The acute effects of smoke are well known to anyone who has stood downwind of a bonfire. More subtle are the effects of low levels of particulate matter over a long period.
Eleven cohort studies in Europe provide data on the relationship between 10 micron particles (PM10) and heart attacks: nine point to a positive relationship and two seem to suggest that increasing air pollution is associated with fewer heart attacks. But all of those results could have been chance.
This study puts them all together in a meta analysis and strengthens the view that even at levels current in western Europe, higher levels of PM10 (but not PM2.5) are associated with more coronary events. There's a good Forest plot showing each study and its weight in the meta analysis. The authors tested for (statistical) heterogeneity and felt able to combine all the studies into one estimate - the meta analysis.
That Forest plot summarises a huge amount of work: modelling the exposure at the home of every member of every cohort by extrapolation from the air pollution monitoring sites; gathering information on obvious confounders such as smoking habit from everyone in the cohorts; obtaining data on the outcome from hospital and mortality databases; combining all the separate studies in the meta analysis. All of this is carefully described.
You can approximate the effect of standing in bonfire smoke for several years by living in Beijing. The PM10 level in this study from Beijing was 146; in most of the European locations (Rome and Turin excepted) it was 10 - 20.