A new study has revealed which New Zealand cities have the worst levels of air pollution.

Urban Environment Specialist, professor Simon Kingham told Tim Dower the long term impacts of air pollution are largely unknown.

"It affects certain people more so it tends to affect people who are more susceptible, so older people and people with respiratory conditions. So for a young, reasonably healthy person in the short term it may not affect them but in the longer term, it may."

"This is where we know less about it, we don't know whether exposure to a healthy, young person at a young age actually makes them more susceptible later on. We certainly shouldn't take it lightly and it can be more than one or two days a year, it can be 10 or 20 days a year in some cases."

However, he said hundreds of people a year die from air pollution so it shouldn't be taken lightly. 

The South Island has some of the worst levels especially during winter. 

"In South Island towns where people use wood to heat their homes through fires and wood burners in summertime it's not a problem because you're not using them but in winter time it can be a problem."

He said the level of air pollution is highly dependant on where you live as well.

"What happens is we have thermal inversions, which is where the pollution gets trapped lower down, it can't escape because of the atmospheric conditions."

"The pollution is emitted by chimneys and then it can't escape into the upper atmosphere...[because] of the different temperatures of the air it can't get through and also the conditions are very still so it tends to happen on very still days because obviously if it's windy it gets carried away."

However, Kingham said wood burners are also at fault.

"The other part is obviously people are using their wood burners and that's why regional councils have a requirement on them to minimise pollution and that's why they try to encourage people not to use wood burners because if we don't emit the pollution, and we don't burn wood in our homes and use cleaner fuels then we won't have pollution problems."