Arsenic in water is a naturally occurring toxic element that dissolves into groundwater as it passes through certain types of rock and soil. It has no smell, no taste, and no color, which means you won’t know it’s there unless you test for it. The current U.S. safety limit is 10 parts per billion (ppb), a standard the EPA set in 2001 after lowering it from the previous limit of 50 ppb. Even at concentrations below that threshold, long-term exposure carries health risks.
How Arsenic Gets Into Water
Arsenic is a trace component in many rocks and sediments. Groundwater picks it up naturally as it flows through volcanic and granitic rock formations, dissolving small amounts along the way. This is the primary route of contamination for most people, and it’s why private wells drilled into bedrock are especially vulnerable.
Human activity adds another layer. Mining operations, industrial processes, and the historical use of arsenic-based pesticides and wood preservatives have all introduced the element into soil and water. Animal feed additives containing arsenic compounds have also contributed. In areas with both geological sources and industrial history, concentrations can be significantly higher.
Inorganic vs. Organic Arsenic
Arsenic exists in two broad chemical forms, and the distinction matters. Inorganic arsenic, the type found in drinking water, is highly toxic. Organic arsenic, the kind present in seafood, is far less harmful and passes through the body more easily. When people talk about the dangers of arsenic in water, they’re referring to the inorganic form.
Why You Can’t Detect It Without Testing
Unlike contaminants that change the color, smell, or taste of your water, arsenic is completely invisible to your senses. Clear, good-tasting water can still contain dangerous levels. The only way to know your exposure is to have your water tested, and this is especially important if you rely on a private well, since public water systems are required to monitor and treat for arsenic while private wells are not.
To get your water tested, contact your state laboratory accreditation program for a list of licensed labs in your area. Order a test kit specifically for arsenic and ask the lab whether they’re accredited for drinking water compliance samples. One important detail: make sure the lab’s reporting limit for arsenic is 5 ppb or lower. Some labs use a 10 ppb reporting limit, which means water containing 9 ppb could come back labeled “not detected” even though it’s close to the federal limit. Collect your sample from the tap you drink from most often, typically the kitchen sink, and follow the kit’s instructions carefully.
Health Effects of Long-Term Exposure
Arsenic interferes with enzyme reactions throughout the body, which is why chronic exposure can damage nearly every organ system. The health effects don’t appear overnight. They develop over years of drinking contaminated water, and the severity depends on the concentration and duration of exposure.
The strongest links are to cancer. Chronic ingestion of inorganic arsenic increases the risk of skin, lung, and bladder cancer. There are also associations, though less firmly established, with cancers of the liver, kidney, and prostate.
Skin changes are often among the earliest visible signs. People with heavy chronic exposure may develop patchy darkening of the skin and small, hard growths called keratoses, most commonly on the palms and soles of the feet. These changes can appear before other symptoms and sometimes serve as a warning sign of prolonged exposure.
The cardiovascular system is also affected. A condition known as “blackfoot disease,” a severe form of gangrene in the extremities, was documented in communities in Taiwan that drank heavily contaminated well water for years. There are also limited associations with high blood pressure and heart disease more broadly.
Other documented effects include:
- Liver damage: Chronic exposure can lead to a form of liver scarring and, rarely, a type of liver cancer called angiosarcoma.
- Kidney damage: Long-term exposure has been linked to chronic kidney insufficiency.
- Nerve damage: Arsenic can destroy nerve fibers, leading to numbness, tingling, and weakness in the hands and feet.
- Blood disorders: Anemia and low white blood cell counts are common findings in chronic arsenic toxicity.
- Diabetes: There is a reasonably strong association between chronic exposure and the development of type 2 diabetes.
- Cognitive effects in children: Follow-up studies of children who consumed arsenic-contaminated milk over time found increased rates of cognitive deficits, epilepsy, brain damage, and severe hearing loss.
Where Arsenic Levels Are Highest
In the United States, arsenic concentrations in groundwater tend to be highest in the western states, where volcanic and granitic geology is more common. Parts of the Southwest, the northern Great Plains, and New England have documented elevated levels. Globally, Bangladesh, India (particularly West Bengal), Vietnam, Argentina, and Chile face some of the most severe arsenic contamination, affecting tens of millions of people who depend on groundwater for drinking.
Even within a single neighborhood, arsenic levels can vary dramatically from well to well depending on the underlying rock. Your neighbor’s test results tell you very little about your own water.
Removing Arsenic From Your Water
If testing reveals arsenic above 10 ppb, or even approaching that level, treatment is worth considering. Several technologies can reduce arsenic, but their real-world performance varies more than manufacturers’ claims might suggest.
A USGS study of 11 homes in Ohio tested three types of systems: single-tap reverse osmosis units, whole-house oxidation and filtration systems, and combined anion exchange with reverse osmosis. The results were humbling. Arsenic removal ranged from just 2% to 90%, with a median of 65%. That means the typical system removed about two-thirds of the arsenic, but some barely made a dent.
The takeaway is that installing a filter isn’t enough on its own. You need to retest your treated water to confirm the system is actually working. Maintenance matters too. Filters that aren’t replaced on schedule lose effectiveness. If your raw water arsenic concentration falls between 10 and 30 ppb, both reverse osmosis and oxidation/filtration systems can achieve similar reductions, so the choice often comes down to whether you want a single-tap solution or whole-house coverage.
For very high concentrations, a single treatment method may not bring levels below the safety standard, and combining technologies or finding an alternative water source may be necessary.