High pollution levels may make people more susceptible to stroke, research suggests.
Pollution damages health
Scientists at Kaoshiung Medical University, Taiwan, found higher hospital admission rates in the city when pollution was high.
Two common pollutants - particulates and nitrogen dioxide - seemed to be particularly important.
Writing in the journal Stroke, the researchers said the problem was worse when temperatures topped 20C.
The researchers collected data on 23,179 hospital admissions from 1997 to 2000 in Kaohsiung, Taiwan's second city, and an area of heavy industry.
They compared air pollution levels on the dates of admissions with air pollution levels one week before and one week after admissions.
A rise in levels of both pollutants was linked to a significant increase in the number of people admitted to hospital with either of the most common type of stroke - one caused by a burst blood vessel in the brain, the other by a clot blocking blood flow to the brain.
The higher the pollutant level, the greater the risk of both types.
For every particulate (PM10) change of 66.33 micrograms per cubic metre the risk of being admitted to hospital with a stroke caused by a burst blood vessel in the brain rose 54%.
A similar risk increase was produced by every extra 7.08 parts per billion of nitrogen dioxide.
Previous research has shown a link between air pollution and death rates from respiratory and heart disease. However, the link with stroke has been far from clear.
Lead researcher Professor Chun-Yuh Yang said: "This study provides new evidence that higher levels of ambient pollutants increase the risk of hospital admissions for stroke, especially on warm days.
"In hot weather, we recommend that people avoid pollution, stay inside and use an air conditioner if needed."
On cool days, researchers noted a link between carbon dioxide levels and ischemic (clot) stroke admissions, but believe this may have been a finding by chance.
Many experts suspect that air pollution makes the blood more sticky, making it tougher for the heart to pump it round the body, and increasing both the risk that it may clot, and that the blood vessels will be damaged.
Professor John Reid, head of the Cardiovascular Research School at Glasgow University, warned against making general conclusions on research based on pollution in a very hot sub-tropical climate.
He said: "I would also have some concerns about the fairly glib superficial statement that people should keep out of rush hour traffic jams and stay inside with the air conditioner.
"This is not really a very practical approach to stroke prevention. If factors associated with air pollution are really involved in the causation of stroke, then what it is doing is activating inflammatory or other mechanisms which precipitate stroke rather than being a direct cause."
A spokesperson for the Stroke Association said: "There have been a number of studies in the past looking at environmental factors in relation to stroke though none have been particularly conclusive.
"This is an interesting area and we welcome any research that helps increase knowledge of possible risk factors."
The main sources of nitrogen dioxide air pollution are emissions from vehicles and from power plants and other fossil fuel-burning industries.
Particulate air pollution is a term used to describe mixtures of solid and liquid particles that are suspended in the air. These particles vary considerably in size, composition and origin.