Water pollution affects humans through every route of exposure: drinking, bathing, eating contaminated seafood, and even breathing mist near polluted lakes. The consequences range from immediate gastrointestinal illness to long-term cancers, developmental delays in children, and neurological damage. In 2019 alone, 1.4 million deaths worldwide were attributed to unsafe water, sanitation, and hygiene, with over one million of those from diarrheal disease.
Waterborne Infections
Bacteria, viruses, and parasites in contaminated water cause the most immediate and visible health effects. In the early 1900s, cholera and typhoid spread primarily through drinking water and caused severe gastrointestinal illness, often fatal. Modern water treatment has largely eliminated those outbreaks in wealthy nations, but the spectrum of waterborne illness has widened. Today, contaminated water is linked to respiratory illnesses, neurological problems, skin conditions, bloodstream infections, and gastrointestinal disease.
One particularly dangerous category involves bacteria that grow in biofilms, the slimy layers that coat the insides of pipes and plumbing systems. While biofilm-related infections account for a small percentage of waterborne disease cases, they cause the majority of hospitalizations and deaths from waterborne illness in the United States. These infections can be especially dangerous for people with weakened immune systems or chronic lung conditions.
Globally, the burden falls hardest on communities without reliable water treatment. Sixty-nine percent of all diarrheal deaths in 2019 were caused by unsafe water and sanitation services. Another 356,000 people died from acute respiratory infections linked to unsafe hygiene practices that same year.
Heavy Metals in Drinking Water
Lead, arsenic, mercury, cadmium, and chromium can enter water supplies through corroding pipes, industrial runoff, mining operations, and natural geological deposits. These metals share a common mechanism of harm: they generate destructive molecules inside your cells, overwhelm your body’s natural defenses against that damage, and disable enzymes your cells need to function.
Lead is one of the most well-studied threats. The EPA considers no level of lead safe and sets its safety goal at zero. When lead builds up in the body, it interferes with the enzymes that produce hemoglobin, potentially causing anemia. In children, even low-level exposure causes deficits in attention span, learning ability, and IQ. Adults face kidney problems and high blood pressure. Lead typically enters tap water through corroded household plumbing, not from the water source itself, which is why older homes with lead service lines carry higher risk.
Arsenic contamination, common in groundwater in many parts of the world, causes a condition called arsenicosis with prolonged exposure. Symptoms include distinctive skin changes like darkened patches and thickened skin on the palms and soles, along with lung disease and increased cancer risk. The EPA’s maximum contaminant level for arsenic is 0.010 mg/L, but the safety goal is zero because any amount carries some risk.
Cadmium contamination was responsible for one of the most infamous pollution disasters in history. In Japan, mass contamination of food and water supplies caused “itai-itai disease,” a condition marked by painful bone deterioration, kidney failure, and lung disease. Chromium, meanwhile, can accumulate in the body over time and has been linked to cancers of the lungs, bladder, kidneys, and other organs, as well as neurological and kidney disorders.
PFAS and Other Synthetic Chemicals
Per- and polyfluoroalkyl substances, commonly called PFAS or “forever chemicals,” represent a newer category of water contaminant. There are nearly 15,000 of these synthetic chemicals, and they earned their nickname because they persist in the environment indefinitely. They also accumulate in human tissue. You take in more than you excrete, so your body’s burden grows over time with repeated exposure.
The health effects of PFAS exposure are wide-ranging. They alter metabolism and weight regulation, increasing the risk of childhood obesity. They suppress the immune system’s ability to fight infections. Two of the most common types, PFOA and PFOS, have been specifically identified as hazards to immune function. In adolescents, PFAS exposure has been linked to decreased bone mineral density over time. In girls, it may delay the onset of puberty, which itself is associated with higher long-term risk of breast cancer, kidney disease, and thyroid disease. There is also evidence of increased cancer risk and negative effects on birth outcomes.
These chemicals enter water supplies from industrial discharge, firefighting foam, and the breakdown of consumer products. Because they don’t degrade naturally, contamination tends to build over decades rather than clearing on its own.
Contamination Through the Food Chain
You don’t have to drink polluted water directly to be affected by it. Mercury released into waterways gets converted by microbes into methylmercury, a far more toxic and absorbable form. This compound magnifies as it moves up the food chain. By the time it reaches large predatory fish, concentrations can be seven orders of magnitude higher than in the water itself. For many people, eating fish is the primary route of mercury exposure. The health consequences include impaired brain development in children and increased cardiovascular disease risk in adults.
PCBs, industrial chemicals banned decades ago but still lingering in waterways, follow a similar pattern. They’re attracted to fat, so they accumulate in fish tissue at levels up to a million times higher than the surrounding water. When pregnant women eat PCB-contaminated fish, it can cause reproductive complications and neurobehavioral effects in their children. Even in adults, neurological effects correlate directly with how often contaminated fish is consumed.
Cancer Risk From Water Treatment Itself
There’s an ironic twist to water safety: the chlorine used to kill dangerous pathogens can react with organic matter in the water to create disinfection byproducts. The most studied group, trihalomethanes, has been consistently linked to increased bladder cancer risk. Research also suggests a connection to colorectal cancer. One analysis estimated that the lifetime cancer risk from disinfection byproducts for the 279 million Americans served by community water systems is approximately 3 in 1,000. That risk is far smaller than the danger of drinking untreated water, but it’s a measurable trade-off that drives ongoing efforts to reduce these byproducts while maintaining disinfection.
Nitrates and Infant Health
Nitrates enter water supplies primarily through agricultural fertilizer runoff and septic system leachate. For most adults, moderate nitrate levels aren’t immediately dangerous. But for infants under six months, high nitrate levels can cause methemoglobinemia, sometimes called “blue baby syndrome,” a condition where the blood loses its ability to carry oxygen effectively. The EPA’s maximum contaminant level for nitrate is 10 mg/L measured as nitrogen. If your well water approaches or exceeds that level, it should not be used to prepare infant formula.
This is primarily a concern for families using private wells, which aren’t subject to the same testing requirements as public water systems. Agricultural areas with heavy fertilizer use tend to have the highest groundwater nitrate concentrations.
Harmful Algal Blooms
Water pollution doesn’t just add toxic substances to water. It also fuels biological events that create new toxins. Excess nutrients from fertilizer runoff and sewage cause explosive growth of cyanobacteria (blue-green algae) in lakes, reservoirs, and coastal waters. These blooms produce multiple toxins that can harm you through skin contact, inhalation, or swallowing.
Skin contact or breathing in droplets near a bloom can cause rashes, eye and nose irritation, sore throat, and coughing. Swallowing bloom-contaminated water or eating fish and shellfish exposed to it can cause stomach pain, vomiting, diarrhea, headache, muscle weakness, dizziness, and liver damage. There is also growing concern about a compound called BMAA produced by cyanobacteria, which animal studies have linked to harmful effects on the brain, raising questions about a possible connection to neurodegenerative diseases in humans.
These blooms are becoming more frequent and widespread as nutrient pollution increases and water temperatures rise, making them an escalating public health concern rather than a rare event.