An increasing proportion of US smokers smoke ≤10 cigarettes per day (CPD) or do not smoke every day, yet the health effects of low-intensity smoking are poorly understood. We identified lifelong smokers of <1 or 1-10 CPD and evaluated risk of incident cancer among 238,525 cancer-free adults, aged 59-82, in the NIH-AARP Diet and Health Study. A questionnaire administered in 2004-2005 assessed CPD during nine age-periods (<15 to ≥70). We estimated hazard ratios (HR) and 95% confidence intervals (CI) using multivariable-adjusted Cox proportional hazards regression with age as the underlying time metric. Of the 18,233 current smokers, (7.6%), 137 and 1,243 reported consistently smoking <1 CPD and 1-10 CPD, respectively. Relative to never smokers, current smokers who reported consistently smoking 1-10 CPD over their lifetime were 2.34 (95% CI = 1.86-2.93) times more likely to develop smoking-related cancer. Current lifetime smokers of <1 CPD were 1.89 (95% CI = 0.90-3.96) times more likely to develop tobacco-related cancer, although the association did not reach statistical significance. Associations were observed for lifelong smoking of ≤10 CPD with lung cancer (HR = 9.65, 95% CI = 6.93-13.43); bladder cancer (HR = 2.22, 95% CI = 1.22-4.05); and pancreatic cancer (HR = 2.03, 95%CI: 1.05-3.95). Among lifelong ≤10 CPD smokers, former smokers had lower risks of smoking-related cancer with longer time since cessation and longer smoking duration. Lifelong <1 and 1-10 CPD smokers are at increased risk of incident cancer relative to never smokers and would benefit from cessation, providing further evidence that even low-levels of cigarette smoking cause cancer.
Keywords: cigarette; incident cancer; lifetime smoking; low-intensity smoking.