Does respiratory health contribute to the effects of long-term air pollution exposure on cardiovascular mortality?

Background: There is growing epidemiological evidence that
short-term and long-term exposure to high levels of air pollution
may increase cardiovascular morbidity and mortality. In addition,
epidemiological studies have shown an association between air
pollution exposure and respiratory health. To what extent the
association between cardiovascular mortality and air pollution is
driven by the impact of air pollution on respiratory health is
unknown. The aim of this study was to investigate whether
respiratory health at baseline contributes to the effects of
long-term exposure to high levels of air pollution on
cardiovascular mortality in a cohort of elderly women. Method: We
analyzed data from 4750 women, aged 55 at the baseline
investigation in the years 1985 - 1994. 2593 of these women had
their lung function tested by spirometry. Respiratory diseases and
symptoms were asked by questionnaire. Ambient air pollution
exposure was assessed by the concentrations of NO2 and total
suspended particles at fixed monitoring sites and by the distance
of residency to a major road. A mortality follow-up of these women
was conducted between 2001 and 2003. For the statistical analysis,
Cox' regression was used. Results: Women with impaired lung
function or pre-existing respiratory diseases had a higher risk of
dying from cardiovascular causes. The impact of impaired lung
function declined over time. The risk ratio (RR) of women with
forced expiratory volume in one second (FEV1) of less than 80%
predicted to die from cardiovascular causes was RR = 3.79 (95% CI:
1.64 - 8.74) at 5 years survival time and RR = 1.35 ( 95% CI: 0.66
- 2.77) at 12 years. The association between air pollution levels
and cardiovascular death rate was strong and statistically
significant. However, this association did only change marginally
when including indicators of respiratory health into the regression
analysis. Furthermore, no interaction between air pollution and
respiratory health on cardiovascular mortality indicating a higher
risk of those with impaired respiratory health could be detected.
Conclusion: Respiratory health is a predictor for cardiovascular
mortality. In women followed about 15 years after the baseline
investigation at age 55 years long-term air pollution exposure and
impaired respiratory health were independently associated with
increased cardiovascular mortality.