Joseph G. Allen is assistant professor at Harvard T.H. Chan School of Public Health and faculty adviser to the Harvard Healthier Building Materials Academy.

They say nothing lasts forever. Nothing, that is, except a group of toxic chemicals that may be associated with testicular cancer, kidney cancer, high cholesterol and suppression of vaccine effectiveness in children. They are now in nearly all of our bodies, are found in the air and water around the globe, and they never go away. They are "Forever Chemicals."

These are stain-repellent chemicals that we use in products throughout our homes, offices, schools, hospitals, cars and airplanes. They are characterized by a fluorine-carbon backbone. And the F-C bond, the Forever-Chemical bond, is quite amazing, representing one of the strongest bonds in all of organic chemistry.

When several F-C bonds are strung together, some really useful industry properties appear, including allowing air to pass through while blocking things such as grease, oil and dirt. This ability to act as a stain repellent is why we apply them to all sorts of products we like to keep clean, from carpets and furniture to camping gear. It's also why we apply it to nonstick cookware — almost nothing can stick to the pan when we have a layer of these chemicals on the surface.

But this property comes with a pernicious dark side. The F-C bond is so strong that these chemicals never fully degrade. Ever. Like, for millennia ever.

Forever Chemicals have been used in products since the 1940s and, to confuse things as only scientists can, the terminology around these chemicals is so precise as to be useless. You might have heard them referred to as "stain-repellent compounds" or "highly fluorinated chemicals." For years, many called them by their infamous toxic poster child "C8," referring to the eight-carbon Forever Chemicals "PFOA" and "PFOS." The most recent technical name for the Forever Chemicals we are talking about — the ones in consumer products and building materials — are perfluorinated or polyfluorinated alkyl substances, or PFASs. Rolls off the tongue, doesn't it?

(There is another set of Forever Chemicals, called hydrofluorocarbons, that we are not talking about here. These are used as refrigerants and, because they are potent greenhouse gases, are being phased out of use under the latest amendment to the Montreal Protocol.)

It might be one thing if Forever Chemicals stayed put once in our consumer products and building materials. But that's simply not the case. Forever Chemicals escape from the products we put them in and from the industrial facilities that produce them. The latest high-profile Forever Chemical to come under scrutiny is GenX, a nonstick chemical in cookware that Chemours, the manufacturer behind Teflon, was allegedly discharging into Cape Fear River, the drinking-water source for hundreds of thousands of people in North Carolina. We now find Forever Chemicals all over the globe, from the blood of polar bears to the middle of the Pacific Ocean.

We also find them inside all of us; in the most recent survey of the U.S. population, one set of these Forever Chemicals showed up in the blood of more than 98 percent of Americans. My colleagues published a paper in 2016 showing that 6 million Americans have these chemicals in their drinking water above the "safe" limit set by the Environmental Protection Agency. That might be an underestimate, because regulators are considering lowering that limit.

Public-health scientists often describe the wicked game of replacing one harmful chemical with an equally harmful chemical as "regrettable substitution." But Forever Chemicals are worse. We don't swap one for one. They are more like weeds in a garden; as soon as we remove one from the market, 10 more appear. We have largely eliminated the use of PFOA and PFOS, but there are thousands of new variants of Forever Chemicals in use.

And it may get worse. In every chemical with a carbon-hydrogen bond (the fundamental unit of organic chemistry), you can theoretically replace the "H" with an "F," creating a Forever Chemical. Thus, the number of Forever Chemicals that can be made is close to infinite. Scientists could study these indefinitely and not make any progress. It's job security that I don't want.

We need a market solution to this problem. Thankfully, there is already action across several sectors. In the health-care industry, Kaiser Permanente announced in 2016 that it is banning Forever Chemicals in furniture and materials in its building projects. Manufacturers HermanMiller and Shaw are offering products without them as well. Even consumer product retailers are getting involved: Columbia Sportswear partnered with hip-hop artist Macklemore in 2016 to introduce a rain jacket with no added Forever Chemicals.

Universities have a role to play, too. Harvard recently announced new Green Building Standards requiring that we no longer purchase furniture and other materials containing Forever Chemicals. We are using our campus as a living laboratory to test out better products and create the scientific evidence that demonstrates that when we act, we see an improvement in environmental quality in our buildings.

Collectively, from all sectors, we need to send clear market signals that we all want products that adhere to green science principles, such as designing for degradation. Because nothing should last forever, including our patience.