Why it’s a bad idea to microwave plastic

It might be convenient to nuke your leftovers in a takeaway container. You shouldn’t . . .

RACHEL THOMAS

We know plastic sucks, but heavy plastic packaging remains a mainstay of microwavable foods – think steamed puddings, packet rice and those instant meals you keep in the freezer.

But despite the temptation or back-of-the-packet instruction to pop these in the microwave as-is, experts say don’t.

The danger lies in the chemicals used in plastic and the leaching risk that happens when heat is added. This is why you should transfer food to a pyrex or glass dish before microwaving it.

Plastic is made from chains of molecules (polymers) which create a hard, brittle substance, so other chemicals called plasticisers are added to make things bendy and soft. Plasticisers are chemicals such as bisphenol A – commonly known as BPA – and phthalates (pronounced ff-thal-ates).

When heat is added to plastic, its molecules change and move and can let go of the plasticisers holding them together. Those plasticisers have to go somewhere – so if they’re next to food, that’s where they’ll end up – on a highway to your gut.

So what’s so bad about eating a bit of BPA and phthalates?

This is where things get interesting. These chemicals are all around us in things like meat, oils, make-up, food cans, tattoo ink – and some exposure is considered safe. But at certain levels, phthalates and BPA chemicals can interrupt basic processes in the body.

The major effect is on our hormones, with both BPA and phthalates considered

‘endocrine disrupting chemicals’ (EDCs). In simple terms, these chemicals mimic hormones, in particular the female hormone, estrogen.

The endocrine system controls nearly all our bodily functions – the way we grow, age, reproduce and move. So while there are documented effects on things you might expect, such as sperm count, there are also known effects on obesity, allergies and asthma, blood pressure, depression, autism and motor function.

Phthalates have also been linked to birth defects, infertility and increased risk of learning, attention and behavioural disorders in children.

Now, we can’t put this all down to microwaving plastic or EDCs alone, but it is another source of exposure. Luckily, it’s exposure we can control.

Many countries (but not New Zealand) have banned phthalates and BPA in baby formula products, children’s toys and bottles.

In the EU, some bans on phthalates extend to makeup.

Higher temperature and fat levels matter too. The higher the temperature, the more plasticisers move. And the greater the fat content of the food – think bacon or salmon – the greater the chances of leaching.

Last year, Massey University research measured the level of phthalates in urine samples from 600 New Zealanders – half adults and half children. The research team didn’t ask participants whether they microwaved food, but found those who ate warm food from plastic containers more than twice a week had higher levels of phthalates in their urine.

A big question mark remains over whether plasticisers increase cancer risk. But the safest advice is: don’t heat your food in plastic – unless there is no other choice.

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This story was written with expert advice from toxicology Professor Ian Shaw and public health Professor Andrea t ‘Mannetje. It was reviewed by The Whole Truth: Te Ma¯ramatanga expert panel member Associate Professor Lisa Te Morenga.