IMM thesis: Environmental factors and cardiovascular disease

This thesis attempts to address some of the key consequences of urbanization, namely how air pollution, road traffic noise and low greenness associates with the risk of cardiovascular disease (CVD). In single exposure models, these factors have been suggested to increase the risk of both myocardial infarction (MI) and stroke, but few studies have explored them simultaneously. Furthermore, socioeconomic deprived groups have been suggested to face a higher environmental burden and may also be more vulnerable to environmental pollution. This could in turn lead to increased health disparities. 

In this thesis, we explored potential socioeconomic differences in population exposure to particles (PM₁₀ and PM_2.5), nitrogen dioxide (NO₂), road traffic noise and urban greenness (Normalized Difference Vegetation Index, NDVI in 500 m radius around the residence), and assessed their separate and combined associations with incidence of MI and stroke. Through a Nordic collaboration (NordSOUND), we were also able to address whether transportation noise is associated with obesity; a potential pathway linking noise with CVD.

Can you tell us about some interesting results?

The thesis is based on 20,407 women, born between 1914 and 1948, included in the Swedish Mammography Cohort (SMC). All women resided in Uppsala County, Sweden, and were followed for 20 years (1997 and 2017). 

In study I, we investigated time-trends in individual exposure by linking the participants home-addresses to spatiotemporally assessed air pollution, road traffic noise and greenness. Furthermore, we used sociodemographic data from registers to assess populational differences in environmental exposure. We thus prospectively assessed the associations between a set of individual and area-level sociodemographic variables and residential exposure to particles, nitrogen dioxide, road traffic noise and greenness. Our results provide evidence of a mixed pattern of environmental exposure inequalities across sociodemographic groups in urban settings. For example, elderly women, those unmarried and women with a high education appeared to be most exposed to air pollution, road traffic noise and low greenness.

In study II, pooled cross-sectional data from the NordSOUND project including eleven Nordic cohorts and up to 177,566 participants with overweight and obesity data, were used to assess exposure-response relationships of long-term exposure to road traffic, railway and aircraft noise, and overweight, obesity and central obesity. Our findings provide evidence of an association between road traffic and railway noise and all obesity markers. For road traffic noise, we observed thresholds of around 50-55 and 55-60 dB for obesity and central obesity, respectively, with a 10% risk increase per 10 dB thereafter.

In study III and IV, we used time-varying data from the SMC in conjunction with register data on MI and stroke to assess associations of multiple environmental exposures and incidence of MI and stroke, respectively. In multi-exposure models, taking both individual- and area-level socioeconomic factors into account, we observed an inverse association between residential greenness and MI, thus suggesting greenness to be a protective factor for incidence of MI. Furthermore, accounting for co-exposures, PM_2.5 was associated with an increased risk of total and ischemic stroke. We also found some evidence of a higher vulnerability in certain groups of the population, for instance in women with low education.

What further research is needed in the area?

Identifying factors in the environment that may harm as well as benefit cardiovascular health in urban settings are relevant for prevention and necessary to build sustainable and equal societies.

With this thesis, we shed some light on the complex relationships between environmental and sociodemographic factors and their impact on cardiovascular health. The interplay between air pollution, road traffic noise and urban greenness is, however, not fully understood and must be clarified. Furthermore, sociodemographic factors may play multiple roles in the relationship between environmental factors and public health, such as confounders or effect modifiers, and are highly context, city and population dependent. These complex relationships clearly merit further research, also utilizing other methods (e.g. mediation analyses) and preferably performed in populations with a higher exposure contrast than in the present thesis.