Some of New Zealand's smallest towns have worse air pollution than Auckland - and Timaru tops the list, according to data released by the World Health Organisation (WHO).
The international organisation released a report of outdoor air quality in 1600 cities across 91 countries. It measured annual mean concentrations of fine particulate matter: particles smaller than 10 micrometres per cubic metre of air (PM10).
The numbers for New Zealand come from the Ministry for the Environment.
Timaru recorded the highest PM10 value of 28 micrograms per cubic metre, nearly twice that of Auckland's 15. Christchurch came in second at 23, followed by Rotorua on 20.
WHO Air Quality Guidelines recommend annual mean PM10 values of 20 micrograms per cubic metre.
Air pollution in Christchurch and Wellington deteriorated between 2010 and 2012. Christchurch went from 17 to 23, and Wellington increased from 11 to 13.
Not all of the New Zealand cities measured in 2012 had also been measured in 2010. Of those that were, Auckland and Hamilton both stayed the same at 15 and 13 respectively.
Dunedin decreased from 25 to 18 between 2010 and 2012.
The WHO said in most cities with enough data to compare to previous years, air pollution was getting worse. Contributing factors included reliance on fossil fuels, dependence on private vehicles and inefficient use of energy in buildings.
WHO Department of Public Health spokesman Dr Carlos Dora said cities could take action to improve air quality.
"We cannot buy clean air in a bottle, but cities can adopt measures that will clean the air and save the lives of their people," he said.
Such measures could include energy efficient housing, compact urban development "well served" by public transport, and street design that is appealing and safe for pedestrians and cyclists.
BY THE NUMBERS
2012 annual mean concentrations of PM10 (micrograms per cubic metre)
Lower Hutt 10
Upper Hutt 10