After several reports in the early 1990s showing excessive mortality associated with unexpectedly low concentrations of particulate matter, substantial research efforts have been devoted to investigating particulate matter health effects and elucidating the related pathophysiological mechanism. Despite remaining knowledge gaps, epidemiological and toxicological studies have provided reasonably coherent evidence supporting the significance of particulate matter-mediated acute cardiovascular effects.¹ Among the mounting volume of literature on air pollution epidemiology, multi-city ecological studies using time-series or case-crossover analyses have reported fairly consistent associations between increased concentrations of particulate matter over short periods of one or several days and increased cardiovascular mortality and morbidity (eg, hospitalisations and emergency visits). However, scientific data linking air pollution to stroke, including ischaemic stroke and intracerebral haemorrhage (ICH), remain relatively scant and inconclusive.²

In this issue, Yamazaki et al³ present a case-crossover study examining the acute effect of particles on mortality from stroke among elderly people (aged ⩾65 years) across 13 Japanese cities in 1990–1994. They found that …