The major cause of air pollution in Otago is classified PM10 - referring to minuscule particles of 10 microns in diameter or less.
Otago Regional Council monitoring in Albany St, Dunedin, showed PM10 levels reached 51mcg per cubic metre of air on March 31.
National Environmental Standards state PM10 levels should not exceed 50mcg per cubic metre of air, for any 24-hour period, more than once a year.
ORC environmental information and science director John Threlfall said PM10 particles were produced mostly by household fires.
Lesser amounts also came from vehicle and industrial emissions.
Dr Threlfall said the March 31 reading was of concern because it went against the usual trend.
PM10 levels were normally at their highest in winter when Dunedin residents used combustion heating the most.
In March, the weather was warmer and household fires were used less.
That left industry and vehicles to blame for the breach.
Dr Threlfall said the summer had been particularly dry this year and he believed particles from boiler chimneys at nearby factories, as well as dust from the quarry, construction at Forsyth Barr Stadium and vehicles on surrounding streets were significant contributors to the breach.
"Even sea spray may have contributed enough to put it over the limit."
He said the council was working with the GNS institute at the University of Otago to pinpoint the source of the pollution.
A review of air discharge consents at industries in the area would also be carried out, and some emissions levels might be reduced, he said.
Although it was a problem area, Dr Threlfall said breaches in air quality in Dunedin were usually only temporary occurrences.
Higher winds helped to clear air pollution while rain caused dust to settle.
The ORC had recorded as many as six breaches a year in Dunedin in recent years, and Dr Threlfall said it was important the levels remained below 50mcg.
The health effects of high levels of PM10 range from minor nose and throat irritations, to more serious effects such as aggravation of existing respiratory and cardiovascular disease, and premature death.
Periods of high PM10 levels result in increased hospital admissions and school absences.
Children, adults with obstructive lung disease, asthmatics and elderly people are most sensitive to particle pollution.
