High hourly levels of air pollution, more than double the risk of one type of stroke, suggests research published ahead of print in Occupational and Environmental Medicine.
Currently, the risk of respiratory and cardiovascular problems tends to be linked to the average daily amount of air pollution, rather than variations in hourly levels.
The researchers assessed data on stroke deaths in people aged 65 years and older, occurring between January 1990 and December 1994 in 13 major urban areas in Japan.
Levels of air pollutants, including nitrogen dioxide, particulate matter, and photochemical oxidants, were monitored hourly at various sites in each of the 13 areas.
When the two sets of data were put together, a pattern emerged for intracerebral haemorrhage - where a blood vessel bursts inside the brain - for the warmer months between April and September.
This showed that high hourly rates of particulate matter (in excess of 200 ug/m3) around two hours before death were associated with a more than doubling in the risk of death from a bleed into the brain.
The findings held true, irrespective of the average daily level of air pollutants.
No such effect was found for ischaemic stroke, where the arteries in the brain become furred up and narrowed as a result of fatty deposits.
This is possibly because the time lag between the start of this type of stroke and death is rather longer than that for a bleed into the brain, say the authors.
Pervious research shows that the effects of air pollution act quickly on the body, say the authors, with inhaled particles detectable in the blood within 60 seconds. Peak levels can stay in the blood for up to an hour.
Their findings prompt the authors to suggest that preventive measures should be based on average hourly measures rather than just average daily measures alone.