Cancer Clusters and Environmental Contamination: How Communities Found the Truth

In 1979, Ann Anderson had already buried one child to leukaemia and was watching her second die of it. She lived in Woburn, Massachusetts, a town whose eastern neighbourhoods seemed to her to have an unusual number of childhood leukaemia cases. Nobody in official medicine took her seriously at first. She started knocking on doors.

What Anderson found — eventually, with the help of a minister and then epidemiologists and eventually a famous civil trial — was that the water supplying her neighbourhood had been contaminated with trichloroethylene and tetrachloroethylene from industrial dumping into the soil near two municipal wells. The case became the subject of a book, A Civil Action, and later a film. It became one of the defining stories of environmental justice — of a community doing the work that institutions wouldn't do.

Cancer clusters are both real and regularly misidentified. The statistical methods for evaluating whether an apparent clustering of cancer cases in a geographic area reflects genuine environmental causation or random variation are genuinely difficult, and many apparent clusters don't survive rigorous scrutiny. But some do. Woburn did. Hinkley, California did. Camp Lejeune did. Understanding how cluster investigations work — and how to check whether you've lived near known contamination — is a practical environmental health skill.

The formal definition of a cancer cluster is: a greater-than-expected number of cancer cases that occurs within a group of people in a geographic area over a defined time period.

The challenge is "greater than expected" — and calculating what's expected is harder than it sounds.

The statistical problem Cancer rates vary naturally across regions for reasons unrelated to environmental contamination: age distribution of the population, differences in screening rates, genetic variation, socioeconomic factors, smoking rates, and random fluctuation. A small community that happens to have older residents, or that has higher screening rates for a particular cancer, will appear to have elevated incidence without any environmental cause.

The human tendency toward pattern recognition — seeing clusters in random distributions — means that perceived cancer clusters are far more common than genuine ones. State health departments receive hundreds of cluster inquiries per year and confirm environmental aetiology in only a small fraction.

What makes a cluster investigation compelling: • Multiple cases of a rare cancer (not common cancers like breast or lung, which occur at background rates that produce apparent clusters by chance) • A plausible exposure route — contaminated water reaching identified households, proximity to a specific facility with known carcinogen releases • A biological mechanism linking the proposed exposure to the specific cancer type • A temporal relationship between the exposure and disease onset consistent with the known latency for that cancer

The CDC and state health department process Formal cluster investigations begin with state health departments, which collect the data, apply standardised statistical methods, and determine whether the case rate exceeds expected levels after controlling for confounders. The CDC provides technical assistance for complex investigations.

Three cluster investigations have generated the most evidence, the most litigation, and the most lasting public health impact.

Woburn, Massachusetts The Woburn cluster investigation produced two important outcomes: the first community-driven, scientifically rigorous cancer cluster investigation in the US (led by Harvard epidemiologist Phil Brown in partnership with the community), and a CERCLA enforcement action that became a landmark in Superfund litigation. The scientific evidence for a causal link between the contaminated wells and childhood leukaemia was contested for years but ultimately supported by subsequent analysis. The contaminated sites remain in EPA's Superfund database.

Hinkley, California The case made famous by Erin Brockovich: Pacific Gas & Electric contaminated the groundwater of the town of Hinkley with hexavalent chromium — a known carcinogen — from a gas compressor station. The subsequent cluster investigation found elevated rates of multiple cancer types in Hinkley residents. A 1996 legal settlement of $333 million remains one of the largest direct-action lawsuit settlements in US history. IARC classifies hexavalent chromium as a Group 1 carcinogen.

Camp Lejeune Marines stationed at Camp Lejeune, North Carolina between 1953 and 1987 and their families consumed drinking water contaminated with TCE, PCE, benzene, and vinyl chloride at concentrations far above safety limits. Studies have since documented elevated rates of multiple cancers, neurological disease, and adverse reproductive outcomes in Camp Lejeune residents. The Camp Lejeune Justice Act of 2022 allows affected persons to file claims against the government — the first time such claims were permitted after decades of legal barrier.

For anyone who has lived near industrial facilities, military installations, or known Superfund sites, several public databases allow systematic checking of contamination history.

EPA Superfund Site Database The National Priorities List (NPL) database at epa.gov/superfund/search-superfund-sites-where-you-live allows you to search for Superfund sites by ZIP code or state. Each site listing includes the contaminants of concern, the extent of contamination, the current cleanup status, and often a health assessment or health consultation from the Agency for Toxic Substances and Disease Registry.

EPA FRS and ECHO The Facility Registry Service and Enforcement and Compliance History Online (ECHO) databases allow you to search for regulated industrial facilities near any address — including their reported releases, compliance history, and any enforcement actions.

ATSDR's PFAS site tracker The ATSDR maintains a database of military installations and industrial sites with known PFAS contamination, with associated health studies and community involvement information.

EJScreen for community context As described in the environmental justice article, EJScreen combines proximity-to-hazardous-facility data with demographic information to generate cumulative environmental burden scores.

Connecting to PollutionProfile PollutionProfile's Historical Exposure Recorder cross-references your residential history against these databases automatically — flagging any Superfund sites within a defined radius of your historical addresses, the contaminants associated with each, and the proximity and timing of your residence relative to the contamination timeline. This automated cross-referencing surfaces contamination connections that most people would never find through manual searching.