Global estimates suggest that 1-5 trillion plastic bags are consumed every year. For scale, 5 trillion bags equates to nearly 10 million bags per minute and, if tied together, these would reach around the world seven times every hour. The excessive use of plastic bags, coupled with concerns over their presence in our natural environment, led to many jurisdictions banning single-use plastic bags, including here in Aotearoa New Zealand from 1 July 2019. Since the ban on single-use plastic shopping bags was announced by the Ministry for the Environment, there has been a lot of public interest in whether the evidence shows that the alternative options are really a better choice for the environment. Is a single-use plastic bag better or worse than alternatives such as a paper bag, a heavier reusable plastic bag or a cotton bag?

There is no clear answer to this question. All bags have environmental impacts from resource extraction, the amount of material used, how it is produced and how it is managed at end-of-life.

Examples of plastic bags that were banned in New Zealand from 1 July 2019. Credit: Ministry for the Environment.

The most environmentally favourable bag choice will be dictated by the environmental impacts of most concern and how many times a person is likely to reuse the bag.

To date, most LCA studies on shopping bags have not accounted for litter – but litter is one of the major concerns driving plastic bag bans. A recent LCA study comparing HDPE (#2), LDPE (#4), PP (#5), paper and biodegradable plastic bags introduced a littering indicator to model littering, by calculating the relative risk between the different options, rather than assigning a final impact.[1]

The study found that the order of environmental favourability for the climate change impact (determined as part of an LCA study) was the opposite to the order determined by the littering potential indicator (see the table below). A reusable plastic bag had the lowest impacts for climate change and all impact categories, but was the second-worst for littering potential.

Comparison between bag types ranked from best to worst for global warming potential and littering potential

Global warming potentialLittering potential
BESTReusable LDPE bag (at least 10 uses) Reusable PP woven bag with 40% recycled content (at least 20 uses)BEST
Single-use HDPE bag (1 use + bin liner)Single-use recycled paper bag (1 use)
Single-use biodegradable bag (1 use) Single-use biodegradable bag (1 use)
Reusable PP woven bag with 40% recycled content (at least 20 uses)Reusable LDPE bag (at least 10 uses)
WORSTSingle-use recycled paper bag (1 use)Single-use HDPE bag (1 use + bin liner)WORST

Ultimately, the study demonstrates that there are discrepancies between the best bag choices depending on whether you prioritise ‘climate’ vs ‘litter’, but overall, the best environmental outcomes will come from extensive reuse of a bag and appropriate disposal at its end-of-life. Further complexities around bag choice would come from adding other considerations such as health and economics, into the decision.This study is one of the first to factor in littering impacts to a bag LCA. Further studies in different contexts and inclusion of other bags (e.g. cotton reusable bags) are needed to address limitations in this initial study, including factoring in that biodegradable bags act the same as single-use plastic bags and could also be used as a bin liner. Use as a bin liner would reduce the likelihood of litter, as the liner would be disposed in landfill with the bag of waste.

There are discrepancies between the best bag choices depending on whether you prioritise ‘climate’ vs ‘litter’, but overall, the best environmental outcomes will come from extensive reuse of a bag and appropriate disposal at its end-of-life

The social aspects of a product ban are not factored into studies of life cycle environmental impacts and littering impacts. Once awareness had been raised by those campaigning to ban the bag, the changing practice was largely driven by the forces of social contagion and has provided educational value in terms of raising awareness around plastic pollution and helping shift people’s attitudes towards sustainability thinking, which is a critical part of the cultural transformation required for rethinking plastics.

References
[1] Civancik-Uslu et al., “Life Cycle Assessment of Carrier Bags and Development of a Littering Indicator,” Science of The Total Environment 685 (2019)

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