How many times do you need to use a cheap re-usable bag to offset its impact?

Question

An op-ed in the Wall Street Journal argues that many re-usable grocery bags are made using more oil than old-fashioned "single-use" plastic grocery bags.

This makes sense, as quite a bit more material is required to make a sufficiently sturdy bag, and many of the lower-quality bags are made of plastic, nylon, or polyester.

Obviously bags made of canvas, burlap, or something similar are best, but if I'm at the check-out line, don't have a reusable bag, and opt to buy a cheap one from the store, how many times will I have to use it before it's better than just using the single use bags?

Answer 1

Summary: According to this independent study if you look at global warming potential, for a sturdy LDPE bag you need to use it at least 4 times, for an even sturdier non-woven PP bag with inserts it's at least 11 times, and for a cotton bag at least 131 times. These numbers are higher if you reuse your single-use HDPE bags for other purposes

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In 2006 the governmental Environment Agency of the UK issued an independent study to find out which type of shopping bags are most environmentally friendly. They compared a conventional light-weight bag made from HDPE with:

• A paper bag
• A thick-gauged or heavy-duty LDPE bag
• An even more durable bag made from non-woven PP, including stiffening inserts
• A cotton bag

The study concluded that:

• The paper, LDPE, non-woven PP and cotton bags should be reused at least 3, 4, 11 and 131 times respectively to ensure that they have lower global warming potential than conventional HDPE carrier bags that are not reused. The number of times each would have to be reused when different proportions of conventional (HDPE) carrier bags are reused are shown in the table below.

The full report is available here: Life cycle assessment of supermarket carrierbags: a review of the bags available in 2006

However there are some caveats:

• Although the study did investigate other impacts, the table shown only takes global warming potential into account.
• This particular study is about 6 years old and uses data on bags that were used 10 years ago. New production techniques used today may lead to different results.
• The result of a life-cycle assessment study done in the UK may not translate to other countries. To investigate this you need to carefully check all the geographical dependent data that was used.
• Other life-cycle assessment studies have different conclusions, but some of the industry-sponsored studies are known to make assumptions that tend to favor certain types of bags.

Answer 2

The cheapest reusable bags round here are made of the same plastic as disposable bags. In this case the number of uses required is simply the ratio of the weights. Without getting the scales out I think this is around 10-20 times. They're also bigger so that's not even 10-20 shopping trips unless you only ever buy a little. I know I've had a few hundred uses out of such bags in the past (a design with a season and year, used minimum twice a week for two years) so it's perfectly possible to do quite well.

A major reason for reusable bags (and the reason we now have a charge on disposable bags here) can't be quantified in quite the same way: plastic pollution, especially ending up in the sea. Number of items is an issue as well as total quantity in this case, and fewer bags being used means fewer to cause problems.