For years, disposable bowls have been a stalwart ally of the fast-casual restaurant. Beige, earthy-looking and made from molded plant fibers, these receptacles appear less wasteful than single-use plastic, lending an aura of social responsibility to the eateries that use them. Some varieties are even certified compostable, which means they’re guaranteed to break down in commercial composting facilities, if not the backyard leaf pile. And while only a few chains actually run composting programs, these bowls still feel lighter-touch somehow – even when they’re simply shipped to the landfill. They suggest a higher-minded way of eating, one based on a form of packaging that’s almost as ephemeral as our encounters with it.

But fast-casual bowls have a troubling secret: virtually all of them contain worrisome chemicals that never biodegrade, polluting soil, water and our bodies in the process. The truth is that, though you might only handle your salad bowl for five minutes, traces of it will stick around in the environment for ever.

That was the central finding of an investigative feature I published last week in the New Food Economy, a non-profit newsroom covering the forces shaping American food, where I’m deputy editor. I spent months interviewing more than a dozen scientists, food service industry experts, packaging professionals and waste management leaders. Everyone I spoke to said the same thing: the familiar bowls all contain PFAS, or per- and polyfluoroalkyl substances, a broad class of “forever chemicals” that persist indefinitely and have been linked to troubling health effects.

We didn’t just take them at their word. The New Food Economy did its own testing of bowls from eight different Manhattan restaurants, including multiple locations of Chipotle, Dig and Sweetgreen – restaurants that run ambitious composting programs and claim their bowls have significant environmental benefits. The results were striking. All 18 bowls tested contained high levels of fluorine, according to Graham Peaslee, the University of Notre Dame chemist who conducted our test. Peaslee and other scientists I spoke to say this leaves no doubt that these bowls were manufactured using PFAS, confirming what industry leaders had told me: that all takeout containers made from molded plant fibers contain these chemicals. This widespread use of PFAS in “compostable” bowls had been a closely guarded industry secret, and had not been reported previously. You can read my investigation here.

PFAS do a great job protecting bowls from heat, moisture and grease. Without them, the plant-based material would be much more delicate, falling to pieces quickly during contact with a salad or hot meal. But that usefulness is a faustian bargain. Speaking broadly, PFAS are manmade compounds characterized by powerful carbon-fluorine bonds, a kind of molecular armor that makes it impossible for enzymes and microbes to attack. This makes them great at repelling grease, but it’s also why they don’t break down – and why they’re so concerning for public health.

Thankfully, the most concerning varieties of PFAS – PFOS and PFOA, which have been linked to a broad range of serious health effects – have been banned or phased out by industry. But the newer varieties approved for use in food packaging by the FDA have risks of their own, concerns summarized by German government scientists in the peer-reviewed journal Environmental Sciences Europe. First, they’re just as persistent as the older varieties, remaining in the environment for unknown periods of time. They also take days or months for humans to excrete, meaning they accumulate quickly in our bodies. While the scientists write that these compounds carry a “risk of adverse effects on humans and the environment, which will rise with increasing exposure”, we do not yet know what those effects may be. Unlike PFOS and PFOA, which have been extensively evaluated, many of the newer varieties of PFAS have simply not been studied for potential health impacts.

That’s concerning. It means that, when it comes to “compostable” bowls, we literally don’t know what we are eating. It’s not just that the compounds being used haven’t been properly studied. Troublingly, several of the restaurants I spoke to would not – or could not – tell me which specific compounds were present in their bowls, something our test did not measure.

Still, the scientists I spoke to were concerned by any use of PFAS in food packaging. While many pesticides and solvents are considered toxic at parts-per-million levels in drinking water, PFAS tend to be considered dangerous at parts per trillion levels – a much lower threshold than other chemicals of concern. While dozens of PFAS are still approved by FDA for food contact applications, the agency is currently reviewing whether its existing rules are sufficient to protect public health.

Before I revealed that they contain PFAS, plant-based bowls seemed to be a sustainable solution to the restaurant industry’s overwhelming takeout waste problem. Many are made from a renewable byproduct of the sugar-making process – bagasse, the pulpy fiber that is left over after sugarcane is processed. Compared with plastic and foam, which tend to be made from virgin petroleum, this raw material can have a much lower carbon footprint. Unfortunately, it’s now clear that the benefit comes with a catch. This is what environmentalists call a “regrettable substitution”: when a promising new replacement technology comes with unintended consequences of its own.

The good news, for now, is that one of the industry’s key players has taken action. The Biodegradable Packaging Institute (BPI), a leading third-party certifier that assesses compostability claims, will change its stance on fluorinated chemicals starting on 1 January 2020. Currently, a bowl – or any other food packaging product – can contain PFAS and still be certified compostable by BPI. After the new standard kicks in, a product will need to contain less than 100 parts per million total fluorine to be certified. For context, the bowls we tested averaged 1,670 parts per million fluorine – more than 16 times that.

Starting in January, then, if you want to avoid food packaging with PFAS, just make sure to buy products certified compostable by BPI. That sounds simple – but may prove difficult. As of this writing, BPI does not certify any molded fiber products that fall below the threshold. Rhodes Yepsen, BPI’s executive director, told me that it will be virtually impossible for a new product to meet the deadline – if one existed, he’d have to know about it already.

In the new year, there will be plenty of options if you want a lunch that’s PFAS free – including BPI-certified paperboard boxes, recyclable plastic containers, and even your own reusable vessel. For now, though, PFAS-free bowls are off the table.