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That study confirmed Keller's point: that a reusable grocery bag made of non-woven polypropylene plastic would have to be used at least eleven times to have a lower carbon footprint than using disposable single-use grocery bags. There were other comparisons in the study, too: Using a paper bag three times would do the trick, while it would take 131 trips to the market with a cotton bag to have a lower carbon footprint - which meant the material used for a reusable bag was critical.
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