The Canterbury Regional Council is trying to get to the bottom of why Christchurch's air quality is so poor this winter.
Photo: RNZ / Nate McKinnon
It has exceeded World Health Organisation levels for clean healthy air six times already.
This time last year the city had not gone over the limit at all.
The council's operations manager Judith Earl-Goulet said the fine particles found on three occasions were most likely from log burners.
But the other three exceedances which involved coarse particles, were harder to explain.
"Three of the readings we've received recently at the Woolston site are actually associated with course material and occurred on warm days.
"Our monitoring sites tell us what's happening with the air quality but they don't tell you where that source is. So that's the bit we need to do a little bit more looking at."
Earl-Goulet said despite the high readings, pollution levels in the city were still much better than a decade ago, before the introduction of tougher rules around log burners.