Can it really be true that plastic bags are the environmentally better alternative?

Responsible use of natural resources 26 April 2018 Marianne Bigum

The environmental impact of shopping bags has long been a subject of discussion amongst the general public and politicians. Opinion and evidence seems to be split down the middle. Some people are insistent that plastic should be avoided at all costs, but some point to the significant water requirements of cotton bag manufacturing.

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A press release from the Danish Environmental Protection Agency (EPA), concludes that the plastic bag is a much better choice for the environment than other types of shopping bags e.g. reusable cotton bags.

The press release was based on a new report from the Technical University of Denmark (DTU), where the researchers conducted a life cycle assessment of the type of shopping bags which today can be purchased at Danish Supermarket. However, if you take a closer look at the report, a different conclusion should have been reached, if it was fully considered that some of the bags are intended for repeated use, which the report and the EPA press release failed to properly include.

"You need to use your cotton bag up to 7,100-20,000 times before it's a better choice" These numbers hit the media and were used to conclude that the plastic bag is the better option for the environment. 

Based on the Danish EPAs press release, media sources were quick to widely repeat: “Forget about the reusable cotton bag. Regular plastic bags are the best choice for the environment". But can the report really support this message? I decided to investigate that by doing something atypical in a time of fast and fake news - I decided to read the report behind the press release.

To quickly sum up: The new report examines all kinds of carrier bags that you can typically buy in Danish supermarkets. These are evaluated in accordance with 15 different environmental impacts (categories) by conducting a life-cycle assessment. For each of these environmental categories, it was then calculated how many times you should use the different types of carrier bags before they match the environmental footprint of a regular plastic bag.

However, the high number of uses reported to the media does not reflect all the environmental impact categories, but simply the highest number among them. This makes a decisive difference in terms of how the results might be seen. The figure mentions only the worst environmental category and says nothing about how the carrier bag performs in the many other categories. The bag could, in principle, be poor in that one category, but, be better on the other 14.

The report identified ozone depletion as the biggest environmental effect of cotton bags. The very high number 7,000-20,000 for cotton bags (conventional and organic respectively) is caused by a chemical used in the processing of cotton. The 20,000 figure indicates that the bag must be used more than 20,000 times before it outperforms the regular plastic bag. However, this is only in this one category, and long before we reach that point, the cotton bag will have outperformed the plastic bag on the other environmental categories.

When it comes to climate change, the organic cotton bag only must be reused 149 times before it has the same climate impact as the plastic bag. If the bag is used 2-3 times a week, it is equivalent to using the cotton bag for just one year before its climate impact is better than the plastic bag. This is not unlikely, is it?

The report from DTU actually shows that with reuse the cotton bag soon outperforms the plastic bag on the majority of the environmental impact categories. 

The cotton bag does indeed have a problem when it comes to water consumption and its effect on the ozone layer. The report therefore included a sensitivity analysis and removed the ozone depleting chemical, and showed that the environmental impact of the cotton bag could be drastically reduced, if this chemical was phased out. Other researchers have since criticized the data used as being old and not representative of today’s cotton processing. 

As for the water consumption of the cotton bags. The functional unit of the environmental assessment is that is considers one regular plastic bag to be filled completely with groceries. The cotton bag you can buy in the supermarkets are slightly smaller than the plastic bag, and the report therefore assumes that you instead of buying one regular plastic bag buy two cotton bags. Basically, the environmental assessment compares using two half-empty cotton bags for each filled plastic bag. If we by better design could get just 2 liters more in the cotton bag, we could save one cotton bag, and cut the environmental impacts of using a cotton bag in half.

Reuse your bag!

At home I have a cotton bag from the Women's Workers' Union in Denmark, it is at least 15 years old. If we had used plastic bags instead, and used each of them 2-3 times before they were disposed of, we would have bought and disposed almost 800 plastic bags by now. The cotton bag is still in debt to the ozone layer, but it has long since outperformed plastic bags in all other categories. In fact, we would have affected the climate by a factor of 14 times more, in terms of CO2, had we not used the cotton bag.

There are consequences for climate change when you continually dispose of plastic bags, and it effects the world's water sources when you use the cotton alternative.
For this reason, make sure you remember to take a reusable bag with you rather than consistently buying new bags, and use it as many times as possible. 

Finally, we must also look at the big picture when thinking about our cotton usage. Each Dane consumes 16 kg of textiles annually, corresponding to 63 fabric bags, so to compensate for choosing our cotton bag, perhaps we could consider buying just one less t-shirt, or making one from some of our textile waste. 

Marianne Bigum is a Senior Consultant at Ramboll, and holds a PhD in life cycle assessments. The post expresses the writer's own views.