People who live beside Auckland's Southern Motorway are subjected to air pollution at nearly double the level of those 130m further away, research shows.
The researchers suggest looking at preventing people from living within 20m of motorways and building more walls to separate the roadways from homes, children's facilities and businesses.
Fixed and bicycle-mounted measuring instruments, used in autumn and winter in Otahuhu, detected pollution levels that peaked beside the motorway from 7am to 9am, coinciding with the morning commuter rush.
The researchers, from Canterbury University's geography department and the National Institute of Water and Atmospheric Research, found similarly high levels of pollution along Princess St, which feeds the motorway, and several other areas of high traffic volume.
Potentially of most concern is their finding of a morning peak of around 140,000 "ultrafine" particles of pollution per cubic centimetre of air. These particles, a 10,000th of a millimetre in diameter, can penetrate deep into the lungs. Particulate air pollution is associated with lung disease and heart problems.
However, the researchers did not investigate the health effects of air pollution and note that their findings cannot be compared with national air pollution standards because of different measuring methods.
They say in the journal Atmospheric Environment that when their data on larger particles and carbon monoxide gas are included, across the study's four daily measurement times, arterial roads with traffic lights appeared to have a greater influence on pollution levels than the busier but more free-flowing motorways.
Beside the Southwestern Motorway in Mangere Bridge, the study's other suburb, the morning peak of ultrafine particle pollution levels was lower than in Otahuhu. In both suburbs, ultrafine particle pollution levels were generally much lower away from the heavy traffic flows.
Some of the houses near high pollution points in Otahuhu are just 5m from the edge of the motorway, said one of the researchers, Dr Woodrow Pattinson.
"Many of the homes are older, from the 1950s, 60s and 70s and don't have double glazing. They have fairly high rates of infiltration of outdoor air. The indoor air is often as bad as what it is outside. In modern apartments with filtration systems it wouldn't be as much of a concern. It's difficult because people need to live somewhere and there is a housing shortage. The best thing we can do for now is to not have sensitive population groups living there."
He said some restrictions were imposed on locating childcare centres near busy roads but he was not aware of any residential housing controls. Some researchers now advocated a buffer zone of 100m between homes and main roads.
Co-researcher Professor Simon Kingham urged authorities to consider not allowing people to live within 20m of main highways.
Dr Pattinson said noise walls helped "to deflect the plume of pollution".
"It would be great if we could have greenbelts or use the land for industrial buildings that properly protect the people inside," he said.
"It is important not to overstate the issue either as Auckland is very coastal so the wind usually flushes out a large proportion of these toxic fumes. However, under certain atmospheric conditions the influence of the motorways is fairly strong."
The study also involved interviews with 104 residents of the two suburbs.
Dr Pattinson said a number of people were worried about "children and family members suffering long-term illnesses because of the polluted air around them".
Who did the study?
Canterbury University and the National Institute of Water and Atmospheric Research.
What did they do?
They measured levels of carbon monoxide and tiny particles of air pollution that can cause asthma attacks, bronchitis and heart problems.
At fixed sites and from bicycles near the motorways in Otahuhu and Mangere Bridge and in the surrounding streets.
What were the main findings?
The highest median level of ultrafine particles from the repeated bike samples was around 140,000 particles per cubic centimetre of air during the morning traffic rush beside the Southern Motorway and in Otahuhu.