filled containers. After 30 days at room temperature, that number climbed to 2.31 micrograms per liter. The same bottles stored in sunlight for 30 days reached 9.05 micrograms per liter on average, with the highest individual reading hitting 11.75 micrograms per liter. That’s a roughly 30-fold increase just from sitting in the sun for a month.
Antimony and Heavy Metals
Antimony is a metalloid used as a catalyst in PET plastic production, and it slowly leaches into the water over time. At room temperature and for short storage periods, antimony levels in bottled water generally stay below the maximum contaminant level of 6 parts per billion. But temperature changes that picture significantly. At 65°C (about 150°F, roughly the temperature inside a car on a hot day), antimony exceeded safe limits within two weeks in some brands. At 80°C, it crossed that threshold in just two to three days.
Aluminum also increased significantly in bottles stored at elevated temperatures, following a similar pattern. Boiling water in or microwaving PET bottles accelerated the leaching process considerably.
Beyond what leaches from the plastic, the water itself can contain heavy metals from its source. A study analyzing 11 bottled water brands found chromium at 5 to 34 micrograms per liter, cadmium at 1.5 to 7 micrograms per liter, lead at 1 to 7 micrograms per liter, nickel at 2 to 29 micrograms per liter, and arsenic up to 3.5 micrograms per liter. These metals occur naturally in groundwater and vary by region and brand, but chronic low-level exposure to lead, cadmium, and arsenic is linked to kidney damage, neurological effects, and increased cancer risk.
PFAS (Forever Chemicals)
Per- and polyfluoroalkyl substances, commonly called “forever chemicals” because they don’t break down in the environment, have been detected in bottled water sold in the United States. A study analyzing U.S. bottled water found 15 different PFAS compounds across the samples tested. The majority were short-chain varieties containing five or fewer carbon-fluorine groups, which are increasingly used as replacements for older, longer-chain PFAS but still raise health concerns.
Notably, the two most infamous PFAS compounds, PFOA and PFOS, were not detected above 4 parts per trillion in a separate FDA survey of 30 bottled water brands. That’s well below the EPA’s health advisory of 70 parts per trillion. However, newer research suggests that even very low levels of PFAS may be concerning, and the EPA has since proposed much stricter limits. The presence of multiple PFAS compounds at low levels also raises questions about cumulative exposure, since people encounter these chemicals from dozens of sources daily.
Disinfection Byproducts
Bottled water is treated before packaging, and that treatment can leave behind chemical byproducts. Trihalomethanes (THMs) form when chlorine reacts with naturally occurring organic matter in water. While THMs are primarily a tap water concern (where levels ranged from 0 to 322 micrograms per liter in one study), they also show up in bottled water. Spring water contained 0.1 to 0.3 micrograms per liter, purified water reached up to 7.6 micrograms per liter, and distilled water ranged from 0.1 to 18.1 micrograms per liter.
The distilled water numbers may seem surprising, but distillation doesn’t always remove volatile organic compounds, and some brands use additional chlorine treatment. Flavored bottled water also showed THM levels up to 5.7 micrograms per liter. While these concentrations are far lower than what’s typical in tap water, they challenge the assumption that bottled water is completely free of treatment-related chemicals.
How Storage Conditions Change the Risk
A recurring theme across nearly every contaminant category is that heat and time make things worse. PET bottles stored in cool, dark conditions leach far fewer chemicals than those left in sunlight, stored in hot warehouses, or forgotten in a car. Antimony, BPA, and aluminum all show steep, measurable increases with rising temperature. The practical takeaway is that a bottle of water sitting on a store shelf in an air-conditioned building is a very different product from the same bottle after a week in a hot trunk.
Sunlight exposure compounds the problem. UV radiation accelerates the degradation of plastic polymers, releasing more particles and chemical compounds into the water. One study noted that even brief outdoor storage with sun exposure increased antimony and other trace elements. The recommendation from researchers is straightforward: store bottled water in cool, shaded environments and avoid keeping it for extended periods, particularly during warmer months.
Does the Bottle Material Matter?
Yes. Standard single-use PET bottles, while showing minimal chemical migration compared to some reusable plastics, still contribute nanoplastics and antimony. Polycarbonate bottles (the rigid, reusable kind often used in water coolers) are the primary source of BPA. A laboratory analysis of reusable bottles made from various plastics found that silicone, polyethylene, and polypropylene showed the highest overall chemical migration, while PET and similar rigid plastics had comparatively lower levels.
Glass bottles largely sidestep the plastic-related contamination issues. They don’t leach BPA, antimony, or plastic particles. They can still contain whatever contaminants are present in the source water itself, including heavy metals and PFAS, but they eliminate the packaging as a variable. For people looking to reduce their chemical exposure from water containers, glass or stainless steel remain the cleanest options.