Arsenic in drinking water damages nearly every organ system in your body, from your skin to your heart, lungs, and bladder. It is the most significant chemical contaminant in drinking water globally, and the effects depend on how much you’re exposed to and for how long. Even at levels considered “low,” chronic exposure over years can raise your risk of cancer, diabetes, and cardiovascular disease.
How Arsenic Damages Your Cells
Once arsenic enters your body, it disrupts how your cells produce energy. Your cells rely on a chain of chemical reactions to convert food into fuel. Arsenic shuts down key enzymes in that process, essentially starving cells of the energy they need to function. As energy production breaks down, cells generate a flood of unstable molecules called free radicals, which damage DNA, destroy proteins, and punch holes in the membranes that protect your cells.
Your body has built-in defenses against this kind of damage, but arsenic overwhelms them. It depletes the antioxidants your cells depend on, leaving DNA exposed. Worse, arsenic also interferes with your body’s DNA repair machinery, so the damage accumulates rather than getting fixed. Over time, this combination of constant damage and impaired repair is what drives cells toward cancer.
Skin Changes Are Usually the First Sign
The earliest visible effects of chronic arsenic exposure show up on the skin, typically after about five years of drinking contaminated water. The hallmark is patchy darkening of the skin, especially on the eyelids, neck, armpits, groin, and nipples. The pattern is distinctive enough that it has its own description: dark brown patches with scattered pale spots, sometimes called a “raindrops on a dusty road” appearance. In severe cases, the discoloration spreads across the chest, back, and abdomen. These pigment changes have been documented in populations drinking water with arsenic levels at or above 400 parts per billion (ppb), which is 40 times the current safety standard.
The other signature skin change is thickened, hardened patches on the palms of the hands and soles of the feet. These growths are usually small, corn-like bumps ranging from about 4 to 10 millimeters across. Many remain benign for decades, but some develop precancerous changes that are indistinguishable from an early form of squamous cell carcinoma. White horizontal bands across the fingernails, known as Mees’ lines, appear in roughly 5% of people with chronic exposure, usually four to six weeks after a significant dose.
Cancer Risk From Long-Term Exposure
The International Agency for Research on Cancer classifies ingested arsenic as carcinogenic to humans. The strongest evidence links it to three types of cancer: skin, bladder, and lung. In fact, the current U.S. and WHO drinking water standards of 10 ppb are based specifically on the risk of lung and bladder cancer. Studies from high-exposure regions like Northern Chile have found dramatically elevated cancer rates, with odds ratios as high as 6.5 for bladder cancer and 4.3 for lung cancer among people with significant long-term exposure.
Some research also suggests links to kidney cancer and prostate cancer, though the evidence for those is less definitive than for the big three. Arsenic-related cancers can take decades to develop, which makes them easy to overlook as a consequence of water contamination. Exposure in the womb or during early childhood has been linked to increased cancer mortality in young adults, meaning the damage can begin long before symptoms ever appear.
Heart Disease and Diabetes
Arsenic’s effects extend well beyond cancer. Chronic exposure raises the risk of heart attacks, and arsenic-induced heart attacks are a significant cause of excess deaths in contaminated regions. Studies in areas with high arsenic levels in drinking water have found that exposed individuals had roughly 1.8 times the odds of developing high blood pressure and 1.7 times the odds of being diagnosed with type 2 diabetes compared to people in areas with clean water. The risk of obesity was also elevated, ranging from 1.3 to 1.9 times higher depending on exposure levels.
The cardiovascular damage is serious enough that in Taiwan, chronic arsenic exposure from well water caused a condition called “Blackfoot disease,” where blood flow to the extremities was so severely compromised that gangrene set in. While this extreme outcome is rare, it illustrates how profoundly arsenic affects the circulatory system.
Effects on Pregnancy and Child Development
Arsenic crosses the placenta and accumulates in fetal tissues. A meta-analysis of studies on the topic found that higher maternal arsenic exposure was associated with a 12% increase in the odds of preterm birth. The mechanisms are varied: arsenic triggers inflammation, causes oxidative stress, disrupts hormones, and interferes with the development of placental blood vessels. When the placenta can’t form properly, it delivers less oxygen and fewer nutrients to the fetus, increasing the risk of fetal distress.
Children who survive arsenic exposure in the womb or during early life face lasting consequences. Multiple studies have found negative impacts on cognitive development, intelligence, and memory. Young adults who were exposed during early childhood also face higher rates of death from cancer, lung disease, heart attacks, and kidney failure.
Acute Poisoning Looks Different
Everything described above relates to chronic, low-level exposure over months or years, typically at doses between 0.1 and 0.5 mg per kilogram of body weight. Acute arsenic poisoning, which occurs at doses of 1 to 3 mg/kg, is a different medical emergency entirely.
Acute poisoning starts with severe gastrointestinal symptoms: nausea, vomiting, abdominal pain, and watery diarrhea that can become bloody. These symptoms can begin within minutes to hours of ingestion. The diarrhea is sometimes described as resembling “rice water.” Dangerous drops in blood pressure follow, driven by fluid loss. Kidney failure can develop within hours to days. Nerve damage causing tingling, numbness, and weakness in the hands and feet typically appears one to three weeks later. Acute poisoning from drinking water alone is uncommon in the U.S. today, but it remains a concern in parts of South Asia and Latin America where well water arsenic levels can be extremely high.
How Much Is Too Much
The EPA set the maximum contaminant level for arsenic in public drinking water at 10 ppb in 2001, replacing an older standard of 50 ppb. The WHO uses the same 10 ppb guideline. This standard was designed to limit cancer risk, but it’s worth noting that no level of arsenic exposure is considered completely safe. Health effects have been observed in populations exposed to levels both above and below the current standard, particularly with decades of exposure.
Public water systems in the U.S. are required to test for arsenic and treat water that exceeds the limit. Private wells, however, are not regulated. The CDC recommends that private well owners test their water at least once a year for basic contaminants, and specifically recommends asking local health or environmental agencies whether arsenic testing is necessary for their region. Arsenic is naturally present in rock formations across many parts of the U.S., with the Southwest, parts of New England, and the upper Midwest being particularly affected.
Removing Arsenic From Your Water
If your water tests above 10 ppb, the most effective home treatment options are reverse osmosis (RO) and nanofiltration systems. Standard carbon filters and basic pitcher-style filters are not effective at removing arsenic. The pores are simply too large relative to the size of arsenic particles.
Reverse osmosis systems have demonstrated removal rates above 99% in some studies, and consistently achieve at least 80% removal across varying water conditions. These are typically installed under the kitchen sink and filter water at the point of use rather than treating the whole house. Nanofiltration is similarly effective and has been recommended for arsenic-rich water sources in countries like China. If you rely on a private well in an area with known arsenic contamination, installing an RO system and retesting your water after installation is the most reliable way to bring your exposure down to safe levels.