Quitting smoking before age 40 years may reduce cancer mortality risk by 90%
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People with a smoking history who quit, especially at younger ages, avoid much of the excess risk for dying of cancer associated with continued smoking, according to a prospective study published in JAMA Oncology.
Additionally, researchers found considerable benefits for those who quit at older ages, reinforcing that it’s never too late to quit smoking.
“The age at which an individual both starts and stops smoking is a major factor in whether that individual is likely to die prematurely, which I think is not emphasized as much as it should be,” Blake Thomson, DPhil, principal scientist in cancer disparity research at American Cancer Society, told Healio.
“As our study shows, those who start smoking at younger ages have much higher likelihood of dying of cancer than those who start later. But starting at older ages still more than doubles cancer mortality rates compared with those who have never smoked,” Thomson continued. “Similarly, it is critical that individuals who smoke understand that by quitting, especially at younger ages, they can avoid most of the excess cancer mortality risk associated with continuing to smoke.”
The study included data of 410,231 participants (mean age, 48 years; 56% women) in the U.S. National Health Interview Survey from 1997 to 2014 linked to the National Death Index, with follow-up through 2015.
Thomson and colleagues categorized self-reported daily smokers by age at initiation (aged younger than 10 years, 10-14 years, 15-17 years, 18-20 years, and 21 years or older) and ex-smokers by age when they quit (15-34 years, 35-44 years, 45-54 years or 55-64 years). They excluded current, nondaily smokers and ex-smokers who quit before age 15 years or after age 65 years. They categorized individuals who quit within 5 years of death as current smokers to limit the bias of quitting because of ill health.
Researchers used Cox regression models to estimate cancer mortality rate ratios for current and ex-smokers vs. never smokers.
During 3.7 million person-years of follow-up, 10,014 cancer deaths occurred between ages 25 and 79 years.
Results showed an overall cancer mortality rate ratio associated with current smoking of 3 (95% CI, 2.86-3.16) compared with non-smokers. Cancer mortality rate ratios based on age at starting smoking were: younger than 10 years, 4.01 (95% CI, 3.33-4.82); 10-14 years, 3.57 (95% CI, 3.29-3.87); 15-17 years, 3.15 (95% CI, 2.94-3.37); 18-20 years, 2.86 (95% CI, 2.65-3.08); and 21 years and older, 2.44 (95% CI, 2.24-2.67).
Cancer mortality rate ratios for smokers based on the age of quitting smoking, compared with never smokers, were: 15-34 years, 0.95 (95% CI, 0.88-1.04); 35-44 years, 1.23 (95% CI, 1.13-1.33); 45-54 years, 1.45 (95% CI, 1.34-1.57); and 55-64 years, 1.88 (95% CI, 1.73-2.05).
“Quitting by age 40 reduces excess cancer mortality risk by about 90%,” Thomson said. “Another way of putting it is that, compared with those who have never smoked, those who smoked but quit by age 40 [years] had about 20% greater cancer mortality rates, and those who kept smoking had about 200% greater cancer mortality rates.”
The researchers wrote that the study helps improve understanding of the relevance of smoking at different ages as it relates to cancer mortality.
“The findings matched our expectations in relation to the benefits of quitting smoking, as older studies and studies conducted in other populations have previously shown the tremendous health benefits of quitting smoking at various ages,” Thomson said. “These findings are extremely consistent with previous findings, confirming the benefits of quitting smoking at younger ages, and ideally before age 40 [years].”
The researchers wrote that the effect of the increase in the federal legal age to purchase tobacco, from 18 to 21 years, on cancer mortality will depend on the extent to which it delays or prevents initiation of smoking.
“In the future, I would like to establish the relevance of smoking initiation and cessation at different ages to site-specific cancers, such as lung cancer,” Thomson told Healio. “Further research on the benefits of quitting smoking is also a key priority. It is important to continue monitoring the relevance of smoking to mortality in the United States, especially as it remains the leading preventable cause of cancer death in this population.”
For more information:
Blake Thomson, DPhil, can be reached at Cancer Disparity Research, Department of Surveillance and Health Equity Science, American Cancer Society, 3380 Chastain Meadows Parkway, Suite 200, Kennesaw, GA 30144; email: blake.thomson@cancer.org.
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