The amphibian's smile suggests a clean, green product but the truth about biodegradable plastic is complicated

It was the cartoon frog that caught my eye. It was smiling - beaming, even - and the lines above the eyes highlighted the fact that this was one happy little amphibian.

As such, it made an excellent mascot for the plastic bag it adorned. The words "100% degradable" that accompanied the image completed the picture: here was a plastic bag you could feel good about using.

Well, a shopper who drew the bag to my attention after she saw it in use at a local food market wasn't feeling good about it at all.

Her worry was that the degradation alluded to was a process of breakdown only into fragments that are invisible to the naked eye. Perhaps, she suggested, they are out of sight, but not out of the waste stream.

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As it turned out, she had a point. The bags are sold by a Tauranga company called Eco-Pal. Its operations manager, Melody Shaw, declined to be interviewed about the distinction between a biodegradable and a degradable bag, but in written answers to emailed questions said that the company's bags were made of "conventional plastic with a pro-degradant additive ... which makes it biodegradable".

This additive, with the brand-name d2w, was developed by an English company called Symphony Environmental; it is composed of "transition metal compounds [of] manganese, iron, cobalt and nickel".

Shaw wrote that the first-stage degradation - into invisible fragments - was followed by the second stage, in which "these tiny fragments are ingested by micro-organisms and are converted to biomass".

Which sounds terrific, except that biodegradation can't occur without oxygen. Even Symphony concedes that d2w bags won't degrade when buried in a landfill where there is no oxygen.

Eco-Pal's website admits as much, acknowledging that degradation in landfill is possible but will take significantly longer than if left in the open environment. Shaw says testing is still being done on the plastic's biodegradability in landfills. Not a moment too soon; Eco-Pal has been selling it in New Zealand since 2005.

The happy little frog could be seen as slyly implying that the product is clean and green, though Shaw said the company was not trying to mislead anyone.

But Eco-Pal was fined $60,000 in 2013 after being found guilty of 15 breaches of the Fair Trading Act for website claims that its rubbish bags were biodegradable and for "giving the impression of environmental friendliness". The judge said its behaviour amounted to serious offending that prejudiced well-intentioned shoppers.

The website has been more modestly reworded and across the top of every page is a disclaimer saying the bags are "designed to reduce the impact of plastic as litter".

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But you don't need to be an environmental activist to work out that the best way to avoid something becoming litter is not to make it.

Shaw says that in the open environment a d2w bag will take between six months and five years to biodegrade. That's a great improvement on the standard plastic bag which takes years to break down and never fully decomposes. But it's a much larger environmental footprint than that of something that never existed.

In any case, biodegradability is not a complete answer. The UK Government-funded Waste and Resources Action Programme says biodegradation of these high-tech plastics releases methane, a greenhouse gas 23 times more powerful than carbon dioxide.

And even a bag that degrades over time has a high environmental impact long before it becomes litter. The Australian Department of the Environment calculates that the amount of petroleum required to make a plastic bag would propel a car 11 metres. In this clean, green country, we use a billion a year; the world uses between 500 billion and a trillion.

A plastic bag that, under certain circumstances, may biodegrade sounds well and good, but making litter that people can feel good about generating rather misses the point: we need to change our mindset about using plastic bags of any sort.

We lag behind many countries, including such outstanding leaders in good government as Eritrea and Burma, in neither banning nor taxing single-use plastic bags. Those agitating for us to catch up face resistance from the bag-makers, such as Symphony, which says taking your own bags to the shops is "not the answer".

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They cost more and they "become a very durable problem when discarded", the super-plastic makers tell us. Worse, they are not hygienic, it warns. Who would have thought that the cotton shopping bag and string kit were such dire health risks?