The contribution of smoking to inequalities in mortality by education varies over time and by sex: two national cohort studies, 1981-84 and 1996-99

Background The contributions of tobacco smoking to overall mortality and socioeconomic inequalities in mortality vary between populations and over time. We determined how these contributions varied by sex and over time in two national New Zealand cohort studies. Methods Poisson regression and modelling were conducted on linked census-mortality cohorts for people aged 45-74 years in 1981-84 and 1996-99 (2.0 and 2.7 million person-years, respectively). Results Contribution to socioeconomic inequalities in mortality. Adjusting for current and former smoking reduced the all-cause mortality rate ratios for men with nil educational qualifications compared with men with post-school qualifications from 1.34 to 1.29 in 1981-84 and from 1.31 to 1.25 in 1996-99, or 16 and 21% reductions in relative inequalities. Equivalent results for women were 1.42-1.41 in 1981-84 and 1.42-1.37 in 1996-99, or 3 and 11% reductions in relative inequalities. Contribution to overall mortality. Using 1996-99 data, we estimated that if all current smokers quit and became ex-smokers, mortality rates would reduce by 11% for men and 5% for women. If everyone was a never smoker (i.e. a historically smoke-free society), mortality rates would have been 26% lower for men and 25% lower for women. Conclusions The contribution of smoking to educational inequalities in mortality was greater for males, and increased over time for both males and females, reflecting the historically differential phasing of the tobacco epidemic by sex and socioeconomic position. Complete cessation of smoking in contemporary New Zealand would reduce both overall mortality and educational inequalities in mortality.