PM10 refers to airborne particles less than 10 microns in diameter easily inhaled into the lungs, and recorded on a daily basis in terms of how many micrograms there are per cubic metre.
Both Arrowtown and Cromwell exceeded the limit 42 times, while Alexandra had gone over the limit 35 times, and Clyde 23 times. Balclutha had exceeded the limit 14 times, Mosgiel 7, and Dunedin 0.
Milton also recorded the highest PM10 emission density of any town.
ORC environmental scientist Deborah Mills said several things contributed to Milton's poor air quality.
Minimum daily temperatures regularly dipped below freezing during winter, which increased the amount of energy needed to keep houses warm, and residents relied ''heavily'' on solid-fuel burning to heat their homes.
Winter temperatures were comparable to those recorded in Central Otago despite Milton's location near the coast.
The ORC's 2016 emissions inventory identified 77% of houses in Milton used solid fuel heating, which was higher than the national average and other Otago towns.
According to the 2013 census, there are 792 houses in Milton.
Hills also blocked Milton from coastal winds, which made the town ''colder and calmer'' than Balclutha.
''Taken together, these factors play a significant role in the poor air quality in Milton.''
As part of its plan to improve air quality, ORC was still offering financial assistance with its ''clean heat, clean air'' programme.
It had also joined a pilot project by the Cosy Homes Charitable Trust to replace up to 50 inefficient or non-compliant heating appliances in cold and damp homes.
An inaugural ''Cosy Homes Expo'' was recently held in Milton to provide information to residents interested in the scheme.
The Clutha District Council had also started offering a warm homes programme under which residents could apply for loans to help with insulation and heating.
Ratepayers could apply for up to $5000 (including GST) to make their home ''warmer, drier and more sustainable'' and pay it off over five years through their rates.
