“Dirty dozens” is a memorable phrase—but it can also be misleading. In discussions about drinking water, it is often used to describe a group of persistent chemicals that remain in the environment for years or decades. The expression may refer to the original “Dirty Dozen” of persistent organic pollutants (POPs), to a shortlist of PFAS, or simply to the wider family of substances known as “forever chemicals”.
These terms are related, but they are not interchangeable. Understanding the difference matters because the risks, regulations and treatment options can vary significantly from one chemical to another.
What does “dirty dozens” mean?
The original “Dirty Dozen” refers to 12 persistent organic pollutants targeted under the Stockholm Convention, an international treaty adopted in 2001. The list included pesticides such as DDT, industrial chemicals such as polychlorinated biphenyls (PCBs), and unintentionally produced pollutants including dioxins and furans.
These substances became internationally recognised because they share several troubling characteristics:
- They persist in the environment instead of breaking down quickly.
- They can travel over long distances through air, water and wildlife.
- They accumulate in the tissues of animals and people.
- Some are toxic to the nervous, immune, reproductive or endocrine systems.
PFAS were not part of the original 12. However, certain PFAS—including perfluorooctane sulfonic acid (PFOS) and perfluorooctanoic acid (PFOA)—now receive similar international attention because of their extreme persistence and widespread distribution.
So, when someone refers to the “dirty dozens” in the context of drinking water, the phrase may be used informally to describe a group of high-concern persistent chemicals. It is important to check which substances are actually being discussed.
Why PFAS are often described as “forever chemicals”
PFAS, or per- and polyfluoroalkyl substances, are a large family of manufactured chemicals. There are thousands of individual PFAS, although only a smaller number have been studied in detail.
Their defining feature is the presence of carbon-fluorine bonds. These bonds are among the strongest in organic chemistry, which gives PFAS useful properties: they resist heat, oil, grease, stains and water. Those same properties make them exceptionally difficult to destroy after they enter the environment.
PFAS have been used in products and industrial processes including:
- Firefighting foams, particularly those used at airports, military sites and industrial facilities.
- Water- and stain-resistant textiles, carpets and upholstery.
- Non-stick cookware and food packaging.
- Industrial coatings, electronics and manufacturing processes.
- Personal care products and some cleaning or polishing formulations.
Some PFAS can move through soil into groundwater. Others can be carried in rivers, wastewater and atmospheric dust. Because many PFAS are highly mobile, contamination may extend beyond the original source. A fire-training site, landfill or industrial facility can therefore affect drinking-water sources several kilometres away.
Which persistent chemicals are found in drinking water?
There is no single global “dirty dozen” list for drinking water. Different countries monitor different substances, based on national regulations, local pollution sources and available laboratory methods.
For PFAS, attention has traditionally focused on compounds such as:
- PFOA: Historically used in the production of fluoropolymers and associated with a range of industrial applications.
- PFOS: Formerly used in firefighting foams, surface treatments and other products.
- PFHxS: A persistent PFAS linked to some industrial and firefighting applications.
- PFNA: A long-chain PFAS detected in environmental and biological samples.
- PFHxA: A shorter-chain PFAS used in some applications and considered more mobile in water.
- GenX chemicals: A group of replacement substances developed for fluoropolymer production, including hexafluoropropylene oxide dimer acid and related compounds.
This is not an exhaustive list. It is also not a ranking of which chemicals are “safe” or “unsafe”. A shorter-chain PFAS may move more readily through groundwater, while a longer-chain PFAS may accumulate more strongly in organisms. Lower bioaccumulation does not automatically mean negligible risk.
Other persistent contaminants may also be present in drinking-water sources, including PCBs, organochlorine pesticides, dioxins, some flame retardants, pharmaceuticals and industrial solvents. Their chemical behaviour and health effects differ from those of PFAS, which is why broad labels can sometimes obscure more than they explain.
How do these chemicals reach drinking water?
Contamination usually begins with a release, but the route into drinking water depends on the chemical and the local environment.
Firefighting foam is one of the clearest examples. Aqueous film-forming foam, or AFFF, was highly effective against fuel fires because it rapidly created a barrier between the fuel and oxygen. However, repeated use at airports, refineries and training grounds released PFAS into soil and groundwater. Once there, the chemicals could migrate towards public supply wells or private boreholes.
Landfills are another important source. Consumer products containing PFAS may release chemicals as they break down or are exposed to water. Leachate can carry PFAS into surrounding soils and groundwater unless it is carefully collected and treated.
Wastewater treatment plants can also receive PFAS from households, hospitals, commercial premises and industry. Conventional treatment is not designed to destroy most PFAS. Instead, the chemicals may remain in treated effluent or become concentrated in sewage sludge.
Persistent organic pollutants can follow different pathways. Some attach strongly to sediments or organic matter, while others travel through the atmosphere. Their long environmental lifetimes mean that contamination detected today may reflect historic use rather than a recent release.
What are the potential health concerns?
Research into PFAS and other persistent chemicals is ongoing, and the evidence is not identical for every compound. Exposure also depends on concentration, duration, age, health status and the combined effect of multiple chemicals.
For some well-studied PFAS, scientific studies have reported associations with:
- Changes in cholesterol levels.
- Reduced antibody response to certain vaccinations.
- Effects on liver function.
- Changes in thyroid and other hormonal pathways.
- Pregnancy-related outcomes, including reduced birth weight.
- Developmental and immune-system effects.
- Increased risk of certain cancers for specific substances and exposure scenarios.
These findings do not mean that every person exposed to a detectable concentration will develop illness. They do show why regulators and health agencies are increasingly applying precautionary limits, particularly for drinking water, where exposure can occur every day.
The body can eliminate some PFAS relatively quickly, while others remain for years. PFOS and PFOA, for example, have biological half-lives measured in years rather than days. Repeated low-level exposure can therefore contribute to a persistent internal burden.
Why testing is more complicated than it sounds
Testing for “PFAS” is not like testing for one contaminant. Laboratories must look for individual compounds, and the number detected depends on the analytical method.
Some methods measure a defined group of PFAS, such as 20 or 30 named substances. Other approaches estimate total organic fluorine or use precursor oxidation to identify chemicals that can transform into more recognisable PFAS. No single test captures every PFAS in every sample.
Results can also be affected by sampling materials. Certain laboratory equipment, tubing and containers may contain fluorinated materials that interfere with analysis. Reliable monitoring therefore requires carefully controlled sampling, validated laboratory methods and appropriate quality assurance.
A “non-detect” result does not necessarily mean that no PFAS are present. It means that the measured concentration was below the method’s reporting limit. That distinction is important when health-based guidance values are extremely low.
How are PFAS removed from drinking water?
Conventional drinking-water treatment methods—including coagulation, sedimentation and chlorination—are not generally effective at removing dissolved PFAS. Advanced treatment is often required.
The most established options include:
- Granular activated carbon: Activated carbon can adsorb many PFAS, particularly longer-chain compounds. Performance depends on contact time, water chemistry, carbon type and the level of competing organic matter. Filters must be replaced or regenerated before they become saturated.
- Ion exchange: Specialised resins can remove PFAS efficiently, including some shorter-chain substances. The spent resin must be managed carefully to prevent the contamination being transferred elsewhere.
- Reverse osmosis: Membrane systems can remove a broad range of PFAS and other dissolved contaminants. They require energy, maintenance and responsible management of the concentrated waste stream.
- Foam fractionation and emerging technologies: These methods may offer useful solutions for concentrated industrial wastewater, but their suitability for municipal supplies depends on the specific water chemistry and treatment objective.
Removal is not the same as destruction. If a filter captures PFAS, the chemicals still exist in the spent carbon, resin or concentrate. A credible treatment strategy must address the entire waste chain, not just the point where PFAS disappears from the water sample.
What can households do?
People concerned about PFAS exposure should begin with reliable information rather than panic. Contact the local water supplier or environmental regulator to ask whether PFAS monitoring data are available. Public reports may identify the compounds tested, the detection limits and the dates of sampling.
Private well owners should consider testing if the property is near an airport, military base, industrial site, landfill, wastewater facility or known firefighting-foam training area. Testing should be carried out by an accredited laboratory using a method appropriate for PFAS.
For point-of-use treatment, certified activated-carbon and reverse-osmosis systems may reduce exposure when correctly selected and maintained. Look for independent certification against relevant PFAS reduction standards, follow the replacement schedule and keep records of filter changes. An old or saturated filter can provide false reassurance.
Boiling water is not a reliable way to remove PFAS. In fact, evaporation can reduce the volume of water while leaving non-volatile PFAS behind. Boiling is useful for killing microbes, but it should not be treated as a solution for chemical contamination.
The regulatory picture is changing
PFAS regulation is developing rapidly. Limits differ between jurisdictions and may be expressed as individual standards, group limits or very low combined concentrations. The United States Environmental Protection Agency has introduced national drinking-water limits for PFOA and PFOS, while the European Union regulates a defined sum of PFAS and a broader total PFAS parameter under its Drinking Water Directive.
In the United Kingdom, monitoring and regulatory requirements are also evolving. Water companies follow guidance and requirements set by the relevant national authorities, including the Drinking Water Inspectorate in England and Wales. Because standards and monitoring lists can change, current official guidance is more reliable than an old online “dirty dozen” graphic.
The central lesson is straightforward: persistent chemicals deserve attention because they do not respect the boundaries between industry, waste, water and public health. A list can help raise awareness, but it should be the starting point—not the end of the investigation.
When evaluating drinking-water safety, ask three practical questions: which chemicals were tested, how sensitive was the test, and what happens to the contamination after treatment? Those questions lead to better decisions than a catchy label alone—and they keep the focus where it belongs: on evidence, prevention and effective protection of water supplies.