The strengths of this study include the comprehensive accounting
for important differences in case management across hospitals
by using detailed clinical data on treatments within the MINAP
database and by allowing each hospital to have its own baseline
hazard. The highly spatially resolved exposure data for a wide range
of air pollutants and noise is another improvement to prior, similar
studies. We are aware of no similar study among MI survivors that
adjusted air pollution estimates for traffic noise and reported the
independent effects of noise on prognosis.
The main limitation of this study was limited statistical power
due to relatively little variation in exposure within each hospital
catchment area. There were several positive associations between
air pollutants and prognosis, even after adjusting for noise; however,
precision of the estimates was limited. Large numbers of events are required for studies of within-city variation in traffic
pollution, which even in Europe’s largest city varied little. Also, we
were not able to examine cause-specific mortality as these data
were not available in the MINAP database. Due to low numbers,
separate analyses by cause of death would have had even more
limited statistical power. Postcode centroid coordinates in MINAP
were rounded for purposes of patient confidentially, resulting in
loss of accuracy of the air pollution and noise exposure estimation
and likely bias towards the null. Complete data on residential
history of participants were not available. However, we explored
the stability of residential postcodes over time among individuals
who were re-admitted to hospital; only 5% of participants moved
between first and second admission in the database. In sensitivity
analyses we adjusted for individual-level smoking and clinical history,
which are likely to be important mediators of the relationship
between an individual’s socioeconomic position and prognosis.
Several studies have shown that area-level measures of deprivation
are more correlated with air pollution exposure than individuallevel
deprivation, thus, adjustment for area-level socioeconomic
factors can essentially remove the correlation between individuallevel
deprivation and air pollution exposure (Goodman et al., 2011;
Naess et al., 2007). Lack of data on individual-level deprivation is
therefore not likely to be an important limitation of our analysis.
Finally, we adjusted for prescribed medication at discharge, but did
not have data on drugs taken during follow-up or other secondary
prevention measures, which may have resulted in some residual
confounding.
In conclusion, these findings provide additional support that
long-term exposure to air pollution is associated with all-cause
mortality and hospital readmission for MI among MI survivors.
These associations were not fully explained by road traffic noise,
which may also have an independent effect on prognosis. There
was no evidence that primary traffic pollutants were more
strongly associated with prognosis compared to pollutants reflecting
regional or urban background. Further investigation using
very large studies enabling multi-pollutant models are required to
further identify which pollutants and sources are primarily responsible
for observed health effects.