Objectives Relationship between low-level air pollution and stroke is conflicting. This study was conducted to document the relationship between outdoor air pollution and ischaemic stroke occurrence.

Design Time-stratified case-crossover analysis.

Setting University Hospital of Nice, France.

Participants All consecutive patients with ischaemic stroke living in Nice admitted in the University Hospital of Nice (France) between January 2007 and December 2011.

Main outcome measure Association (adjusted OR) between daily levels of outdoor pollutants (ozone (O₃), nitrogen dioxide (NO₂), particulate matter (PM10) and sulfur dioxide (SO₂)) and ischaemic stroke occurrence.

Results 1729 patients with ischaemic stroke (mean age: 76.1±14.0 years; men: 46.7%) were enrolled. No significant association was found between stroke occurrence and short-term effects of all pollutants tested. In stratified analysis, we observed significant associations only between recurrent (n=280) and large artery ischaemic stroke (n=578) onset and short-term effect of O₃ exposure. For an increase of 10 µg/m³ of O₃ level, recurrent stroke risk (mean D-1, D-2 and D-3 lag) was increased by 12.1% (95% CI 1.5% to 23.9%) and large artery stroke risk (mean D-3 and D-4 lag) was increased by 8% (95% CI 2.0% to 16.6%). Linear dose–response relationship for both subgroups was found.

Conclusions Our results confirm the relationship between low-level O₃ exposure and ischaemic stroke in high vascular risk subgroup with linear exposure–response relation, independently of other pollutants and meteorological parameters. The physiopathological processes underlying this association between ischaemic stroke and O₃ exposure remain to be investigated.