People living in western Christchurch are breathing the cleanest air in the city.

Environment Canterbury (ECan) has been monitoring the city's air pollution levels for about 20 years.

Air pollution is measured by the amount of particulates in the air below 10 micrometres, with pollution recorded as "high" when the daily concentration of PM10 (particle matter) is greater than 50 micrograms per cubic metre of air.

ECan senior air-quality analyst Teresa Aberkane said the central city and suburbs on the northern, eastern and southern sides had more pollution as they were generally denser areas with older housing and closer to industrial areas.

"The suburbs in northwestern [Christchurch] are going to be cleaner than those on the eastern [side] because the air is flowing from the mountains towards the sea," she said.

For example, St Albans and Opawa had dirtier air than Burnside.

"Anything that is produced on the western edge will move east."

She said the Port Hills acted as a barrier, so pollution collected at the bottom, but wind drifted down the hills overnight and could push that pollution into the central area. Where pollution became concentrated and where it was produced could be different, she said.

Aberkane said ECan used to move the monitoring site around, but new national standards required it to continuously measure the same spots.

Since the new standards, the main monitoring sites have been in Woolston and in St Albans. She said the same trends, of central suburbs and surrounds being higher, should still be true.

"In general terms, we'd expect PM10 concentrations to still be high in Opawa, as they are in Woolston," she said.

- © Fairfax NZ News

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