Tap water contains a complex mix of intentionally added disinfectants, naturally occurring minerals, and trace contaminants that slip through treatment. In the United States, the EPA regulates over 90 chemicals in drinking water, setting legal limits for everything from arsenic to industrial solvents. Some of what’s in your water is there on purpose to keep you safe. The rest arrives from pipes, soil, farms, factories, and even the treatment process itself.
Disinfectants: Chlorine and Chloramine
The most common chemicals in tap water are the ones your water utility adds deliberately. Chlorine or chloramine (a combination of chlorine and ammonia) kills bacteria, viruses, and parasites that would otherwise make water dangerous to drink. These disinfectants are the reason waterborne diseases like cholera and typhoid are essentially nonexistent in the U.S. today.
The EPA allows up to 4.0 mg/L of either chlorine or chloramine in finished drinking water. Most utilities keep levels well below that, typically between 0.2 and 2.0 mg/L. That’s enough to maintain germ-killing power as water travels through miles of pipes to your faucet, but low enough that the taste and smell stay faint. If your water has a noticeable pool-like odor, it usually means chlorine levels are on the higher end of normal, not that anything is wrong.
Disinfection Byproducts
When chlorine reacts with organic matter naturally present in the water source (leaves, algae, soil particles), it creates a secondary category of chemicals called disinfection byproducts. The two main groups are trihalomethanes and haloacetic acids. These form during treatment and are difficult to eliminate entirely. Long-term exposure at high levels has been linked to increased cancer risk, which is why the EPA caps trihalomethanes at 80 parts per billion and haloacetic acids at 60 ppb in drinking water. Your water utility tests for these regularly, and the results appear in your annual water quality report.
Fluoride
Most U.S. water systems add fluoride to reduce tooth decay. The recommended level is 0.7 mg/L, though the legal maximum is 4.0 mg/L. At the recommended concentration, fluoride strengthens tooth enamel without posing health risks. At levels above 2.0 mg/L over many years, it can cause cosmetic staining of teeth in children, and concentrations above 4.0 mg/L raise the risk of bone problems. Whether your water is fluoridated depends on your local utility; about 73% of the U.S. population on public water systems receives fluoridated water.
Minerals and Water Hardness
Tap water picks up dissolved minerals as it moves through rock and soil. Calcium and magnesium are the most abundant, and their combined concentration determines whether your water is “hard” or “soft.” The U.S. Geological Survey classifies water with 0 to 60 mg/L of calcium carbonate as soft, 61 to 120 mg/L as moderately hard, 121 to 180 mg/L as hard, and anything above 180 mg/L as very hard.
Hard water is not a health concern. In fact, the World Health Organization notes that drinking water can be a meaningful source of calcium and magnesium, particularly for people whose diets are low in these minerals. The main downside of hard water is practical: it leaves scale deposits on fixtures, reduces soap lathering, and can shorten the lifespan of water heaters. Aside from calcium and magnesium, tap water also contains trace amounts of sodium, potassium, iron, manganese, and other minerals that vary by region.
Lead and Copper
Lead and copper aren’t present in the water when it leaves the treatment plant. They leach into the water from plumbing materials, including older lead service lines, brass fittings, and copper pipes, especially when water sits in pipes for hours or when the water’s chemistry is corrosive enough to dissolve metal surfaces.
The EPA’s action level for lead is 15 parts per billion. If more than 10% of sampled homes in a water system exceed that threshold, the utility must take steps to reduce corrosion. The health goal for lead, though, is zero, because no level of lead exposure is considered safe, particularly for young children. Copper’s action level is 1.3 parts per million. At levels above that, copper can cause nausea and, over longer periods, liver and kidney damage. If your home has older plumbing, running the cold water tap for 30 seconds to two minutes before drinking flushes out water that has been sitting in contact with pipes.
Nitrates
Nitrates enter water supplies primarily from agricultural fertilizer runoff, animal waste, and septic systems. The EPA limit is 10 parts per million. This standard was set specifically to protect infants, who are vulnerable to a condition called methemoglobinemia, sometimes known as “blue baby syndrome.” In this condition, nitrates interfere with the blood’s ability to carry oxygen. Adults can handle moderate nitrate levels without issue, but in rural areas with heavy farming, well water and even some public systems can approach or exceed the limit, especially in spring and early summer when fertilizer application peaks.
Arsenic
Arsenic occurs naturally in rock formations and dissolves into groundwater. In 2001, the EPA lowered the legal limit from 50 parts per billion to 10 ppb, reflecting growing evidence that long-term exposure increases the risk of cancer, heart disease, and neurological problems. Groundwater-fed systems in the western U.S. and parts of the Midwest and New England tend to have higher natural arsenic levels. Surface water sources like rivers and reservoirs generally have lower concentrations. Even at levels below 10 ppb, some researchers argue the risk is not zero, which makes arsenic one of the more closely watched contaminants in drinking water.
PFAS (Forever Chemicals)
PFAS are a family of thousands of synthetic compounds used since the 1950s in nonstick cookware, waterproof clothing, food packaging, and firefighting foam. They earned the nickname “forever chemicals” because they don’t break down in the environment or in your body. In 2024, the EPA finalized legally enforceable limits for six PFAS compounds in drinking water for the first time. The two most studied, PFOA and PFOS, are each capped at just 4.0 parts per trillion. Four other PFAS compounds or mixtures have limits of 10 ppt or are regulated through a combined hazard index.
These limits are extraordinarily low, measured in the trillionths rather than the billionths or millionths used for other contaminants. That reflects how persistent PFAS are in the body and the growing evidence linking them to thyroid disease, immune suppression, certain cancers, and reproductive problems. Water utilities have several years to comply with the new standards, so your local system may still be in the process of testing and installing treatment.
Volatile Organic Compounds
Volatile organic compounds, or VOCs, are carbon-based chemicals that evaporate easily and can seep into groundwater from industrial sites, gas stations, dry cleaners, and chemical spills. The EPA regulates more than 20 individual VOCs in drinking water. Benzene, for example, has a legal limit of 5 ppb due to its link to leukemia. Trichloroethylene, a common industrial solvent, is capped at the same level. These chemicals are more likely to show up in water systems near industrial areas or Superfund sites than in systems drawing from protected surface reservoirs.
Microplastics
Tiny plastic particles are now detectable in tap water worldwide. A 2025 systematic review and meta-analysis found an average of about 57 microplastic particles per liter of tap water. These fragments come from the breakdown of plastic bottles, synthetic clothing fibers, tire dust, and packaging. There is no EPA standard for microplastics in drinking water yet, partly because scientists are still determining what level of exposure, if any, causes harm. The particles range in size from visible specks down to fragments too small to see without a microscope, and their health effects at typical drinking water concentrations remain an open question.
Pharmaceutical Residues
Trace amounts of medications end up in tap water when people excrete drugs their bodies didn’t fully absorb, or when unused pills are flushed. A nationwide study analyzing treated drinking water found 17 different pharmaceutical compounds present after treatment. Blood pressure medications and antidepressants were the most commonly detected categories. Concentrations are extremely low, typically in the range of single-digit nanograms per liter (a nanogram is one billionth of a gram). To put that in perspective, you would need to drink thousands of liters of tap water to approach a single therapeutic dose of any of these drugs. No current EPA standards exist for pharmaceuticals in drinking water, but several compounds have been flagged for future monitoring.
How to Check What’s in Your Water
Every public water system in the U.S. is required to publish an annual Consumer Confidence Report listing exactly which contaminants were detected and at what levels. You can find yours by searching your utility’s name online or checking the EPA’s database. If you’re on a private well, no one tests it for you. Annual testing for bacteria, nitrates, and pH is a good baseline, with additional tests for arsenic, lead, and other contaminants based on your region.
If your report shows levels close to legal limits for anything that concerns you, point-of-use filters can reduce specific contaminants. Activated carbon filters (including pitcher filters) are effective for chlorine, some VOCs, and taste issues. Reverse osmosis systems remove a broader range of contaminants, including lead, arsenic, PFAS, and nitrates. No single filter removes everything, so matching the filter to the specific problem in your water is what matters most.