Are cotton or paper bags really better than plastic?
If you use cotton bags when you go shopping you probably think you're doing the world a favour.
But if you use it once or twice, stick it in the cupboard then buy a new one, it may be just as bad as using a conventional plastic bag - or worse.
Many Kiwis have called for action on single-use plastic bags. Supermarkets have taken heed and pledged to ban the bag at Countdown and New World stores by the end of next year.
Other countries, such as Kenya, have banned plastic bags altogether. In the United Kingdom and Ireland, levies have seen plastic bag use plummet by over 80 per cent.
But the effect on demand for alternatives is largely unknown. What is known is that all bags, from plastic to paper to polypropylene, are made differently, and how they are made has an environmental impact.
A 2011 study by the United Kingdom Environment Agency found the environmental impact of all types of bags was dominated by resource use and the production stage. In comparison, transport, secondary packaging and end-of-life management had little effect.
The study compared different types of reusable bags to supermarket single-use plastic bags made from high-density polyethylene (HDPE), currently used at Countdown, and calculated how many times each reusable bag would need to be used to be better than a conventional plastic HDPE bag.
LDPE & HDPE
The study found the reuse of plastic bags for shopping or as bin-liners is "pivotal to their environmental performance and reuse as bin liners produces greater benefits than recycling bags".
HDPE plastic bags are used at Countdown. Reduced weight low-density polyethylene (LDPE) bags are used at New World and Pak 'n Save.
The study compared HDPE with heavy duty LDPE bags and found the LDPE bags had to be used at least four times to have a lower global warming potential than HDPE grocery bags that are not reused. So if HDPE bags are used twice for shopping then once as a bin liner, LDPE bags have to be used 12 times.
The study found 64 per cent of the bags' impact on global warming was directly from the extraction and production of the plastic.
The best thing shoppers can do to mitigate the effects of current plastic bags on the environment is to reuse the bags as many times as possible.
The study also looked at woven polypropylene reusable bags, such as those sold at New World and Pak 'n Save.
These bags are made from the same plastic as HDPE and LDPE bags but are much thicker.
The study found shoppers need to use the bags 14 times before they are better than grocery bags.
The study did not look at non-woven polypropylene reusable bags, such as those at Countdown.
Paper bags are often seen as more environmentally friendly because paper is recyclable. But paper bags could be the worst of the lot because of the difficulty in reusing them.
According to a report by the Scottish Government, the process of making paper bags takes almost four times as much water, and releases more than three times as many greenhouse gas emissions than conventional plastic bags.
In 2007, San Francisco banned non-compostable plastic bags but the policy led to an increase in the use of single-use recyclable paper bags.
Only 38 per cent of paper bags were recycled, the rest went to landfill where they take up fives times more space than plastic bags.
Paper bags usually do not biodegrade in landfill because there is no oxygen.
The UK study found paper bags need to be reused three times if they are to have less of an environmental impact than a conventional plastic bag used once, but the study found "no significant reuse of paper bags," not even as bin liners.
Cotton bags are rarely seen in supermarkets but are for sale as an alternative to plastic.
The study found cotton bags need to be used at least 173 times if they are to do less environmental damage than a plastic bag that is used once.
If a plastic bag is reused three times, for example being used twice in the supermarket and then as a bin liner, the cotton bag has to be used almost 400 times to have lower global warming potential than plastic.
This is because of the amount of energy and use of non-renewable resources it takes to extract cotton, make the bags and then ship them.
Victoria University environmental science lecturer Dr Lynda Petherick said manufacturing of cotton bags had "large, negative impacts on the terrestrial ecosystem. In addition, fertilisers get into the waterways and can harm aquatic, and ultimately marine environments.
"The bonus is that the bags can be used more times, and so there should be less waste," she said.
Using a cotton bag 173 times is equivalent to using it once a week for about three and half years. "From what I can gather, people do not commit to using cotton or reusable bags, and so the positives from reducing waste disposal are not realised."
But that of course assumes you're using a new cotton bag - not one recycled from another cotton product.
Jute bags have become a popular option for shoppers with some fashion designers releasing limited edition bags, often with a hefty price tag.
The bags look and feel like canvas but they are made from the plant fibre jute.
The UK EPA study didn't look at jute, but a 2002 study by the Indian Institute of Technology looked at 50 kg jute bags used to transport commodities such as cement, fertilisers and food grains.
It found jute bags use almost 50 per cent more energy and nine times more water than HDPE bags in production. However, jute plants can negate some of this energy consumption through the carbon dioxide consumed when they are grown.
A 2014 study by Dhaka's National Institute of Textile Engineering and Research said one hectare of jute plants can consume about 15,000 kg of CO2 from the atmosphere and release about 11,000 kg of oxygen in the 100 days of the jute growing season.
SO WHAT SHOULD YOU USE?
The study's main conclusion was that reusable bags are better than plastic - but only when they are reused a significant number of times.
Petherick said key to the report was the amount of resources needed to produce reusable bags.
"If the bags last longer, then we don't need to use as many, or dispose of as many," she said. "However, some sort of societal change needs to be instigated, we need to actually start reusing bags. So really, in my opinion, it comes down to reduce, reuse, recycle."
Petherick said it was important to note that the study did not take into account the effect of littering. The World Economic Forum last year warned that by 2050, oceans will contain more plastic than fish. Each year, at least 8 million tonnes of plastic ends up in the world's oceans.
"Plastic bags do not biodegrade and can create a huge problem in marine environments.
"There you have issues such as ingestion by seabirds, entanglement, the breakdown of bags releasing carcinogens and plastic settling on the ocean floor leading to no oxygen in the lower layer of the ocean," she said.
The study also does not take into account the long-term effects of materials degrading in landfill, only the carbon used in disposing of them.
Petherick said there was no perfect solution. "A large component of the study is the survey about whether or not people reuse their plastic bags. Typically people tend to try make it sound like they are doing the right thing, so I am a little suspicious about the rate of plastic bag reuse.
"The LDPE bags don't biodegrade so have landfill implications.
"Education is very important. The crucial thing is getting people to start thinking about how they are using bags, and whether they need to – prevention and minimisation – and then getting the bags to be used more than once."