Scientists are warning the growing push for biodegradable bags could be an environmental disaster in the making.
Kiwi supermarkets and retailers are pushing to get rid of traditional plastic bags, with some offering reusable options and others opting for biodegradable/compostable carriers or paper.
- Warehouse to phase out single-use plastic bags
- Countdown announces first 10 supermarkets to go plastic bag-free
- New World follows Countdown's lead to phase out plastic bags
But there's growing concern the so-called environmentally friendly options aren't much better than single-use bags, if at all.
In a new article, Jesse Harrison of the University of Helsinki says too little is known about how well biodegradable plastics break down in the environment to confidently say they're better.
"Current standards and test methods are insufficient in their ability to realistically predict the biodegradability of filmic carrier bags in these environments, due to several shortcomings in experimental procedures and a paucity of information in the scientific literature," he writes in the journal Royal Society Open Science.
Moreover, existing biodegradability standards and test methods for aquatic environments do not involve toxicity testing or account for the potentially adverse ecological impacts of carrier bags, plastic additives, polymer degradation products or small (microscopic) plastic particles that can arise via fragmentation."
In other words, we don't really know what damaging effects biodegradable bags have once they've been discarded.
Too many unknowns - Kiwi scientists
Kiwi scientists are praising Dr Harrison's work.
"This research helps destroy the thinking a plastic bag with a label 'biodegradable' is safe for the environment," says Thomas Neitzert, engineering professor at AUT.
He says lab testing is no substitute for real-world conditions, and doesn't account for what happens to the plastics once they've been broken down.
"A biodegradable plastic bag is potentially dangerous to marine life from the moment it enters the water until it dissolves into micro- or nanoparticles over many years... The standards are usually underestimating the life of a plastic product by years and sometimes decades."
Fellow professor of engineering Kim Pickering of the University of Waikato says biodegradable bags could be a step in the right direction, but there's still a lot of research to be done.
"It is important to assess how long things take to degrade in real situations and also what they break into and the consequences of that and we need to address such shortfalls."
A single plastic bag can turn into 2 million pieces of microplastics, research released last year found. New Zealanders use about 673 million plastic bags a year - about 150 each.
Previous research has found reusable bags would need to be used hundreds of times before they became more environmentally friendly than single-use bags, because of the resources required in their production.