Ian Stephenson of Dunedin asks :-

I heard that heating plastics can release dangerous dioxins. Are plastics safe to use in microwave ovens?

Ian Shaw, a toxicologist at the University of Canterbury's Department of Chemistry, author of "Is it Safe to Eat?", and editor of "Endocrine-disrupting Chemicals in Food", responded.

Heating plastics in the microwave is not a good idea unless the plastic is specifically designed to be used in that way and does not contain chemicals called plasticisers which are used to make plastics pliable. Plasticisers can move from the plastic into food under microwaving conditions. I’ll explain this in more detail later. First though I will address the dioxin issue. Some plastics contain molecules containing chlorine atoms (e.g. polyvinylchloride – PVC). If these plastics are burned at relatively low temperatures (e.g. in a fire) they form highly toxic, cancer-causing dioxins. On the other hand if they are burned at very high temperatures (e.g. in specially designed incinerators) the dioxins do not form. I do not think that microwaving chlorinated plastics would provide the right conditions to form dioxins.

Now back to the real problems of microwaving plastics. Some plastics contain large amounts of plasticisers, others might contain unpolimerised monomer units - plastics are formed by linking up small molecules called monomers into long chains called polymers. Many plasticisers and some monomers (especially bisphenol-A the monomer of polycarbonate plastics) mimic the female hormone estrogen. Long-term low doses of these estrogen mimics (also called xenoestrogens or endocrine disrupting chemicals -EDCs) can have affects on consumers. They might result in feminisation of males (e.g. reduced sperm count) and it has been suggested that they could influence the development of breast cancer in women – some breast cancers are promoted by estrogen and so estrogen mimics could have the same effect. Estrogen mimics do not only come from plastics there are natural phyto-estrogens in plants such as soy and so we should not only blame plastics for xenoestrogens in our food.

In short, dioxins are not an issue when you microwave plastics, but plasticisers are and it is just possible that some of them might promote (but not cause) breast cancer.