Emerging Contaminants: 5 Chemicals the EPA Hasn't Regulated Yet

Pharmaceuticals, microplastics, 1,4-dioxane, and more. What's in your water that isn't tested.

What Are They

Emerging contaminants are chemicals detected in drinking water that have no federal regulation. The EPA currently regulates about 90 contaminants under the Safe Drinking Water Act, but independent testing has identified over 300 additional substances in US tap water. These unregulated chemicals include pharmaceuticals, industrial solvents, microplastics, and personal care product residues.

The regulatory process for new contaminants is slow by design. Before the EPA can set an enforceable limit, it must go through a multi-year cycle: identify the contaminant, fund research on health effects, conduct a cost-benefit analysis, propose a rule, take public comment, and finalize the standard. PFAS, one of the most well-known emerging contaminants, took over 20 years from initial detection to the first enforceable MCL in 2024.

The gap matters because "unregulated" does not mean "safe." It means the science or the political process has not yet caught up with the contamination. Utilities are not required to test for these chemicals, and most do not. If a substance is not tested for, it will not appear in your water quality report, even if it is present at concerning levels.

Pharmaceuticals

Trace amounts of prescription and over-the-counter drugs have been detected in the drinking water of at least 41 million Americans. These include antibiotics, antidepressants, hormones (including estrogen from birth control pills), anti-seizure medications, and painkillers. They enter the water supply primarily through human excretion and improper disposal of unused medications flushed down toilets.

Conventional water treatment plants were not designed to remove pharmaceuticals. Chlorination and basic filtration reduce some compounds but leave others largely intact. A USGS study detected pharmaceuticals in 80% of streams sampled downstream of wastewater treatment plants, confirming that treated effluent is a major pathway into drinking water sources.

The concentrations are extremely low, typically measured in parts per trillion (nanograms per liter). No individual dose from a glass of water approaches a therapeutic level. However, scientists are concerned about two things: the effects of chronic low-dose exposure over decades, and the "cocktail effect" of multiple pharmaceuticals present simultaneously. Endocrine-disrupting compounds like synthetic estrogen are of particular concern because hormones can trigger biological effects at vanishingly small concentrations.

There is no EPA MCL for any pharmaceutical compound in drinking water. The agency has placed several on its Contaminant Candidate List, but rulemaking has not begun. For now, removal depends entirely on what filter you use at home.

1,4-Dioxane

1,4-Dioxane is a probable human carcinogen found in the drinking water of an estimated 90 million Americans, with no federal MCL. It is a synthetic industrial solvent used in manufacturing paints, adhesives, antifreeze, and cosmetics. It also forms as a byproduct in the production of common household products like detergents and shampoos.

The EPA classifies 1,4-dioxane as a "likely human carcinogen" and has set a health advisory of 0.35 ppb for a one-in-a-million cancer risk. However, this is a non-enforceable guideline. Monitoring data from the EPA's Unregulated Contaminant Monitoring Rule (UCMR3) found detectable levels in approximately 21% of public water systems tested, with some systems exceeding 35 ppb.

What makes 1,4-dioxane especially problematic is that it is extremely difficult to remove from water. It is highly soluble, does not adsorb well to standard activated carbon, and passes through many conventional treatment processes. Advanced oxidation processes (AOP) using UV light and hydrogen peroxide are the most effective treatment at the utility scale, but few plants have installed this technology. At the home level, reverse osmosis provides moderate reduction, and specialized carbon block filters with extended contact time can help, though no home filter is specifically certified for 1,4-dioxane removal.

Microplastics

94% of US tap water contains microplastics, tiny synthetic fragments smaller than 5 millimeters shed from plastic bottles, packaging, synthetic clothing, and tire wear. A February 2026 Ohio State University study found that bottled water contains approximately 3x more nanoplastic particles than treated tap water, making the switch to bottled water counterproductive.

The health effects of ingesting microplastics are still being studied, but early research has linked exposure to chronic inflammation, endocrine disruption, and cardiovascular effects. Nanoplastics (smaller than 1 micrometer) are of greatest concern because they can cross cell membranes and have been found in human blood, lung tissue, and placentas.

There is no EPA regulation for microplastics in drinking water, and no standard testing protocol exists for utilities. The WHO reviewed available evidence in 2022 and concluded that microplastics in drinking water do not appear to pose a health risk "at current levels," while acknowledging that data is extremely limited and more research is urgently needed. For a detailed breakdown of the science and filtration options, see our complete microplastics guide.

Protection

Reverse osmosis is the single best defense against emerging contaminants as a category. RO membranes physically block particles and reject dissolved chemicals based on molecular size, which means they remove compounds that were never specifically tested for, including many pharmaceuticals, 1,4-dioxane at moderate concentrations, and virtually all microplastics. An under-sink RO system like the Waterdrop G3P800 provides broad-spectrum protection.

High-quality carbon block filters are the next best option. Multi-stage systems and filters with extended carbon contact time, such as the Clearly Filtered pitcher, remove a wider range of organic chemicals than basic granular carbon. Look for filters tested against NSF/ANSI 401 (emerging contaminants) and NSF/ANSI P473 (PFAS) for the broadest certified coverage.

Beyond filtration, you can reduce your contribution to the problem. Never flush unused medications down the toilet. Use DEA-authorized drug take-back programs or pharmacy drop-off locations instead. Choose personal care products without 1,4-dioxane (often hidden as a byproduct in ingredients listed as "PEG," "-eth," or "polysorbate"). And reduce single-use plastic in your kitchen to minimize microplastic exposure from sources other than water.

Start by understanding what is in your water now. Check your city's data on CheckMyTap for regulated contaminants, then consider a lab test like SimpleLab Tap Score if you want to screen for unregulated compounds at your specific tap.