The research examined the impact of smog on a group of 93 students at a Christchurch school during the winter of 2004.

Most of the smog in Christchurch, considered New Zealand's worst city for air pollution, is caused by wood fires.

The smog is most noticeable in winter. While there were small decreases in lung function for 26 students who had asthma on days of very high pollution, the study showed students without respiratory problems were not affected.

Senior lecturer in medicine at the University of Otago Michael Epton, who assisted with the study, said there was no safe limit for pollution but healthy people should not be unduly concerned about smoggy days.

''This study shows us that with smaller exposures people with normal lungs have not got major symptoms because of this level of exposure to particulate pollution,'' Dr Epton said.

But he warned that people with some health problems were affected.

''People with asthma, for instance, do show some effects even at this relatively low level of air pollution,'' he said.

''We already know in Christchurch that people with heart and lung disease do get affected by air pollution ... they do end up in hospital more.''

He said it was difficult to say whether exposure to other types of air pollution, such as smog from coal-fired power stations or diesel fumes, would produce similar results.

Features of particulate pollution, whether from wood smoke or industry, were similar but ''one has to be very careful about extrapolating data like this''. AAP