Seventy-four metric tonnes of microplastics have been found in Auckland's atmosphere, the equivalent of three million plastic bottles per year, researchers say.
The University of Auckland research, published in Environmental Science & Technology, found the microplastics were of such small sizes that there was concern they could be inhaled and accumulate in the human body.
Nanoplastics, the smallest particles, can potentially enter cells, cross the blood-brain barrier, and may build up in organs such as the testicles, liver and brain. Plastics have also been detected in the placenta.
"Microplastics have also been detected in human lungs and in the lung tissue of cancer patients, indicating that the inhalation of atmospheric microplastics is an exposure risk to humans," the paper noted.
The levels of microplastics in Auckland's air were much higher than those recorded in London, Paris and Hamburg in recent years, with the study utilising a method which could detect particles as small as 0.01 of a millimetre, researchers said.
Lead author Dr Joel Rindelaub said "The smaller the size ranges we looked at, the more microplastics we saw".
"This is notable because the smallest sizes are the most toxicologically relevant."
In Auckland, the average number of airborne microplastics found in a square metre in a day was 4885, compared with 771 in London in 2020, 275 in Hamburg (2019) and 110 in Paris (2016).
But Rindelaub believed researchers around the world were likely to have dramatically undercounted their airborne microplastics.
"Future work needs to quantify exactly how much plastic we are breathing in," he said.
"It's becoming more and more clear that this is an important route of exposure."
The experiment was carried out over nine weeks during September, October and November in 2020.
Microplastics enter the environment from various sources, with weathering and ageing breaking plastic down into ever smaller particles.
For example, fibres dispersed by washing synthetic clothes, fragments shed by car tyres and washed by rain into the ocean, and bottles floating down rivers.
Polyethylene (PE) was the major substance detected in the University of Auckland study, followed by polycarbonate (PC) and polyethylene terephthalate (PET).
Polyethylene and PET are packaging materials while PC is used in electrical and electronic applications. All three are also used in the construction industry.
"Over the last 70 years, 8.3 billion metric tons of plastic have been produced globally," the paper stated. "Only nine percent have been recycled, with the rest either incinerated or released into the environment."
The researchers said waves breaking in the Hauraki Gulf may also be transmitting water-borne microplastics into the air, with more microplastics detected after winds from the gulf picked up speed, likely leading to bigger waves and more transmission.
"The production of airborne microplastics from breaking waves could be a key part of the global transport of microplastics," Rindelaub said.
"And it could help explain how some microplastics get into the atmosphere and are carried to remote places, like here in New Zealand."
The study also noted when winds passed over the Auckland city centre, the microplastics downwind were larger, indicating the plastics had gone through less environmental ageing and came from a closer source.
Researchers believed the study was the first to calculate the total mass of microplastics in a city's air.
The paper, co-authored by professor Kim Dirks, Dr Patricia Cabedo Sanz and associate professor Gordon Miskelly, called for standardisation of reporting metrics so studies of airborne microplastics could be better compared.