Across the Indo-Gangetic plains, millions of families draw drinking water from hand pumps installed decades ago. Although the water carries no odor or discoloration, in the Gangetic-plain districts of West Bengal, Bihar, Uttar Pradesh and Assam, that water often contains arsenic at concentrations far exceeding the World Health Organization guideline of 10 micrograms per liter (10 µg/L). The problem is not limited to India. Across southern Asia, more than 140 million people are estimated to drink arsenic-contaminated groundwater. In India, between 20 and 30 million people may be chronically exposed. Public health experts describe this as the largest mass poisoning in history, albeit a slow-moving one.
For a long time, arsenic was seen only as a cause of skin lesions and rare cancers. We now know that the health toll is much broader. The strongest evidence comes from neighboring Bangladesh, whose hydrogeology mirrors eastern India. The Health Effects of Arsenic Longitudinal Study (HEALS), following tens of thousands of adults over many years, demonstrated a clear dose–response relationship between arsenic exposure and mortality. Individuals exposed to higher arsenic concentrations had significantly elevated risks of death, not only from cancers of the lung, bladder and skin, but also from cardiovascular diseases.
At high exposure levels, lung and bladder cancer risks have been observed to double or even triple compared to low-exposure populations. Equally concerning, cardiovascular mortality — heart attacks and strokes — rises substantially with increasing arsenic exposure, sometimes accounting for as many or more arsenic-attributable deaths as cancer.
Indian data, though less extensive, indicate elevated rates of lung and bladder cancer in exposed populations in arsenic-affected districts of West Bengal, along with increased prevalence of peripheral vascular disease — sometimes termed “blackfoot disease” in its most severe form. Surveys have also shown higher burdens of hypertension and ischemic heart disease in heavily exposed communities.
At the national level, cancer registry data compiled by the National Cancer Registry Program do not indicate that the entire Gangetic plain as India’s highest-incidence cancer region. That is not surprising because arsenic exposure is geographically patchy, registry coverage in rural districts is incomplete, and arsenic-related cancers often emerge decades after exposure begins. Furthermore, when arsenic doubles the risk of a relatively uncommon cancer, the absolute numbers may still appear modest in aggregated statistics. District-level studies show that in villages and blocks with high arsenic concentrations, cancer and cardiovascular risks are measurably elevated relative to low-exposure areas.
Estimating precise mortality in India is difficult because arsenic does not appear on death certificates. Lung cancer may be attributed to smoking, heart attacks to diet or diabetes. Yet, extrapolating from cohort data and exposure prevalence suggests that arsenic likely contributes to several thousand excess deaths annually in India, many from cardiovascular causes that remain invisible in the public narrative.
The roots of the arsenic crisis lie in geology and microbiology, not industrial dumping. Sediments eroded from arsenic-bearing rocks in the Himalayas are carried downstream into the Ganga-Brahmaputra delta. For years, scientists believed arsenic was released deep underground. Field research in south and southeast Asia transformed that understanding.
Investigators showed that arsenic mobilization often begins near the surface. Arsenic arriving from upstream binds to iron oxides, essentially rust particles, in river sediments. When these sediments are rapidly buried, oxygen disappears. In oxygen-poor conditions, soil bacteria shift their metabolism, using iron compounds instead of oxygen. In dissolving the iron minerals, they also release the arsenic bound to them. The dissolved arsenic then migrates downward into aquifers tapped by tube wells.
This process is natural and has likely operated for centuries. But the impact on human health likely began in the 1970s and 1980s, when millions of shallow tube wells were installed across eastern India to reduce cholera and diarrheal disease from contaminated surface water. Although the intervention saved lives from infection, it exposed communities to arsenic.
Arsenic does not kill swiftly, but increases lifetime risk. A villager exposed in her twenties may develop heart disease in her forties or cancer in her fifties. The latency obscures causality. The deaths blend into the background burden of non-communicable disease. Bangladesh, for example, responded with nationwide well-testing campaigns, colour-coding contaminated wells and installing deeper community wells. Although exposure persists in some regions, awareness and mitigation have reduced reliance on highly contaminated sources.
The most compelling evidence that action matters comes from a 20-year follow-up study of nearly 11,000 adults in Bangladesh published late last year. Researchers found that individuals who reduced their arsenic exposure by switching to safer water sources experienced up to a 50 percent lower risk of death from heart disease, cancer and other chronic conditions compared with those who continued drinking contaminated water. Lowering arsenic exposure can substantially reduce mortality, even among people who had already been exposed for years.
For India, the lesson is that centralized treatment and reliable piped water provide the most durable protection. Household filters and rainwater harvesting can help, but they depend on maintenance, monitoring and affordability. Although India has expanded rural water infrastructure under the Jal Jeevan Mission, which aims to provide piped water to every rural household, much remains to be done to make sure that piped water is drinkable. But if enhanced and properly managed, the mission could eliminate one of the largest environmental health threats in the country.
The question now is whether safe drinking water becomes a universal guarantee, rather than a village-by-village contingency. The wells that once shielded communities from cholera now expose them to a different hazard. Ending this slow poisoning will require the same clarity of purpose that drove the original groundwater revolution — only this time guided by a fuller understanding of the risks that lie beneath our feet.
Ramanan Laxminarayan is president, One Health Trust. The views expressed are personal
