What comes out of your shower depends on where you live.

EPA's 4.0 mg/L limit applies to the yearly average of monthly results, and utilities may raise levels for a time to deal with contamination. Free chlorine fades from water left standing: DC Water says the taste and smell go within a few hours in an open pitcher in the fridge.

Utilities use chloramine because it lasts longer in the pipes, and some switched to meet by-product limits (EPA). It does not fade like chlorine: Pinellas County notes chlorine leaves standing water within hours to days, while chloramine may take weeks. It must be removed for kidney dialysis and aquariums.

In a CDC study of 31 adults, blood levels were highest after a 10-minute shower and lowest after drinking 1 liter of tap water in 10 minutes. A Rutgers study estimated skin uptake in a shower about equals inhaled uptake. Both were small and measured exposure, not health effects.

HAA5 covers chloroacetic, dichloroacetic, trichloroacetic, bromoacetic and dibromoacetic acid. In lab tests on human skin they crossed about 50 to 200 times more slowly than THMs, and inhaled shower spray carried under 1% of the dose from drinking.

Chlorine dioxide is limited to 0.8 mg/L and its by-product chlorite to 1.0 mg/L. WHO notes that at typical doses chlorite stays below about 0.2 mg/L, and that chlorine dioxide can be tasted or smelled from 0.2 to 0.4 mg/L. It can also leave chlorate, covered under other by-products.

In EPA's 2013 to 2015 national survey, 38.6% of systems had at least one chlorate sample above EPA's 210 µg/L reference concentration, a level set for lifetime exposure. WHO guidelines: chlorate 0.7 mg/L and dichloroacetonitrile 0.02 mg/L (provisional), NDMA 0.1 µg/L.

The USGS calls water soft at 0 to 60 mg/L (as calcium carbonate), moderately hard at 61 to 120, hard at 121 to 180, and very hard above 180. It describes hardness as a nuisance, not a health concern: more soap is needed to get things clean, and scale builds up in pipes over time.

Above 0.3 mg/L, iron can give water a rusty color, sediment and a metallic taste, and leave red-brown stains on laundry, porcelain and dishes. Connecticut's health department says iron in drinking water is not directly linked to adverse health effects. Iron bacteria, also not a health threat, can form slime in toilet tanks and plumbing.

EPA's health advisory sets 0.3 mg/L for a lifetime of drinking, to protect against possible nervous system effects, and advises the same level for infants under 6 months even over 10 days. Above 0.05 mg/L, manganese can turn water black to brown, cause black staining and give a bitter metallic taste.

Lead gets into tap water mainly when pipes, solder or fixtures that contain lead corrode. It is regulated by a treatment technique: if over 10% of tap samples exceed the action level, a system must act. The 2024 Lead and Copper Rule Improvements require most systems to replace lead service lines within 10 years.

EPA lists stomach upset from short-term drinking above the action level, and liver or kidney damage from long-term drinking; people with Wilson's disease should ask their doctor if their water exceeds it. Above 1.0 mg/L, copper can give a metallic taste and blue-green staining.

Long-term drinking above the limit is linked to skin damage, circulatory problems and higher cancer risk, EPA says. Public systems must meet the limit, but private wells are the owner's responsibility to test and treat; USGS found high-arsenic wells across parts of the West, Northeast, Midwest and Atlantic Southeast.

The federal limit of 0.1 mg/L (1991) covers chromium-3 and chromium-6 together. EPA's 2024 IRIS assessment classified chromium-6 as likely to cause cancer when swallowed, based on animal studies. California's 0.010 mg/L chromium-6 limit took effect in 2024, phased in 2026 to 2028. EPA has not proposed a federal one.

EPA limits in mg/L: barium 2, selenium 0.05, antimony 0.006, cadmium 0.005, beryllium 0.004, mercury (inorganic) 0.002, thallium 0.002. Each is based on long-term drinking: EPA links cadmium and mercury to kidney damage and barium to higher blood pressure.

The limit protects babies under six months, who can become seriously ill (blue baby syndrome) from formula made with high-nitrate water. Boiling concentrates nitrate rather than removing it. A USGS study found nitrate above the limit in 4.4% of private wells sampled in 1991 to 2004, and in nearly 25% of wells in intensively farmed areas.

EPA's enforceable limit of 4.0 mg/L, set in 1986, guards against bone disease; the 2.0 mg/L guideline guards against tooth discoloration. Where fluoride is added, the US Public Health Service has recommended 0.7 mg/L since 2015. EPA began an accelerated review in 2025. Utah banned adding fluoride that year, and Florida ended it under a 2025 law on water additives.

Above 500 mg/L water tastes salty and leaves deposits, and dissolved solids can add to plumbing corrosion. USGS found TDS above the guideline in 15% of private wells sampled in 1991 to 2004. Road salt use has roughly tripled since the 1970s, and chloride is rising in US streams and in many aquifers.

Perchlorate can reduce iodide uptake by the thyroid. After a 2023 court ruling, EPA proposed a national limit in January 2026, while stating it judged the benefits would not justify the costs; a final rule is due by May 2027. California has enforced a 6 µg/L limit since 2007. Cyanide, from metal and plastics plants, is capped at 0.2 mg/L.

Radon entering from soil is usually a much larger risk than radon from water, and surface water rarely has a radon problem. USGS found radon above 4,000 pCi/L in 4.4% of private wells sampled, and uranium above its limit in 1.7%. EPA advises testing indoor air first, then well water if air levels are high.

The April 2024 rule set the first national PFAS limits: 4.0 ppt for PFOA and PFOS, 10 ppt for PFHxS, PFNA and GenX, and a mixture index. Systems report levels from 2027 and must comply by April 2029. In May 2026 EPA proposed dropping the four other limits and allowing PFOA and PFOS exemptions to 2031; neither is final.

EPA sets enforceable limits for many pesticides; atrazine's is 3 µg/L. In a USGS study of groundwater used for public supply, pesticide compounds were found in 41% of 1,204 wells, with atrazine and four of its breakdown products among the most common, but none exceeded human-health benchmarks.

A 1996 breath study found that drinking, breathing and skin contact each added to TCE exposure from daily water use, and that doses inhaled or absorbed through skin spread through the body, while swallowed doses were metabolized first. A USGS survey of US domestic wells found at least one VOC in 65% of low-level samples but levels above a health benchmark in 1.2%.

EPA sets enforceable limits for dozens of synthetic organic chemicals, including PCBs and dioxin, whose limit equals 30 picograms per liter. PCB production was banned in the US in 1979, and PCBs cling to soil and sediment. 1,4-dioxane has no federal limit; EPA's lifetime health advisory is 0.2 mg/L.

WHO's review found treated drinking water usually well below 0.05 µg/L (50 ng/L), more than 1,000 times below the lowest therapeutic doses, and judged health effects very unlikely. A USGS-EPA study of 25 US treatment plants with wastewater-affected sources detected 47 different drugs in source water, with medians below 113 ng/L.

Disinfectants react with natural organic matter to form by-products such as trihalomethanes and haloacetic acids, faster in warm water, so levels tend to peak in summer. Under EPA's by-product rules, surface-water plants using conventional filtration must remove 15% to 50% of TOC, depending on how much is in the source water and its alkalinity.

EPA lists foaming agents among its secondary, non-enforceable standards; the noticeable effects are a frothy, cloudy look, bitter taste and odor. Foam usually comes from detergents when water is agitated or aerated, as in many faucets. In groundwater, detergents mainly signal contamination of sewage origin.

EPA says total coliforms are not a health threat in themselves; they show whether treatment and pipes are working. A positive sample triggers repeat tests and an E. coli test. E. coli in a repeat sample breaks the federal limit, which EPA rules treat as a possible acute health risk requiring public notice.

Legionella and nontuberculous mycobacteria (NTM) are bacteria found naturally in water that can grow in building plumbing, including showerheads. Most healthy people exposed to Legionella do not get sick. CDC lists higher risk for people 50 and older, smokers, and people with chronic lung disease or a weakened immune system.

Both can cause diarrhea, vomiting and cramps when swallowed. Because Crypto resists chlorine, surface water plants rely on filtration as well as disinfection, and since 2006 systems with higher-risk sources must add more Crypto treatment. Utilities that do not filter must address Crypto through watershed protection and, under the 2006 rule, Crypto-specific disinfection.

Enteric viruses can cause diarrhea, vomiting and cramps. Surface water systems must remove or inactivate 99.99% of viruses. Groundwater systems that do not treat to that level must test their source after a positive coliform sample, under the 2006 Ground Water Rule. CDC notes water can carry norovirus when it is not treated properly.

In EPA's 2018 to 2020 national survey, total microcystins were found in 8 of 35,000 samples, from 7 of 3,485 systems. In August 2014 a bloom near Toledo's Lake Erie intake led to a two-day do not drink advisory for nearly 500,000 customers. Boiling does not destroy these toxins and can concentrate them.

Turbidity, the cloudiness particles cause, is limited at treatment plants because higher levels often travel with germs: water from plants using conventional or direct filtration must stay at or below 0.3 NTU in 95% of monthly samples. Sediment picked up later in pipes has no federal limit. EPA's iron guideline, 0.3 mg/L, lists rusty color and sediment as signs.

A 2018 study of 159 tap water samples worldwide found human-made particles, mostly fibers, in 81%; US samples averaged 9.24 per liter. WHO said in 2019 that microplastics in drinking water don't appear to pose a health risk at current levels, but more research is needed; its 2022 review again set out data gaps.

EPA bases its limit on a higher risk of benign intestinal polyps from long-term drinking. WHO (2021) found no consistent evidence that swallowed asbestos is harmful and set no guideline value; levels are usually under 1 million fibers per liter, higher where asbestos-cement pipe is used.

Connecticut's health department says most people smell it from about 0.5 mg/L, and at 1 to 2 mg/L water smells of rotten eggs and corrodes plumbing, tarnishing silver and staining fixtures. If only the hot tap smells, the water heater is the likely source. A rare cause is sewage, so a bacteria test is advised then.

Noses are very sensitive to them: USGS says the public can typically detect about 30 nanograms per liter of geosmin and 10 of 2-MIB, parts per trillion. A study in Environmental Science and Technology notes neither has known adverse effects. They are different compounds from the toxins some cyanobacteria make.