The main conclusions indicated by the seven studies are the following:
● The material type and weight of a shopping bag are important characteristics for determining its environmental impacts. A bag with the same material but double the weight has double the impact, unless it is reused more times or used to carry more goods. The LCAs in the meta-analysis indicate that a SUPB weighs approximately 6 g in China, India, Singapore and the US, but 18-20 g in Finland, Spain and the UK.
● The number of times a bag is used directly influences its environmental impacts. For instance, if a bag is used for shopping twice instead of once, it has only half the environmental impact per shopping round.
● The technology and material/energy use of production processes influence the impact of bags. For example, the climate impact of paper bags varies greatly, depending on what fuel is used in the pulp and paper production.
● The waste-management process also influences the environmental impact of bags. Paper bags that end up in landfills cause emissions of methane with high climate change effect, while plastic bags are relatively inert. On the other hand, incineration of used plastic bags affects the climate through emissions of fossil carbon dioxide (CO2), while the CO2 emitted from incineration of paper bags is part of the natural carbon cycle. The environmental impacts of biodegradable bags are reduced if the bags are composted, while most other bags benefit from material recycling.
● Considering the impacts from all life cycle stages, the environmental ranking of bags varies between different environmental categories. The SUPB is a poor option in terms of litter on land, marine litter and microplastics, but it scores well in other environmental impact categories, such as climate change, acidification, eutrophication, water use and land use. The overall environmental ranking will depend on what environmental aspects are given the highest priority. In this context it might be important to note that bags are responsible for a significant share of the litter, but a very small share of the total climate change when compared with other products and commodities.
● Reusable bags can be environmentally superior to SUPBs, if they are reused many times. For example, a cotton bag needs to be used 50-150 times to have less impact on the climate compared to one SUPB. A thick and durable polypropylene (PP) bag must be used for an estimated 10-20 times, and a slimmer but still reusable polyethylene (PE) bag 5-10 times, to have the same climate impacts as a SUPB. This requires not only durability of the bags, but also consumers to reuse each bag many times.
● Paper bags contribute less to the impacts of littering but in most cases have a larger impact on the climate, eutrophication and acidification, compared to SUPBs. However, they can be better for the climate if the SUPB is heavy, the paper mills use renewable fuel, the paper bags are reused multiple times, and/or the waste bags are incinerated rather than deposited at landfills.
● Single-use polyethylene bags based on renewable resources are better for the climate, compared to conventional SUPBs; however, they cause the same problems related to impacts of littering and are likely to cause more acidification and eutrophication.
● Biodegradable bags decompose and contribute less to the impacts of littering, compared to conventional SUPBs; however, the LCA results indicate they might be the worst option when it comes to climate impacts, acidification, eutrophication, and toxic emissions.
● A plastic bag with a prodegradant additive (oxo-degradable bag) has almost identical impacts compared to the conventional SUPB. It has less visual impact as litter, because it degrades into small fragments. However, the problem of microplastics might still remain, and the prodegradant can cause problems in recycling processes