Disentangling contextual effects on cause-specific mortality in a longitudinal 23-year follow-up study: impact of population density or socioeconomic environment?

BACKGROUND: Various studies have investigated urban/rural differences in cause-specific mortality. A separate body of literature has analysed effects of socioeconomic environment on mortality. Almost no studies have attempted to disentangle effects of population density and socioeconomic environment on mortality, beyond the effects of individual characteristics.
METHODS: Considering all individuals living in the region of Scania, Sweden, from 1970-93, we performed 10 year mortality follow-ups on (i) individuals aged 55, (ii) individuals aged 65, and (iii) individuals aged 75 years at baseline. Cox multilevel models adjusted for individual factors allowed us to investigate the independent effects of population density and median income in the parish of residence on mortality from ischaemic heart disease (IHD), lung cancer, and chronic obstructive pulmonary disease (COPD) among individuals who had lived in the same parish for at least 10 years prior to mortality follow-up.
RESULTS: In females, as in males, after adjustment for individual and contextual socioeconomic status, we found a dose-response association between population density and mortality from lung cancer and COPD in all age groups investigated, and from IHD especially in the youngest age group. Overall, the population density effect was the strongest on lung cancer mortality. Median income had an additional impact only in 2 out of 16 subgroups of age x gender x cause of death.
CONCLUSIONS: In our region-wide study conducted at the parish level, contextual disparities in mortality were dominated by the population density effect. However, it may be unwise to conclude that truly contextual effects exist on mortality, before identification of plausible mediating processes through which urbanicity may influence mortality risk.