Asked by: Roy Kendrick, Bury

Plastic bags have only been around since 1957 so this is obviously just an estimate. Polythene is degraded fairly effectively by ultra-violet light, but plastic bags in landfill are quickly buried and so don’t get much light. Bacteria and fungi have a much harder time digesting it. Polythene doesn’t exist in nature so soil microorganisms that are well adapted to breaking it down haven’t had time to evolve yet.

In 2008, Daniel Burd, a high-school student from Canada, found that a brew of Pseudomonas and Sphingomonas bacteria, together with yeast and sodium acetate, could digest 43 per cent of a plastic bag in six weeks. But this culture needs to be kept at 37°C, so it would probably need to be used in an industrial reprocessing plant rather than an ordinary landfill.

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