But it's not so clear.
One of the many lessons of the cancer cluster in Toms River - the subject of a new book, Toms River: A Story of Science and Salvation, by journalist Dan Fagin - is how hard it is to prove that one exists, or to pinpoint a cause.
The task is time-consuming, expensive, and often inconclusive.
After a $10 million investigation, scientists found a link to both the Toms River plant and a nearby farm where chemicals were illegally dumped. A confidential settlement - estimated at more than $35 million - with some of the families of cancer victims was reached in 2001.
The difficulty of proving a cancer cluster - as shown in two ongoing Pennsylvania cases - is a cautionary tale for all who live in today's chemical-laced world, many say.
Why is it so difficult?
For one, cancer is common. Just because someone sees a lot of it doesn't mean there's a cluster.
In the United States, half of all men and one in three women will be disgnosed with cancer. Nearly 80,000 Pennsylvanians and 50,000 New Jerseyans will be diagnosed this year, the American Cancer Society estimates.
Cancer isn't just one disease, but many, with different causes. Most cancer is caused by life choices: smoking, obesity, not exercising, excess drinking.
Exposure to pollutants is thought to cause few cancer deaths - 4 percent from occupational exposures, 2 percent from chemicals and other carcinogens, including naturally occuring radon.
Even so, when a pollution-related cluster exists, the effects can be horrific. In Toms River, at least 69 children developed illnesses, such as leukemia and brain cancer. Some died.
Some links are known; when a lung cancer called mesothelioma is diagnosed, asbestos is the likely cause.
Other links - proving that X chemical caused Y cancer - are elusive.
Worst of all is when you have disease and must search for a cause, said Trevor Penning, a cancer expert at the University of Pennsylvania. "Then you begin to have an expensive conversation."
Toms River officials didn't know at first what chemicals were involved - even in the drinking water from municipal wells.
Even with a known exposure, some will get cancer, and some won't.
It can take a long time for cancer to develop, so re-creating a person's exposure - from drinking water to pesticides - is like "looking for footprints in the sand 10 to 20 years after the wind has stopped blowing," said Boston University epidemiologist Richard Clapp, who was on the Toms River case.
In recent years, residents suspected clusters in Collegeville, near two plants using the degreaser TCE trichloroethylene, and in Philadelphia's Eastwick section, close to a toxic landfill. Residents in Mount Laurel also thought something in the water or soil was sickening them.
But studies showed no evidence of any clusters.
Still, Fagin thinks many more exist than we know - since no one is looking.
"We shouldn't wait until disaster strikes," he said. "We ought to proactively search for these patterns . . . and protect people before lives are lost."
Gene Weinberg, a cancer epidemiologist with the Pennsylvania Department of Health, said he gets about 25 calls a year that require data analysis. He checks the state cancer registry to determine types and rates of cancer, and other area statistics.
But going beyond - to in-depth comparisons of those with and without the cancer, and hundreds of interviews - is rare. The state does not have the money.
One lesson of Toms River is that we need to know more about how carcinogens cause cancer, said Timothy Rebbeck, a Penn cancer epidemiologist.
Conversely, even if a cluster can't be proved, it doesn't mean there's not a problem - say, contaminated drinking water - said Daniel Wartenberg, a Rutgers University epidemiologist.
Another lesson of Toms River is that clusters, however rare, do exist. At least two Pennsylvania sites are under scrutiny.
About $5 million in federal funds has gone to to investigate cancer in three northeastern Pennsylvania counties, where an unusually high number of people have a rare blood disease.
Drexel University received $499,000 to do lengthy interviews with hundreds of people - sickened and not. "We're crunching through that right now," said Arthur L. Frank, chair of the Department of Environmental and Occupational Health.
In Ambler, officials are studying an excess of mesothelioma cases linked to former asbestos manufacturing there. In the 19002 zip code, the overall cancer rate was lower than statewide, but the rate of mesothelioma was three times higher.
These days, even with the legacy of Superfund sites, many think the next frontier of toxicity isn't as much industrial pollution - today's laws are more stringent - as everyday chemicals that people come into contact with.
More than 80,000 chemicals are in use today; few have been independently evaluated for safety.
"We've gotten into this habit of churning out a fire hydrant of new chemicals," said Clapp. "It's all about lower cost . . . . There has to be a way of doing this without creating toxic chemicals."
"A lot of this has to do with the hubris of science . . . and thinking that things can't cause cancer, and then being tragically surprised," said Mark Cuker, a Cherry Hill lawyer. He and colleagues litigated the Toms River case and now represent residents in Paulsboro, where a derailed train spilled vinyl chloride last year.
The $91 million cleanup at the Ciba-Geigy site in Toms River began in 2004. It was such an attraction that a viewing platform was built. It ended in 2010.
Recovery wells began treating groundwater in 1996. The pumps will run until 2025.
The advocacy group that resident Linda Gillick formed to help Ocean County children with cancer, Ocean of Love, is still active.
The mother of a child who developed a rare cancer, she said Toms River shows that "you cannot depend on government to protect you." And "the companies continue to do what they do. If not here, then in other places."
At the end of his book, Fagin takes readers to another children's hospital and another mother of a child with leukemia.
She's in China.
Contact Sandy Bauers at 215-854-5147, email@example.com, or follow on Twitter @sbauers. Read her blog, "GreenSpace," at www.philly.com/greenspace