Air pollution: how much NO2 are city dwellers breathing?

The study focused on Wellington's busy Adelaide Rd, which is travelled by 20,000 cars each day and is seeing housing redevelopment that could attract 2000 more residents. Photo / NZME

A unique study which tested 50 homes along one of Wellington's busiest roads shows New Zealand has comparatively low levels of a key air pollutant.

But researchers behind the new findings say city planners who allow development in traffic-heavy areas still need to consider the threat of nitrogen oxide (NO2) becauseit can pose a health risk at any level.

"Air pollution is an area where there has been a huge amount of work in recent years that has shown an increasing range of adverse health effects - and it is probably the largest global environmental health risk," explained the study's lead author, Dr Caroline Shaw, of Otago University's Department of Public Health.

Despite that, it wasn't generally considered a large health issue in New Zealand – perhaps because enough research hadn't been done here.

Shaw said it was known that New Zealand had "hot spots" of air pollution, which was likely to become a bigger problem with climate change and denser cities filled with old and polluting vehicles.

"Hence there is the potential for unintended health risks of inner-city housing intensification as a result of higher numbers of people being exposed to air pollution," she said.

"So, given all of these factors, we were interested in whether there was any association between indoor and outdoor air pollution in residential housing."

In urban areas, outdoor NO2 emissions usually came from cars, trucks and buses – and the pollution directly from vehicles has been linked to deaths, hospitalisations and respiratory diseases like asthma.

The pollutant also posed a health risk inside our homes, where NO2 mostly flowed from smoking and using gas for cooking, hot water and heating.

For a case study, Shaw and her team picked the Adelaide Rd area in Wellington's Mt Cook, as it was bisected by a road travelled by some 20,000 cars and 600 buses each day, and was scheduled for redevelopment.

While the busy spot has historically been a light industrial area, that was changing with construction of low-rise apartments and town houses – and another 2000 people potentially moving in over the next few decades.

In 50 local homes, Shaw's team deployed indoor and outdoor NO2 monitoring equipment for two weeks, and then surveyed residents to find out more about the characteristics of each household.

Their study found that, compared with many other countries, levels were low – and outdoor NO2 was only slightly higher than that indoor.

There was also a weak link between increases in outdoor NO2 and that recorded inside.

But Shaw said there still important take-home messages for authorities.

"Even though New Zealand has low NO2 levels, it is still important for councils to consider urban air pollution, especially since there are known health impacts at low levels," she said.

"There's a balance between housing regeneration and development and the potential for increased numbers of people, especially children, to be exposed to traffic related air pollution in areas near busy roads."

There were effective ways cities could lower that risk – namely through electrifying vehicles, encouraging commuters to walk or bike, planting more trees along roadsides, widening distances between roads and homes, and banning unflued indoor gas heaters.

"These approaches need to be embedded into decision making around land use, building design and transport systems."