The link to lung cancer was especially high. The group that smoked less than one cigarette a day over their lifetimes had nine times as high a risk of dying from the disease than nonsmokers, while those who smoked one to 10 cigarettes a day had a 12-fold increased risk.
“The message is that there is no safe level of smoking,” said Maki Inoue Choi, NCI researcher and lead author of the study, which was published in JAMA Internal Medicine.
The researchers said in the study that there was “a common perception, particularly among young people,” that smoking a few cigarettes a day or a week was safe, adding, “These smokers may substantially underestimate” the risks.
Previous studies have shown that the duration of a person's smoking habit, independent of the number of cigarettes smoked a day, is the most critical factor in the risk of getting a smoking-related disease, the researchers said.
Over the past several years, smoking rates have declined in the United States and many other countries, reflecting increased awareness about the health damage caused by cigarettes. At the same time, a growing proportion of total smokers use fewer than 10 cigarettes a day, said Neal Freedman, another NCI researcher involved in the study. Yet, the health effects of this “low-intensity smoking” have not been well studied.
Such smoking was once viewed as a temporary practice for people who were trying to quit. But more recent studies have indicated that many smokers stick to this pattern for years.
The study analyzed data for almost 300,000 adults in the NIH-AARP Diet and Health Study. Participants were age 59 to 82 at the start of the study, which found that health risks were lower for those who had quit, especially at earlier ages.
Researchers relied on participants remembering their smoking histories over many years, a limitation of the study, because memories often aren't accurate. Other limitations, the authors said, were that most participants were older and white, and the data didn't provide detailed information about patterns among people who smoked less than one cigarette a day. For example, it wasn't clear whether they smoked just on the weekends or every other day.