Background
While the excess in lung cancer risk among lower socioeconomic status individuals has been widely described, the magnitude of this association across lung cancer subtypes, as well as histotype-related long-term incidence trends, are inconclusively reported.

Aims
We explored the variation in the incidence of the three main lung cancer histotypes (i.e. squamous cell carcinoma, small cell carcinoma and adenocarcinoma) by socioeconomic status (SES, i.e. upper and lower white collar, upper and lower blue collar, and farming/forestry/fishing) in the adult population of four Nordic countries (i.e. Sweden, Norway, Finland and Denmark).

Materials & Methods
We have used data from the Nordic Occupational Cancer Study (NOCCA), computing age-standardized incidence rates per 100,000 person-years truncated at ages 50–69 years, by sex, histotype, country and SES, for the period 1971–2005. We estimated relative risks and the corresponding 95% confidence intervals through Poisson regression models, including terms for SES, age, sex and country, as indicated.

Results
A clear socioeconomic gradient, with a progressive increase in lung cancer risk as SES level decreases, was observed in all subtypes and in both sexes. Favourable lung cancer incidence trends were seen among men for squamous cell and small cell carcinomas, although for adenocarcinomas rates were increasing everywhere except for Finland. Among women, upward temporal trends were seen in all SES groups and for all subtypes, although rates increased to a greater extent for low, compared to high, SES, especially in Denmark and Norway. Farmers showed comparatively lower risks compared to other SES categories.

Discussion
This prospective cohort study shows that substantial socioeconomic inequalities in the incidence of the most important lung cancer histotypes exist in the Nordic Countries, and that these inequalities are on the rise, especially among women.

Conclusion
Smoking habits are likely to largely explain the observed social gradient for lung cancer histotypes in both sexes.