Over the last 60 years, the American Cancer Society has enrolled millions of volunteers in long-term studies that helped identify smoking, obesity, and air pollution as contributors to various cancers.

Now, the organization is conducting its most ambitious cancer-prevention study ever.

But finding enough participants hasn't been easy.

"To a certain extent, we're seeing a generational shift," said Alpa Patel, director of the latest study. "People are busier, and their lifestyles have changed."

That's why the cancer society is partnering with the Blue Cross Broad Street Run to use the enormously popular 10-mile race (more than 30,000 slots sold out in five hours) as an opportunity to recruit runners, their families, and friends.

Anyone between age 30 and 65 who has not had cancer — and would like to make a simple but important contribution to medical science — can enroll in the study May 4 or 5 at Lincoln Financial Field, during the prerace Not for Runners Only Expo.

The latest cancer-prevention study, the third of its kind, was launched in 2006 with the aim of signing up 500,000 volunteers. That goal has been scaled back to 300,000 — including 1,300 in this region — but only half that many have enrolled so far.

Such "observational" studies are designed to follow people for a long time — in this case, up to 20 years — to better understand factors that cause or prevent cancer.

As in the society's two previous cancer-prevention studies, participants fill out a detailed questionnaire about their health and habits; the survey is updated every few years. The latest study also requires volunteers to provide a blood sample and waist measurement. Some people may be put off by those prerequisites, acknowledged cancer society spokeswoman Lynn Ayres.

With blood samples in storage, researchers will be able to look for genetic contributors to cancer, as well as lifestyle and environmental factors, Patel said.

Observational studies, which follow people doing what they normally do, cannot prove cause and effect. But this type of research is great for generating or bolstering a theory. For example, in 1952, when smoking was being advertised as a healthful habit, the cancer society began following 188,000 men who provided detailed information about their smoking habits. By 1955, the study had compelling circumstantial evidence for what is now irrefutable: Smoking dramatically increases the chance of death from lung cancer and heart disease.

That paved the way for the society's first major cancer-prevention study, which followed nearly one million men and women in 25 states from 1958 through 1972. In addition to solidifying the link between smoking and lung cancer, that study was the first to connect obesity to shortened lives.

The second cancer-prevention study, launched in 1982, has yielded many important findings. Among them: Low-tar cigarettes are not safer than regular ones, and regular aspirin use reduces colon cancer risk.

Now, Patel said, researchers are looking at factors that didn't exist in the past, such as cellphone use, and factors that have become more common. "We've demonstrated that obesity is related to 10 types of cancer," she said. "Now we have a generation of people who have been overweight or obese for a much longer part of their lives. What are the implications?"

The Broad Street Run, managed by the City of Philadelphia and sponsored by Independence Blue Cross, has raised funds for the cancer society for 29 years, but this is the first time the partners are collaborating on research recruitment, organizers said.

The Inquirer is presenting a daily profile of participants in the May 6 Broad Street Run on Page B2. See full coverage at www.philly.com/broadstreetrun.

Contact Marie McCullough at 215-854-2720 or mmccullough@phillynews.com, or follow on Twitter @repopter.

Cancer Study

Anyone age 30 to 65 who has never had cancer is invited to join the American Cancer Society's latest cancer prevention study. Enrollment will be held during the Broad Street Run's pre-race Expo on May 4from 11 a.m. to 6 p.m, and May 5 from 9 a.m. to 5 p.m., at Lincoln Financial Field, One Lincoln Financial Field Way & 11th Street. For more information or to pre-enroll (which will shorten the 20-minute enrollment process), go to cancer.org/cps3 or call 888-604-5888.