Microwave Meltdown

The most common source of microwaves in day-to-day life is the microwave oven. Injury may result from:

• direct thermal injury due to dielectric heating
• indirect thermal injury due to super-heating of fluids
• mechanical trauma —
  e.g. fragments of of an exploding superheated foodstuff or dropping a microwave oven on your toe!

Another potential hazard is exposure to toxic gases. This can result from the use of inappropriate food containers. For example, polymer fume fever can result from overheating and melting of teflon aka polytetrafluoroethylene (PFTE).

In ‘simple physician-ly’ terms, this is how dielectric heating works:

The microwave energy is absorbed by polar molecules. These molecules rotate and collide as a result. Thus the electromagnetic energy is converted into kinetic energy and generates heat. Polar liquids like water heat up more quickly in the microwave than either polar solids (e.g. ice) or less polar liquids such as oils.

Despite their safety mechanisms, microwave ovens are not fool-proof. There are reports of people managing to sustain direct thermal injury due to dielectric heating. Occasionally people somehow manage to burn their hands in a microwave oven… or, horrifically, a small infant…

Clinically, it is important to realise that:

Injuries sustained by dielectric heating resemble electrical burns in that the degree of internal thermal injury, and resulting necrosis, may be under-estimated due to the lack of external signs of injury.

Super-heating occurs when a liquid is heated to beyond its boiling point.

This can occur in undisturbed liquids in a smooth container that are free of nucleation sites as the formation of bubbles in the boiling process is suppressed by surface tension. Superheating is not limited to ‘pure’ fluids — it also occur in ‘impure’ fluids, like a cup of coffee, infant milk formula, or even an egg. When a superheated liquid is disturbed, rapid boiling may occur, and the result may be a violent explosion… Sanjay Arora MD found this out at last year’s USC Essentials Trauma Review course (see free video here).

Microwave-heated food or drink items are also prone to causing internal thermal injuries following premature ingestion. This can occur because the food or drink may seem cool externally yet be internally superheated.

Most microwave manufacturers advise letting food stand for 2 minutes to allow adequate diffusion of heat — the more viscous the fluid, the longer the time required.

Standing time is particularly important with items that have fluid centres and a solid outer shell — examples include eggs, jelly-filled donuts and bottles of infant formula.