Chlorine and Disinfection Byproducts
Utilities add chlorine or chloramine to kill bacteria, which is essential for preventing waterborne disease. But when chlorine reacts with organic matter in the water, it creates disinfection byproducts (DBPs) like trihalomethanes (THMs) and haloacetic acids (HAAs) [1]. The EPA regulates THMs at 80 parts per billion, but long-term epidemiological studies have associated even regulated levels with increased bladder cancer risk [2].
Lead
There is no safe level of lead exposure [5]. Lead enters tap water not from the source but from the infrastructure — lead service lines, solder joints, and brass fixtures in homes built before 1986. The EPA's action level is 15 ppb, but the CDC and American Academy of Pediatrics emphasize that any detectable lead poses a risk to children's neurological development [5]. The 2014 Flint, Michigan crisis made national news, but EWG testing shows detectable lead in water systems serving millions of Americans [3].
PFAS (Per- and Polyfluoroalkyl Substances)
PFAS are synthetic chemicals used in nonstick coatings, food packaging, and firefighting foam since the 1950s. They earned the name "forever chemicals" because they don't break down in the environment or the human body [4]. In 2024, the EPA finalized the first national drinking water standard for six PFAS compounds at 4 parts per trillion for PFOA and PFOS. Prior to this, there was no enforceable federal limit. PFAS exposure is associated with thyroid disease, immune suppression, reproductive harm, and several cancers [4].
Microplastics
Microplastics (particles under 5mm) have been detected in tap water worldwide, including US municipal systems. A 2018 study found microplastic fibers in 94% of US tap water samples tested. Research on health effects is still emerging, but concerns center on the particles themselves and the chemical additives they carry (phthalates, bisphenols, flame retardants) [2].
Pharmaceuticals and Hormones
Trace amounts of prescription drugs — antibiotics, antidepressants, synthetic estrogens — enter the water supply through human excretion and improper disposal. Conventional water treatment does not fully remove them. Detected concentrations are typically in the low nanogram-per-liter range, well below therapeutic doses, but the effects of chronic low-level exposure and chemical mixtures are not yet well understood [2].
How Standards Are Set — and Where They Fall Short
The EPA's National Primary Drinking Water Regulations set Maximum Contaminant Levels (MCLs) for about 90 contaminants [1]. These MCLs are legally enforceable, but they represent a balance between health goals, treatment feasibility, and cost — not purely health-based thresholds. The EPA also publishes Maximum Contaminant Level Goals (MCLGs), which are non-enforceable health targets. For lead and several carcinogens, the MCLG is zero, meaning any exposure carries some theoretical risk [1].
The EWG Tap Water Database compiles utility testing data and compares results against both legal limits and their own health guidelines, which are often 10–100x stricter than MCLs [3]. For example, the legal limit for chromium-6 (hexavalent chromium) is 100 ppb based on a 1991 standard, while California's public health goal — based on more recent cancer data — is 0.02 ppb.
The 2021 review by Levin et al. in Environmental Science & Technology quantified the population-level disease burden from regulated contaminants in US drinking water, estimating over 100,000 cancer cases attributable to tap water contaminants over a lifetime, with arsenic, DBPs, and radioactive elements as the largest contributors [2].
For PFAS specifically, Cordner et al. (2021) systematically reviewed epidemiological evidence across 40+ studies and found consistent associations between PFAS blood serum levels and thyroid disease, ulcerative colitis, testicular cancer, kidney cancer, and pregnancy-induced hypertension. The strength of evidence varies by specific PFAS compound and health endpoint, but the overall pattern supports a precautionary approach to exposure reduction [4].
The CDC emphasizes that lead risk is heavily infrastructure-dependent. Homes with lead service lines, which the EPA estimates at 6–10 million nationwide, face the highest exposure. Running cold water for 30 seconds to 2 minutes before use and using certified filters are the primary recommended mitigations [5].