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Warming food in a microwave oven is not dangerous for your health. Physics says so. It doesn’t change healthy food into unhealthy, or even cancer-causing food. It doesn’t cause cancer if you stand too close. But it might knock out your WiFi. Physicists understand this, and are perfectly comfortable warming up food in a microwave oven. But why?

I’ll use “microwave oven” when talking about the appliance, and “microwave” when talking about the electromagnetic radiation (which isn’t a scary term, as we’ll discuss).

As I understand, the primary concerns with using a microwave oven are an increased risk of getting cancer, and destroying the nutritional content of the food being microwaved. We’ll tackle both of those separately.

But let’s start by understanding the term “electromagnetic radiation”. 

Charged things are surrounded by electric fields. Moving charged things create magnetic fields. Accelerating charged things radiate energy by creating both electric and magnetic fields. This radiating energy is in the form of electromagnetic radiation.

The term “radiation” to a physicist is a broad term implying waves or particles are carrying energy away from a source. Sometimes it’s very low energy. Sometimes it’s dangerously high.

These last two sentences are essentially the divide between physicists’ and non-physicists’ understanding of microwave ovens. Non-physicists generally consider all radiation to be dangerously high in energy, or rather focus on the negative effects of dangerously high-energy radiation and improperly ascribe those effects to low-energy radiation as well.

Microwaves are a type of electromagnetic radiation produced by accelerating charges, as discussed above. Microwaves are an example of electromagnetic radiation in the very-low-energy range.

If microwaves are one type, what are other types of electromagnetic radiation?

Electromagnetic radiation is the technical term for “light”. Typically, “light” refers to the type of electromagnetic radiation that humans can see with their eyes. But that’s only a small selection of the possible types of electromagnetic radiation.

From low to high energy, we have: radio waves, microwaves, infrared, visible, ultraviolet, X-rays, and gamma rays. 

Note that there is a continuum of energy across electromagnetic radiation spectrum, but we break the continuum into those categories above. This is similar to how we discuss someone’s age with categories like baby, toddler, child, teenager, young adult, adult, senior. Some terms have sharp divides while others sort of fade into the next without clear delineation.

What do we mean by “energy of the radiation”?

To answer that question, we need to get into some simple quantum mechanics. The amount of energy in the low-to-high list above is per particle of light. Each light particle for radio waves is lower energy than the light particle of microwaves, which is lower than infrared, and so on.

Microwave particles carry less energy than visible light particles as you can see in the electromagnetic spectrum below. Notice the energy increases from left to right. The numbers are the wavelengths: the shorter the wavelength the higher the energy.

If that’s true, why don’t we use visible light to warm things up? 

We do! We use heat lamps and solar ovens for food, and generally put things in sunlight to warm.  

But visible light gets absorbed right at the surface and warms slowly from that outer area. Microwaves, on the other hand, penetrate a few centimeters into food and get absorbed throughout. This causes some warming at the surface and some warming in the middle. This is why we use microwaves to cook our food. It’s faster because the heating happens throughout the object, rather than just at the surface and waiting for the warming to reach the middle over time.

(Cold spots in microwaved food are due to a different phenomenon not important to this discussion.)

If microwaves can warm the insides of food better than visible light, do radio waves do the same? 

Yes, but we’d need a bigger “radio wave oven” box to use the radio waves effectively, and we would have to be careful not to interfere with other uses of radio waves, like actual radio communication.

The size of a microwave oven is small enough for home kitchen use, and we’ve carved out a bit of the electromagnetic spectrum for microwave oven use. Sometimes it gets too close to our WiFi part of the electromagnetic spectrum, which is why your internet might go out while using the microwave — your computer starts trying to pick up internet from your microwave oven rather than your router.

Okay, how about cancer?

Loosely, cancer is caused by DNA communicating a wrong message about cell growth. This communication is done by molecules in the DNA which are made of atoms of carbon, oxygen, nitrogen, or hydrogen. Some radiation is of high enough energy to cause an atom of carbon, for example, to act like nitrogen instead. This affects the DNA communication and can lead to cancer by the DNA telling cells to “Grow, grow, grow!” instead of “Chill, we’re good.”

This atom-behavior-changing process is called “ionization” and radiation at high enough energy is called “ionizing radiation”. This is the dangerous stuff. 

It takes a certain amount of energy to ionize an atom. In the electromagnetic radiation categories, ultraviolet, X-rays, and gamma rays are of high enough energy to ionize atoms. Visible, infrared, microwaves, and radio waves are too low in energy to ionize atoms.

Ultraviolet light is a common cause of skin cancer. It’s best to avoid sunlight in large doses since the sun outputs large amounts of ultraviolet light. 

X-rays are used for medical imaging since we as a society have decided that the benefit of knowing when a cavity or broken bone has developed outweighs the risk of cancer if exposed once a year or so. Gamma rays should be avoided at all costs since they are the highest energy. They are found near some nuclear reactor cores and rain down on our atmosphere causing cosmic rays (not part of the electromagnetic spectrum), which can also cause cancer.

Other types of ionizing radiation that are not part of the electromagnetic spectrum are alpha, beta, and neutron radiation. These, along with gamma, are due to decay of radioactive nuclei. 

Of all these types of radiation, only one can cause something to become radioactive after exposure, and that’s neutrons. Unless you’re near a nuclear reactor or atomic bomb explosion, you do not need to worry about neutrons contaminating anything you have contact with.

The point of the radiation discussion above is that the particles of light that make up microwaves are very, very weak. They don’t have enough energy to do damage to DNA, and so don’t cause cancer — they aren’t ionizing. Microwaves are weaker than visible light, and we comfortably sit under household light bulbs hours each day without concern for cancer due to their light. Standing near a microwave oven will not cause damage to your DNA to cause cancer.

Okay, cancer isn’t a concern, but how about nutritional content being changed by the microwave oven?

Yes, that certainly happens, but no more than warming food on the stove, grill, or in a conventional oven.

Microwave ovens heat food. Heating food — no matter the method — changes protein structures and kills bacteria. THIS IS GOOD! We want to kill harmful bacteria. That’s the whole reason we cook our meat and boil water to disinfect it. 

If you boil, steam, fry, sear, grill, or microwave, you’re heating your food to either kill bacteria or because you prefer the texture and consistency of it being warm. Nothing about microwaves changes this. The only difference is that microwaves can go past the surface into the food and get absorbed in the middle.

Grilling, and all the other examples above, warms food from the outside only, and takes more time than a microwave oven for the energy to reach into the middle of food being grilled. If you prefer the charred outside of grilled food, by all means, skip the microwave oven. But you should know that the National Cancer Institute recommends avoiding charred meats, which contain cancer-causing compounds. They recommend using a microwave prior to grilling to reduce these risks.

If nutrients are broken down by using a microwave oven, they will be broken down by using any other warming method as well.

What should be avoided in the microwave?

Warming food in some types of plastic containers can leach harmful chemicals into your food. This is true for any warming, but is most relevant to microwave ovens because other methods would completely melt the container before the food has a chance to warm since the container is on the outside. Avoid plastic containers in microwaves.

In summary: 
 * Microwave ovens work by warming food from both the inside and surface, rather than just the surface. 
 * Microwaves are a type of light made up of particles with too low of energy to do damage to DNA and lead to cancer. 
 * If any type of warming changes the nutritional content of your food, that will happen in microwave ovens as well. 
 * Don’t warm food in plastic containers since warming can leach harmful chemicals into your food.
 * Wear sunscreen (not related to microwave ovens).

To explore similar physics quandaries and puzzles, check out The Hyperfine Physics Podcast, co-hosted by yours truly!