Peter Jennings, who was 67 when he died, quit smoking 20 years ago but admitted starting again after 9/11. That might have further damaged lungs that were not yet healed.

Jennings, the face of ABC News for more than two decades, quit smoking 20 years ago. But he admitted starting again after the Sept. 11, 2001, terrorist attacks.

According to the American Lung Association, about 87 percent of lung cancer cases are caused by smoking, and 40 percent to 50 percent of new cases may occur in former smokers.

Number One Cancer Killer

Because most lung cancers are diagnosed at a late stage, the five-year survival rate is only 15.2 percent, compared with 63 percent for colon cancer, 88 percent for breast cancer and 99 percent for prostate cancer, according to the lung association.

In 2005, lung cancer will take about 163,500 American lives and will maintain its place as the number one cancer killer, outpacing deaths from the second, third, fourth and fifth most common causes of cancer deaths combined, Dr. Bill Solomon, and associate professor of medicine at SUNY Downstate Medical Center in New York City, told HealthDay last spring.

Ninety percent of people who are diagnosed with lung cancer will eventually die of the disease, added Dr. Jay Brooks, chairman of hematology/oncology at the Ochsner Clinic Foundation in New Orleans.

Not everyone who smokes will get lung cancer and not everyone who quits will be protected. Why? No one knows for sure. People who smoke have a 10- to 15-fold greater risk of developing lung cancer than those who never light up, experts say. And, for the most part, the more you smoke -- or smoked in the past -- the greater your accumulated risk.

"The risk does decline with time after you stop but those numbers aren't clear," Dr. Norman Edelman, chief medical officer of the lung association, told HealthDay.

Other Environmental Factors, Genes

A landmark study published last year in the British Medical Journal found that cigarette smokers die an average of 10 years sooner than nonsmokers. At least half, and possibly up to two-thirds, of people who smoke from youth on are eventually killed by their habit, a quarter of them in middle age, the study reported.

But the study also found that quitting can offer big advantages. Stopping at age 50 cuts the risk of dying in half, while quitting at age 30 almost eliminates the risk.

But individuals are still, well, individuals. Jennings, who was 67 when he died, quit smoking 20 years ago but admitted starting again after 9/11. That might have further damaged lungs that were not yet healed, Brooks said.

There were likely other factors at play as well.

"We're gambling with other things in the environment or genes or both," Edelman said. "The fact that Jennings smoked for a long period of time increased his risk of getting lung cancer. How much that risk was dissipated by stopping for 20 years I don't know, but it was obviously not entirely."

Always Good to Quit

The ABC News anchor joins a long list of celebrities who smoked and developed lung cancer, including Yul Brynner, Nat King Cole, Gary Cooper, Walt Disney, Duke Ellington, Betty Grable and George Harrison.

Lung cancer isn't the only pitfall of smoking. "Smoking is also responsible for many cases of bladder cancer in males, head and neck cancer, esophageal cancer and pancreatic cancer," Solomon said.

Actor Michael Landon's four-pack-a-day habit likely contributed to his fatal pancreatic cancer. Humphrey Bogart and Sammy Davis Jr. both were smokers and both developed throat cancer.

Experts advise that if you're an ex-smoker with a cough, get to a doctor and get screened.

If you're a smoker, stop.

"Quitting is good. It's always good to quit, no matter how long you've smoked," Edelman said. "You'll reduce your risk of lung cancer, reduce the degree to which you have chronic obstructive pulmonary disease, reduce your risk of other types of cancer and of heart disease. The data is very clear. Even if you're 75, you can benefit from stopping."

More information: For more on quitting smoking, visit the American Lung Association.