New Zealand has some of the sweetest, purest, clearest clean air anywhere in the world.
Most of the time.
In most places.
This is the story of those other times and places.
New Zealand is harbouring four guilty secrets about its air – four situations where our current way of life is putting our health at risk unnecessarily. The good news is that they can all be dealt with using a little science and a little will.
So, what’s number one? I often ask people what they think is the main cause of air pollution in New Zealand. Probably the most common answer is road traffic.
If you’ve experienced Auckland’s motorways this may not come as a surprise. Us kiwis seem to have an addiction to motor transport. We use our cars like extensions of our legs. We’re also getting more and more fond of having all sorts of goods delivered to our door by vans and trucks. Admittedly both petrol and diesel vehicles have become a lot cleaner over the last few decades, but we have a lot more of them and we use them an awful lot more than we used to. At some point between the fifties and seventies our urban planners seem to have adopted the assumption that we will all be driving cars in the future, and should therefore design our streets and neighbourhoods to enable this to happen most efficiently. This seems to have meant designing out any effective alternative – not only walking, but also urban forms that suit public transport. Which basically means many kiwis have to drive (or feel they do) in order to go anywhere or get anything done. And thus, our traffic pollution problem was locked in for a century. 
I’ve spent nearly two decades monitoring and mapping traffic pollution in nearly every town and city across the country. My work shows that, despite improvements in the 1990s and 2000s, levels of the traffic-related pollutant nitrogen dioxide are above the World Health Organisation’s guideline (10 mg m-3) across much of Auckland (https://www.nzta.govt.nz/assets/resources/air-quality-monitoring/docs/review-of-the-national-air-quality-monitoring-network-NIWA-january-2021.pdf). What seems to be somewhat less well known, however, is that Auckland’s level of traffic pollution is barely any worse than all of our other towns and cities (https://www.nzta.govt.nz/assets/resources/air-quality-monitoring/docs/National-NO2-Monitoring-Network-Annual-Report-2007-2023.pdf). That’s down to New Zealand’s natural windiness. Traffic pollution in NZ doesn’t accumulate in stagnant air and mutate into a chemical smog, as afflicts many cities overseas, but leaves the city within the hour. But that also means that the city’s size has only a minor impact on the level of traffic pollution experienced by its citizens. Much more important is the local density of its roads and buildings.
And now we come to one of the paradoxes of 21st century air quality. Our traditional, sprawling, low-density, low-rise quarter-acre plot urban design keeps a good distance between most homes and major roads – allowing for lots of dispersion on the wind and generally good air quality where most of us live. But the larger distances between our homes and virtually anything else force us to drive nearly everywhere. And in our town and city centres – where there is still plenty of driving because that’s what our low-density sprawl incentivizes as well as disincentivising any other form of transport – more densely-collected and taller buildings (forming what we call “street canyons”) shelter trafficked streets from the wind, reducing dispersion and concentrating the pollution.
Basically, street canyons and traffic – especially diesel traffic – don’t mix. Until very recently, one of the most polluted places in New Zealand was Auckland’s main city centre thoroughfare – Queen Street. It is worth noting that Queen Street had a tiny fraction of the traffic volumes of most of Auckland’s major roads and especially its motorways. But the vehicles that did go along Queen Street were disproportionately likely to be diesel powered – either buses or vans – driving along the bottom of deep concrete and brick canyons.
Now Queen Street is cleaning up. The canyon formed by the buildings is still there – and probably will be for a century or more. It’s the vehicles, especially the diesels, that have been encouraged by various means to find alternative routes that are less damaging to air quality, or have been replaced by cleaner technologies like hybrid and fully electric. The positive impact on air quality has been dramatic. This could readily be achieved tomorrow in every other major town and city across the country, especially if citizens express that they want it.
The gains brought about by the combination of new vehicle and fuel technologies, and the legislation that requires and supports their uptake in NZ, have been substantial. But like many “simple” solutions they leave us with some trickier problems still to solve. In general, we find that traffic pollution is worst where people tend to congregate the most – on dense city streets with dense buildings, places like Queen Street and its equivalents in every town and city. This is particularly a challenge where we are trying to create more density to correct the errors of previous vehicle-dependent sprawl and the high emissions that locks in. Increasingly low or zero emission zones will need to be created either intentionally, or indirectly through other initiatives (like shared streets), if we want to claim that our city streets aren’t to remain a glaring exception to New Zealand’s excellent air quality. Low emission zones (e.g. petrol/diesel-free zones) established in small but critical locations (like Queen Street) not only incentivize vehicles to take other routes, they also incentivize clean vehicle purchases by individuals, businesses, bus operators and logistics firms; make city streets more attractive and can change the whole trajectory of a city’s development.
So, are the members of the public I talked to correct? Is road traffic our number one air quality problem?
Annual mean NO2 at Queen Street (from LAWA)
Like so many of the best questions in science – it depends! It depends both on how we judge “worst”, and also what data we have to make that judgement. The science is continually evolving and recent findings are leading us to accept that road traffic emissions have more extensive, harmful and pervasive impacts than was previously believed. During the first COVID-19 lockdowns levels of traffic pollution in Auckland plummeted overnight by two-thirds to a quarter of normal. This shows that traffic pollution is within our power to control.
But although road traffic is clearly important, I still think it’s unclear whether it is the biggest risk we face. Our main challenge may be closer to home. But that’s for next time.
