Microwave accidents costing close to $500k a year

ACC is spending close to $500,000 a year supporting people with injuries from microwave ovens.

How do I know this? It started with a painful mistake.

I copped a burn that thankfully didn't require ACC assistance, but did spark questions about what is microwave safe.

It happened at work, in the kitchen, using a mug I picked up - our workplace made the wise and noble decision to replace paper cups with an assortment of crockery that expands as people add to it with their own.

But the fetching zebra-striped mug I selected, left me with a nasty red stripe on my finger when I picked it up from the microwave.

Seventy seconds on full power, half-full of milk and the handle felt red hot. yes the handle.

Handles are supposed to stay cool. I went to absurd lengths to prove it - calling in the help of McDiarmid Institute physicist Geoff Willmott at Auckland University to check my crockery against some control mugs.

Geoff and his trusty FLIR infrared imaging camera scanned the offending, unbranded zebra mug. After two minutes in a microwave, hall-full of water, the handle was around 95 degrees Celsius - hotter than the water inside or most of the rest of the mug.

After a further two minutes, the handle hit 117 degrees Celsius - hotter than the boiling water inside and sure to cause injury.

Willmott says some materials have properties that lead to runaway heating in a microwave oven, like a mini greenhouse effect.

The hotter they get, the hotter they can get as they absorb more and more microwaves. That's one of two possible causes of my mad mug fail.

"Firstly it was poor materials and poor design of the mug in the first place so that the handle heated up and it was directly heated. The second is that glazing has failed and the water's got into the porous interior and heated up the handle that way," said Willmott.

Water in the handle? Why would that matter? It helps to understand how microwave ovens work.

"Microwaves are a form of electromagnetic radiations, so we've got a whole lot of light waves bouncing around inside the microwave and they're at the right wavelength that they like to heat up water molecules in particular so that's what we're doing with a microwave oven; heating up all the water molecules in your food."

To do some basic scientific comparison, Willmott checked temperatures after nuking a trusty McDiarmid Institute-branded coffee mug from his office kitchen.

The soft purple glow on the FLIR screen showed a comfortable 30-odd degrees Celsius even after two minutes in the same microwave at full power.

"Most glass or ceramics are pretty much microwave safe on the face of it, but it seems like we've found a little gap in that armour here," Willmott said, preparing to bin the offending zebra-print mug-mare.

Neither mugs carried the familiar three wavy lines that are on some products.

"Manufacturers of some plastics will put 'microwave safe' on some plastics to show that it won't leach into your food," said Willmott, who suggested not microwaving plastics without that label and urges caution regardless.

Burns are a leading cause of microwave-related injury along with sprains and strains from moving ovens or dropping them and most happen at home.

"When you're at home you drop your guard a fair bit more than you do when you;re at work. and some things like safety get forgotten about." said ACC injury prevention manager James Whitaker.

"if you're at home and you want to avoid injury just slow down a little bit, think of the risks, make smart choices and you'll be sweet."

And if you do get burned - Hato Hone St John advice is to cool it under running water for 20 minutes and seek medical help if required.