Metabolic syndrome in the SCAPIS cohort - Investigating associations at low level exposure to ambient air pollution

Studies on long-term exposure to air pollution at high levels suggest an association with increased risk of metabolic syndrome (MetS). We aimed to study this relationship at very low levels, less well-studied, yet of public health importance.

The Swedish CArdioPulmonary bioImage Study (SCAPIS) consists of randomly selected subjects aged 50-64 years from six large cities in Sweden (n = 30,154). Participants underwent medical examination and answered a health-related questionnaire at enrolment. MetS was defined using established criteria. Residential exposure to PM_2.5 and NO₂ was modelled annually for each subject over 10 years before enrolment. Adjusted prevalence ratios (adj PR) were calculated both by quartiles of exposure and per 1 μg/m³ increment in PM_2.5 and NO₂, with adjustments for age, lifestyle and socioeconomic factors.

Full information on exposure, covariates, and MetS was obtained for 13,997 (90.26 %) females and 12,978 (88.61 %) males, respectively. The 10-years mean exposures of PM_2.5 and NO₂ before enrolment were 6.57 and 11.47 μg/m³, respectively. The prevalence of MetS was 28.75 % in females and 38.20 % in males, respectively. Quartile analysis of PM_2.5 did not reveal significant increased adj PR for MetS, neither in females nor in males. For females the adj PR per 1 μg/m³ was 0.91 (0.89-0.94) and for males 0.95 (0.92-0.97), respectively. Similar, but somewhat weaker, associations were seen for NO₂.

Some inverse associations between air pollution and MetS were observed, but these results should be interpreted with caution due to limitations in the cross-sectional design. Results should not be interpreted beyond our studied exposure range.