It is hoped Dunedin's good air quality record this winter will be retained even though the city's air exceeded the National Environmental Standard once.
On Saturday, July 27, the central Dunedin air quality station recorded 56mcg of airborne pollutants per cubic metres of air.
Under the national PM10 (very small particle) standard, only one reading above 50mcg is allowed per year.
Apart from that one high reading, Dunedin's air quality recordings had been below 50mcg throughout winter.
Environmental information and science director John Threlfall said at a committee meeting yesterday an investigation into the cause of the high reading led to the discovery there had been an equipment failure at a nearby industry's boiler, which led to a ''stream of pollutants'' leaving a stack without being filtered.
The company closed the operation until the problem was resolved.
As a result of the investigation and with additional evidence on wind speed and direction, the council was applying to the Ministry for the Environment for an ''exceptional circumstance'' exemption.
If this was granted, it meant the exceedance would not be included in the total for the year.
''While, with only one failure, we still meet the NES, it would be nice to have a zero.''
If the request is accepted, it would be the second time since testing began in Dunedin no exceedances of the national standard had been recorded.
Under the standard, a site had to comply with the standard for five years before its considered not a polluted airshed, he said.
It was an area more affected by industrial emissions than home heating and under World Health Organisation guidelines, Dunedin had been beneath the ambient standard of 20mcg in 2012 and 2013.
