Timaru's air pollution levels are the worst in Australasia, new figures released by the World Health Organisation (WHO) show.

Timaru's annual mean levels of PM10 particles and PM2.5 particles were both above the air pollution level recommended as safe by the WHO.

PM10 and PM2.5 particles are used to measure air quality levels. The particles are so small that they can get into the lungs, potentially causing serious health problems.

JOHN BISSET/FAIRFAX NZ Smog over Timaru.

As urban air quality declines, the risk of strokes, heart disease, lung cancer, and chronic and acute respiratory diseases including asthma, increases for the people who live in them, the WHO states.

Christchurch was the worst of New Zealand's major cities, while Wellington and Auckland's air was found to be much cleaner.

While the level of air pollution in Timaru presented "some risk", Parliamentary Commissioner for the Environment Dr Jan Wright said there was no reason to panic.

SUPPLIED Dr Jan Wright, Parliamentary Commissioner for the Environment

Timaru had a PM2.5 annual mean level of 15 micrograms per cubic metre, and a PM10 level of 28 micrograms per cubic metre.

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The WHO guidelines state that levels of 10 and 20 micrograms per cubic metre respectively are considered safe.

Wright released a report into New Zealand's air quality in 2015. She found that a lot of progress had been made in improving air quality, but there was still real evidence of harm to New Zealanders' health.

Following the release of the updated WHO air quality database, Wright said an increasing reliance on heat pumps rather than woodburners would improve the country's air quality in the future.

"Air quality over time has got a whole lot better in New Zealand, and will continue to do so.

"People get the idea that if you're below the guidelines then everything's fine, and if you're about then everything's terrible. That's not the case at all."

Environment Canterbury (ECan) measures dirty air in PM10, particulate matter less than 10 micrometres in diameter. Readings higher than 50 are considered to be high pollution levels.

Timaru has recorded two high pollution nights this year, with levels of 55 and 51 respectively. Washdyke has recorded one high pollution night.

From September this year, Timaru cannot have any more than three high pollution nights a year to meet the national air quality target.

Timaru had 26 high air pollution readings in 2015.

ECan air spokeswoman Katherine Trought said ECan was concerned about the high levels of PM10 in Timaru as there was no safe level for air pollution.

Due to the combined effects of high emissions and South Canterbury's weather and landscape, during calm winter nights pollution could be trapped in a layer of cold air near the surface, she said.

Winter air pollution was usually worse in the evenings as that was when most people used their log burners, and the rapid cooling of the ground created a layer of cold air that trapped emissions.

"Everyone has a role to play in improving winter air quality, and as well as wood burner users, we are working with industry, land managers, farmers and developers. This winter, we're asking wood burner users to do their bit too."

MP for Rangitata, National's Jo Goodhew, said she was "heartened" by ECan's educational approach. Goodhew said people sometimes visited her Timaru electorate office about the issue, including some who were concerned about having to change their fireplaces.

According to the new WHO database, levels of ultra-fine particles of less than 2.5 microns were highest in India, which has 16 of the world's 30 most polluted cities.

China had improved its air quality since 2011 and now had only five cities in the top 30. Nine other countries, including Pakistan and Iran, had one city each in the worst 30.

The most polluted city in Australia was Geraldton, north of Perth.

New Zealand's air quality (micrograms per cubic metre)

Christchurch

PM10: 21

PM2.5: 10

Dunedin

PM10: 18

PM2.5: 9

Wellington

PM10: 10

PM2.5: 5

Auckland

PM10: 14

PM2.5: 6

* An earlier version of this city said Timaru's air quality was the worst in Oceania. This was incorrect, it is the worst in Australasia.