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A fortnight of still, freezing days has boosted air pollution levels in Central Otago towns.
With the average temperature well below 1degC in Alexandra, Arrowtown and Clyde most days for the past week, and Cromwell only a fraction warmer, log burners and other heating fires have been going almost non-stop.
National Environmental Standards for air quality were set by the Government in 2004 and the target was that the level of particulate in the air should not exceed 50 micrograms per cubic metre of air, for any 24-hour period, more than once a year.
All four towns had high air pollution days in the past week, Arrowtown going over the limit every day, Alexandra on five days out of seven, Clyde on four days and Cromwell on two days.
"Most of the air pollution and the emissions that cause it are as a result of domestic heating, " Otago Regional Council air quality scientist Deborah Mills said.
"All of those four towns have similar climates, with cold temperatures and very still and very calm [air at] this time of year," Ms Mills said.
"Sometimes you'll get a clear sunny day, but the inversion layer means that the particulate stays trapped near the surface."
There was little air flow, and the towns all sat in valleys.
So far this winter, there have been 26 high air pollution days in Alexandra, 27 in Arrowtown, 18 in Clyde and Cromwell and 16 in Milton.
Although new regulations had been introduced to improve air quality, especially in smog-prone parts of the district, it would take a few years before the effects of this made a noticeable difference to air pollution levels, she said.
"We're heading in the right direction but we won't see things change overnight.
"It's a hard issue and it'll take a few years before you notice some reduction in air pollution levels."
Subsidies are available for people in Alexandra, Clyde, Cromwell, Arrowtown and Milton to improve insulation in their homes and change their domestic heating to clean heating appliances.
In the meantime, it was important to use only dry wood in log burners, as that burned "cleaner", and to avoid burning rubbish in burners and open fires, Ms Mills said.