Industry says it’s not a problem. Policymakers can’t decide. But scientists are adamant: highly persistent chemicals are always a hazard.

Many people think that, so long as scientists haven’t proven a chemical to be toxic, its persistence is not a problem. But time and time again, history has shown this view to be wrong. Highly persistent substances have built-in potential to cause harm, even when their toxicity might appear to be low.

Within the scientific community, there is what one researcher calls an “entirely valid consensus” that highly persistent contaminants deserve special attention. Let’s have a look at why this is.

If it’s not toxic, what’s the problem?

The problem, as the late US defence secretary Donald Rumsfeld might have said, is the known unknowns – things we know that we do not know.

A long-lived chemical’s effects are difficult to predict because we cannot know for sure how it will behave long into the future. Scientists struggle to identify population-wide effects in laboratory testing, mainly because exposure times might need to be very long.

Evidence of toxic effect is therefore not essential for a highly persistent chemical to ring alarm bells. We cannot reliably predict its long-term effects. Scientific knowledge will always remain uncertain and incomplete. So highly persistent chemicals are always a hazard.

Also, over the long term, industry will release more chemicals, creating complex and unpredictable mixture effects from multiple exposures.

So what about sand? Or metal?

Nice try! Sand is indeed highly persistent, but it is not generally “bioavailable” – able to be ingested, inhaled, or absorbed into people, plants or animals. Many metals and other naturally occurring substances (arsenic, asbestos) are also highly persistent. In some cases, however, human activity can cause them to become bioavailable, and therefore potentially hazardous.

In this discussion of persistence, however, we are talking about “novel entities” such as man-made organic substances, engineered nano-materials, and plastics. When we talk about highly persistent substances, we mean chemicals that did not exist before people invented them.

The problem is the known unknowns, things we know that we do not know

Are we being over-cautious?

Scientific uncertainty alone is not enough to ban a chemical. The known unknowns become an issue when the chemical is highly persistent and won’t degrade.

Here are some powerful historical examples from which we can learn a thing or two:

PFAS

When these extremely persistent “forever chemicals” were first invented, nobody knew what their health effects might be. The first sign of a problem was with liver toxicity in the 1960s. Sixty years on, unexpected health effects are being discovered all the time. As knowledge of their toxicity has improved, regulators have repeatedly and drastically revised downwards levels once considered safe – a pattern observed across many highly persistent substances.

GenX

DuPont began the commercial development of GenX as a safer replacement for PFOA, after concerns emerged about the toxicity of this PFAS. GenX went on sale in 2009. Animal studies then started to show health effects on the liver, kidneys, the immune system and embryos. In 2019, the EU decided it was a substance of very high concern. Chemours challenged this decision, but in 2023 it was upheld by the European Court of Justice. The following year, the EPA included GenX in its first maximum contaminant levels for PFAS in drinking water – 15 years after this chemical went on sale.

TFA

Similarly, the presence of the short-chain PFAS trifluoroacetic acid (TFA) has grown dramatically in the environment since the 1990s. The EU is currently evaluating the chemical as a potential reproductive toxicant.

PCBs

Polychlorinated biphenyls (PCBs) first entered the market in the 1950s, where they found multiple uses. After researchers found high concentrations in wildlife, PCB production ceased in the early 1990s. By 2004, governments had agreed internationally to ban them. Because of their persistence, the problem of PCB pollution will continue far into the future.

DDT

From 1945, DDT (dichlorodiphenyltrichloroethane) rapidly became one of the most commonly used pesticides worldwide. Nobody believed it would be harmful – the inventor even got a Nobel Prize. However, its persistence led to a buildup throughout the food chain. Rising DDT levels were found in humans all around the world. And by the mid-1950s, evidence emerged to show the chemical may be harmful, particularly to reproduction. In response to these concerns, by the early 1980s, most countries banned it.

To sum up, the toxicity of highly persistent chemicals can be underestimated for decades. The damage can’t be undone — and contamination spreads worldwide before we even know the risks. By the time risk analysis catch up with the emerging evidence, highly persistent chemicals may already have caused widespread and long-lasting harm.

The toxicity of highly persistent chemicals can be underestimated for decades

How persistent is highly persistent?

Good question. A chemical’s persistence in the environment can vary depending on whether it is in water, soil or sediment. It also depends on temperature, acidity, oxygen availability and sunlight. Different mechanisms (microbial, hydrolysis or photolysis) for breaking down the chemical take place at varying speeds.

The Stockholm Convention on Persistent Organic Pollutants (POPs) defines a persistent chemical as one that takes more than 6 months to reduce by half in soil or sediment, and two months in water. The EU’s REACH regulation defines such substances as “very persistent”. Highly persistent chemicals are ones that exceed these thresholds.

In fact, chemicals with very high and very low persistence are relatively easy to identify. It is the ones in the middle that are more problematic – data are scarce and results can be scattered and inconsistent. Uncertainties with measuring the persistence of this middle group do not imply that nothing can be done, simply that scientists must exercises judgment.

But society needs persistent chemicals!

Sometimes. Often, however, highly persistent chemicals are used in applications that are not important for society to function. Regulators should limit these chemicals to uses that are critical to society and where no alternatives are available. A host of safer alternatives can be found on ChemSec’s Marketplace website, which gathers green chemistry innovations in one place.

Persistence can be a desired property of chemicals, such as pigments in paint. Manufacturers deliberately build persistence into chemical products to provide durability, stability, and long service life. Recognising persistence as a hazard in its own right means we need to design chemicals so they do not remain in the environment. Chemists need to engineer substances to break down into harmless products once the substance has fulfilled its intended function.

This creates a design challenge of how to generate the necessary function and performance in products while avoiding environmental persistence – but it can be done.

Conclusion

Understanding that high persistence is a hazard means recognising that highly persistent chemicals are inherently unsafe, regardless of current toxicity data.

Safety therefore demands that highly persistent chemicals should be regulated — whether they are known to be toxic today or not. Highly persistent chemicals are always a hazard.