Bottled water has gone from a convenience to an alternative drinking-water system, with about a third of Americans choosing it over tap water most or all of the time. Why? More than 90% of those buying bottled water cite “safety” and “quality” as the reasons, but while it’s true that it can indeed be safe, this isn’t always the case.

Much has been written about the trash problem created by the billions of disposable plastic drinking-water bottles sold each year in the U.S. Far less is understood about the contents of those bottles. According to a Government Accountability Office (GAO) report from 2009, the most recent data available, about 70% of the bottled water sold in the U.S. was not subject to Food and Drug Administration (FDA) regulation. That’s because if water is bottled and sold in the same state, as is the case for some smaller labels, it’s considered intrastate commerce and is therefore regulated by the state. This isn’t necessarily a problem–some states’ regulations are stricter than the FDA’s–but the GAO report also said these rules can be less comprehensive than those for tap water, which must comply with the Safe Drinking Water Act, administered by the Environmental Protection Agency.

Bottled water under the FDA’s purview may not get the scrutiny you expect either. This is not a reflection of the Trump Administration’s antiregulation bias. Bottled water was an $18.5 billion (wholesale) industry in the U.S. in 2017, but under Presidents of both parties, FDA oversight has been lightly staffed. Furthermore, the agency allows bottlers to fill bottles with tap water. While the water is usually treated, this is not a guarantee.

This isn’t to say you should worry that every bottle of water you drink is contaminated. Rather, a better understanding of how the industry works is important for public health. Major bottled-water suppliers like Nestlé, Coca-Cola and PepsiCo, as well as prestige brands like Fiji and Perrier, have an incentive to do all they can to sell safe drinking water. At a minimum, they want to make sure that their brands are not injured by a loss of reputation. But there are hundreds of bottled-water brands, and some lesser-known bottlers–working on tiny profit margins–may not share the concern of the most recognizable ones. Because they don’t have to worry about public confidence in their brand name, they can stop using one label and start selling under another without changing the source.

A scientifically rigorous study from 1999 by the Natural Resources Defense Council (NRDC), which tested three samples of more than 100 bottled-water brands, revealed that about a third of the bottles examined had levels of bacteria or chemical contaminants above state or industry standards or guidelines. Erik D. Olson, senior director for health and food at NRDC, believes a similar test today would yield similar results, but says he can’t say for sure without additional monitoring. As an example of ongoing problems, he cites a recent investigation by Consumer Reports that found several brands of bottled water contained potentially unsafe levels of arsenic.

The bottles themselves can also present a health concern, as only a small percentage of all bottled water sold in the U.S. is bottled and shipped in a stable material like glass. Even if the water that fills a plastic bottle is pure, if it sits long enough–and especially if it’s stored in a hot place–there’s a risk of phthalates and other chemical agents in the plastic leaching into the water. Phthalates are endocrine-disrupting agents that pose special threats to pregnant women and young kids.

When contaminated water is found in Flint, Mich.; Newark, N.J.; or Puerto Rico, among other places, millions of bottles are shipped and distributed as a temporary solution. Many people also buy bottled water to consume in their homes, presumably only a few steps from vastly cheaper tap water.

Given the potential risks of bottled water and the pervasive fears about tap, consumers may feel they have nowhere to turn. The answer lies in improving the nation’s drinking-water infrastructure through advanced filtration systems–and publicizing that effort–so tap water becomes a more appealing option. No one likes paying over 300 times more than they have to, so by fixing tap water, a virtuous circle would be created: less trash, a better environment, cheaper water, better national health–and the peace of mind that will come from knowing that drinking water is safe.

Siegel is the author of Troubled Water: What’s Wrong With What We Drink

This appears in the October 07, 2019 issue of TIME.

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