Justin White, of Balclutha Primary School, asks :-

Why do we use plastic for packaging when it takes so long to break down in a landfill?

Alan Happer, an organic chemist at the University of Canterbury, responded.

The real explanation is one of cost and convenience. The obvious alternative to plastic is paper or cardboard - this biodegrades fairly quickly in a landfill and comes from wood, which makes it sustainable. Plastic on the other hand, although in theory biodegradable, breaks down extremely slowly. Furthermore, it comes from petroleum, which is not sustainable, at least in the long term.

Unfortunately paper production not only consumes lots of energy but also gives by-products that can pollute waterways. A lot of money has to be spent to prevent this. Plastic packaging production produces fewer polluting by-products. It also gives a much stronger product for its weight. For example, a supermarket plastic bag is thin, light and strong and unused ones take up very little space. A paper version would need to be thicker and heavier to be as strong. This means that not only would unused ones take up more space, but they would probably end up more expensive to make. (That they don't like getting wet is also a disadvantage.)

On the other hand, cardboard is preferred to plastic for containers because the thickness of plastic required to give the stiffness needed would make them much more expensive than cardboard. Probably equally important though, before they are filled and after they are emptied cardboard containers can be easily collapsed to take up less space for moving around from place to place.

Starting to appear on the scene overseas are plastics that not only can be produced from sustainable sources but will also biodegrade rapidly in a landfill. They are currently more expensive than other plastics, but are expected to become increasingly used by supermarkets instead of the polyethylene bags currently used.