As we discussed in a previous article, microwave ovens don’t destroy atoms – their radiation simply isn’t ionising radiation, while it’s exactly the ionising stuff that is bad for your cells, such as gamma rays and cosmic rays. The latter reside at the high-energy end of the electromagnetic spectrum. We also mentioned that microwave oven radiation isn’t the same as the radiation present in Chernobyl. Microwave ovens are not radioactive. But when do we call something radioactive then? What is radioactivity?
Unstable to the core
Matter is radioactive when the nuclei of its atoms are unstable enough to decay into different types of nuclei, emitting any of the following ionising radiation in the process:
- alpha rays, a stream of clumps of two protons and two neutrons;
- beta rays, a stream of electrons or positrons;
- gamma rays, the higher-energy form of electromagnetic radiation beyond X-rays;
- neutrino, a particle with the smallest mass.
Two well-known examples of radioactive material are plutonium and uranium, mostly associated with nuclear power plants and nuclear weapons. Perhaps less known to be radioactive are, for instance, radon-222, lead-210, polonium-210, and potassium-40. We will explain those numbers in the next section.
So, again, just to be clear, microwave ovens do not pour any of these particles over your food. Nothing gets ‘nuked’. There’s nothing nuclear going on. Your food, however, might very well be radioactive as it’s likely to contain potassium-40.
A cartoon of an atom
Atoms consist of three constituents: electrons, protons, and neutrons. Only the hydrogen atom lacks neutrons, the rest is a composite of all three elements. Figure 1 shows a cartoon of an atom. The vague, yellow band represents the electron cloud. The nucleus has been magnified so that its protons and neutrons become visible.
What makes one atom different from the other – say, calcium from potassium – is the number of electrons, protons, and neutrons. The latter two are called nucleons. The number of protons is most important here: it determines which element of the famous periodic table of elements (type of atoms) we’re dealing with. Potassium has 19 protons – this is crucial. If it were to acquire any other number of protons, it stops being potassium. Calcium has 20 protons, for example. The number 40 in ‘potassium-40’ means that its nucleus has 40 nucleons. In other words, it has 40 nucleons – 19 protons = 21 neutrons. Mind you, there are also potassium-39 (19 protons, 20 neutrons) and potassium-41 (19 protons, 22 neutrons).
All these potassium-versions – same number of protons, different number of neutrons – are called isotopes. So, while an element (type of atom) has a very specific number of protons, its number of neutrons may differ. Potassium-40 is an example of an isotope of potassium. This and the other aforementioned potassium-isotopes all occur naturally. However, humans are capable of synthesizing another 22 isotopes, no less.
The nucleus of potassium-40 is unstable. It decays, as it’s called, mostly into calcium-40, which is a stable atom. During the decay, the potassium atom gains a proton and becomes a calcium atom. In the process, a beta particle, i.e. an electron, and an antineutrino are emitted. This is why the decay is called radioactive: it actively radiates stuff. The electron flies off at great speeds and is potentially ionising, i.e. it is capable of knocking another electron from its atom, thereby potentially destroying molecules such as DNA.
About 0.01% of the potassium mass in our bodies, acquired through food, is of the potassium-40 variety. About 5000 of these atomic nuclei decay every second in an average human body. So, humans possess a radioactivity of 5000 Bq (becquerel, named in honour of Henri Becquerel, who shared the Nobel Prize with Pierre and Marie Curie for their work in radioactivity).
Bananas are also radioactive as they also naturally contain potassium-40. Its decay rate lies at 14 Bq, i.e. 14 decaying nuclei per second. It won’t set off a Geiger counter. A truck full of them probably would.
Ionising radiation and the human body
To assess the extent to which exposure to ionising radiation is likely to have medical consequences, we use units of ‘sievert’ (the symbol is Sv), a measure for the effective dose of ionising radiation in humans.
The International Commission on Radiological Protection, an independent, non-governmental organisation providing recommendations concerning ionising radiation, assessed that 1 Sv represents a 5% chance of developing cancer.
Fortunately, we evolved to absorb safe and small amounts of ionising radiation on a daily basis. The talented and clever Randall Munroe, famous for his xkcd.com cartoons, made a wonderful chart ‘with help from Ellen, a Senior reactor operator at the Reed Research Reactor’. We have blatantly copied their brilliant concept in Figure 2 to concisely give you a feeling for the various amounts of ionising radiation (click to enlarge). However, by all means, do also have a look at their original chart. Note that all these diagrams mainly indicate the orders of magnitude – not exact values as sieverts may vary a little for various human bodies, and sources and locations mentioned. This should, nonetheless, give you an idea how much radiation professionals in the radiation industry are allowed to be exposed to on an annual basis.
No, this isn’t going to be pretty. If anything, it’s pretty bad, actually.
Earlier, we mentioned radioactive elements radon-222, polonium-210, and lead-210. They naturally occur in the soil and air. They are also present in and on tobacco leaves and remain there even after processing. Once inhaled, sticky tar assures that these radioactive elements remain in the small air passageways and lungs indefinitely. Moreover, radon-222 happens to decay into said polonium and lead isotopes. Hence, the latter two build up even more. This is also true for secondhand smoke. Together with toxic substances such as tar, arsenic, nicotine, and cyanide, the ionising radiation emitting decaying nuclei of radon-222, polonium-210, and lead-210 increase chances of developing lung cancer immensely.
According to Little and colleagues (1965, 1967), polonium-210 accumulates in certain ‘hotspots’ in the lungs. Karagueuzian and colleagues (2012) reported an estimated lung dose of 165 mSv per year. To get a feeling of how much ionising radiation a smoker’s lungs (especially the hotspots) receive annually compared to how much ionising radiation a whole body of a professional radiation worker is allowed to be exposed to, see Figure 3. Mind you, this is just about radioactive decay in the lungs. We haven’t even discussed the other toxic compounds. And so, while someone may worry about Wi-Fi signals, mobile phones, and microwave ovens, which, by physics, they oughtn’t, keep in mind, in this Universe, that same physics tells us their smoking habit is really bad. Like, really.
Little, J. B., Radford, E. P., Mccombs, H. L. and Hunt, V. R. (1965) ‘Distribution of polonium-210 in pulmonary tissues of cigarette smokers’, The New England journal of medicine, vol. 273, no. 25, p. 1343 [Online]. DOI: 10.1056/NEJM196512162732501 (Accessed 9 August 2019).
Little, J. B., Radford, E. P. and Holtzman, R. B. (1967) ‘Polonium-210 in Bronchial Epithelium of Cigarette Smokers’, Science, vol. 155, no. 3762, pp. 606–607 [Online]. DOI: 10.1126/science.155.3762.606 (Accessed 9 August 2019).
Karagueuzian, H. S., White, C., Sayre, J. and Norman, A. (2012) ‘Cigarette Smoke Radioactivity and Lung Cancer Risk’, Nicotine & Tobacco Research, vol. 14, no. 1, pp. 79–90 [Online]. DOI: 10.1093/ntr/ntr145 (Accessed 9 August 2019).
Featured photo by Julia Sakelli.
@kjrunia is reading mathematics and theoretical physics (final year) in England, at The Open University, Walton Hall, Milton Keynes. He also works on coding for the Mars Rover of the university’s Planetary Robotics Team.