Several types of water can harm your health, from untreated streams and contaminated wells to water sitting in old pipes or even ultra-purified water stripped of essential minerals. The risks depend on what’s in the water (or missing from it), how long you’re exposed, and who’s drinking it. Here’s a practical breakdown of the water sources and conditions worth understanding.
Untreated Surface Water
Water from lakes, rivers, and streams can look perfectly clear and still be loaded with invisible threats. The main concerns are bacteria, parasites like Giardia and Cryptosporidium, and viruses including norovirus, hepatitis A, rotavirus, and adenovirus. These pathogens cause acute gastrointestinal illness: vomiting, diarrhea, cramping, and sometimes fever. Hepatitis A and E can cause liver inflammation that lingers for weeks.
Municipal water treatment plants remove the vast majority of these organisms through filtration and disinfection. The danger is in drinking water that hasn’t gone through that process, whether you’re camping, hiking, or collecting rainwater without proper treatment. If you’re sourcing water outdoors, boiling it for at least one minute (three minutes at high altitude) or using a filter rated for protozoa and bacteria is the minimum safeguard.
Lead and Heavy Metals in Tap Water
Treated tap water can still pick up contaminants on the way to your glass. Lead is the most well-known culprit. It leaches into water from lead pipes, brass plumbing fixtures, and copper pipes joined with lead solder, especially in homes built before 1986. Lead builds up in the body over months or years. In children, even low-level exposure causes developmental delays, learning difficulties, and behavioral problems. In adults, it affects memory, concentration, kidney function, and the nervous system.
Arsenic is another heavy metal that enters drinking water both from natural geological deposits and from agricultural and industrial runoff. The EPA set the enforceable limit for arsenic at 10 parts per billion, but the health goal is zero. Long-term arsenic exposure is linked to skin changes, circulatory problems, and increased cancer risk. Other heavy metals found in well and tap water include cadmium, chromium, and copper, all of which can damage the liver, kidneys, and intestines at high levels.
Private Well Water
If your water comes from a private well, no federal agency is testing it for you. Public water systems are regulated by the EPA, but private wells are the homeowner’s responsibility. The five major contaminant categories the EPA flags for private wells are microorganisms, nitrates, heavy metals, organic chemicals (from pesticides, solvents, and industrial products), and radionuclides like uranium and radium.
Nitrates deserve special attention if there are infants in your household. Fertilizer runoff and septic system leakage are the primary sources. Nitrate levels above 10 mg/L are dangerous for babies under 12 months because their bodies convert nitrates into a form that prevents red blood cells from carrying oxygen. This condition, called “blue baby syndrome,” can develop rapidly over days, causing bluish skin and shortness of breath. It can be fatal. Adults and older children process nitrates more efficiently, but the threshold for infants is firm: do not use well water above 10 mg/L for formula or baby food.
PFAS-Contaminated Water
PFAS, sometimes called “forever chemicals,” are synthetic compounds used in nonstick cookware, food packaging, firefighting foam, and waterproof clothing. They don’t break down in the environment, and they accumulate in the body over time. In 2024, the EPA finalized the first-ever national drinking water standard for PFAS, setting enforceable limits of 4.0 parts per trillion for the two most studied compounds, PFOA and PFOS. The health-based goal for both is zero.
Those limits are extraordinarily low, which reflects how potent these chemicals are at small concentrations. PFAS exposure has been linked to immune suppression, thyroid disruption, liver damage, and certain cancers. If you’re on a public water system, your utility will be required to test for and reduce PFAS levels. If you’re on a private well near industrial sites, military bases, or airports where firefighting foam was used, independent testing is the only way to know your exposure.
Stagnant Water in Your Plumbing
Water that sits in pipes for days or weeks creates two problems. First, metals like lead and copper leach more heavily into water the longer it stays in contact with plumbing. Second, stagnant water is the ideal breeding ground for Legionella, the bacterium that causes Legionnaires’ disease, a severe form of pneumonia. Legionella thrives in water between 77°F and 113°F, and stagnant conditions accelerate its growth by depleting the residual disinfectant that normally keeps bacteria in check.
This is most relevant if you’ve been away from home for an extended period, or if you have a water heater set below 120°F. The CDC recommends keeping hot water stored above 140°F and ensuring it doesn’t drop below 120°F in circulation. Cold water should stay below 77°F. When returning from a trip, flush your taps for several minutes before drinking.
Distilled and Demineralized Water
Water that’s been stripped of all minerals through distillation or reverse osmosis isn’t toxic in the short term, but drinking it exclusively over months or years creates real nutritional gaps. Drinking water normally supplies meaningful amounts of calcium, magnesium, and other trace minerals. Research published in the Medical Journal of Armed Forces India found that demineralized water used for cooking can leach up to 60% of calcium and magnesium from food, and even higher percentages of copper, manganese, and cobalt.
The downstream effects are not trivial. Low calcium intake is associated with higher fracture risk in children. Low magnesium intake has been linked to pregnancy complications, including preeclampsia, and certain neurodegenerative diseases. If you use a reverse osmosis system at home, remineralization filters or a mineral-rich diet can offset what the water no longer provides.
Bottled Water and Microplastics
Bottled water is generally safe from a microbial and chemical standpoint, but it carries a different concern. A 2024 study published in the Proceedings of the National Academy of Sciences found that a typical liter of bottled water contains roughly 240,000 tiny plastic particles, about 90% of which are nanoplastics small enough to cross cell membranes. That number is 10 to 100 times higher than earlier estimates, which had only measured larger microplastic fragments.
The long-term health effects of ingesting nanoplastics are still being studied, but their ability to penetrate tissues and accumulate in organs is what concerns researchers. The plastic comes primarily from the bottles themselves and the manufacturing process. Tap water also contains microplastics, but generally at lower concentrations than bottled water.
Excessively Alkaline Water
Alkaline water with a pH above 9 is marketed as a health booster, but the claims are thin. Your stomach fluid sits at a pH of 1.5 to 3.5, and any alkaline water you drink gets neutralized almost immediately upon arrival. Even if a large volume temporarily raises stomach pH, your kidneys quickly rebalance your blood chemistry.
Harvard Health notes that water above pH 9 tends to taste bitter and, more importantly, poses a risk for people taking proton pump inhibitors for acid reflux. These medications already raise stomach pH, and stacking alkaline water on top could push blood pH high enough to disrupt potassium levels, particularly in people with kidney disease. For most people, alkaline water is simply an expensive way to drink water that your body will treat the same as any other.
Agricultural and Industrial Runoff
Pesticides and herbicides from farming operations are among the most common organic contaminants in groundwater. The EPA regulates more than a dozen agricultural chemicals in drinking water, including atrazine (from row crop herbicides), glyphosate (the active ingredient in many weed killers), and simazine. Banned chemicals like toxaphene and heptachlor still show up in water supplies because they persist in soil for decades.
These contaminants are most likely to affect people on private wells in agricultural regions, though they occasionally appear in public systems near heavy farming activity. Long-term exposure to organic chemicals at levels above EPA limits can damage the kidneys, liver, nervous system, and reproductive system. If your well is within a mile of active farmland, annual testing for nitrates and pesticides is a practical step.