So far this year, some towns in Otago have breached the national air quality standard 22 times compared to only six times during the same period last year.
All but one of the towns the Otago Regional Council monitors for particulate (PM10) levels have breached the national environmental standard at least once this year, council air quality scientist Deborah Mills says.
The standard requires PM10 values not to exceed 50mcg per cu m of air, for any 24-hour period, more than once a year by 2013.
During the past two weeks, particulate levels had steadily climbed throughout much of Otago, resulting in the first winter breaches for many towns where the regional council monitored, she said.
"While temperatures were still fairly mild during the last week of April, they fell noticeably during the first week of May."
Alexandra, Clyde, Cromwell, Milton, and Mosgiel (excluding the forest fire breach) all had their first breaches after May 5.
Last year some sites breached the standard as early as April.
"Calm winds, particularly overnight, hindered the dispersal of what were most likely increased emissions from domestic heating."
Arrowtown, which often had some of the worst air quality readings during winter, had yet to breach the standard.
Last year its first breach was on May 26.
Milton had its first breach on May 5 (May 3, 2009) and six more since then, while Dunedin had its first (excluding February's forest fire) on March 31 (May 27 in 2009) and four more since then.
"A persistent pattern of light, northeast winds throughout much of the autumn likely moved sea salts and construction dust over the monitoring site," Ms Mills said.
Council environmental information and science director John Threlfall said a recent spate of backyard burning had impacted on Mosgiel's air quality, with the daily average on May 12 reaching 63mcg per cu m.
From 7pm until midnight, hourly readings were at times more than 100mcg - more than double the permitted level.
Breaches had previously happened in May, although it was not common this early in the year, Dr Threlfall said.
"The monitoring results underline the need for people to consider other ways of dealing with autumn prunings and other garden material, such as composting or shredding," he said.
