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Local Anesthesia and Nuclear Physics

A dentist's chair with a cartoon of an atom overlaid.

The other day I needed to go to the dentist for a tooth procedure. As the procedure required going under the gums, he needed to use local anesthesia to make sure I didn’t feel anything in the surrounding area. Unfortunately, he didn’t use enough when working on the right side of my mouth so I felt a small poking motion. When he switched to the left side of my mouth, he decided to use double the amount of local anesthesia, which worked as expected.

Now, have you ever tried to eat when you can’t feel your mouth? It’s actually pretty difficult. So as lunch time rolled around, I began to wonder how much longer before I could regain feeling in my entire mouth. I could feel the right side of my mouth again but not my left side. After all, the dentist had used twice as much numbing agents so shouldn’t it take twice as long to regain feeling? Well, that’s what I expected would happen, but instead, I regained feeling within half the time I was expecting. What’s going on here?

To explain what is going on here, we need to borrow some ideas from nuclear physics and radioactivity. Typically when we think of radioactivity, it is in the context of iridescent green objects and superheroes who gain their powers after an accident. But what actually is radioactivity? Simply, it is when particles are emitted from the nucleus as it undergoes a nuclear transition, such as when the nucleus breaks up. The nucleus is the clump of positively charged particles called protons and neutral particles called neutrons at the center of the atom. In most objects we see, the number of neutrons and protons are in a good balance and the object isn’t radioactive. Sometimes though, the number of neutrons is much larger than the number of protons, which isn’t a good balance. Just like trying to stand on one foot for a long time before toppling over, the atom can stay in a good balance for some time before it breaks apart or decays. During radioactive decay, the atom often releases a particle with a large amount of energy. For example, when carbon-14, a carbon atom with two more neutrons than found in a typical carbon atom in a molecule of carbon dioxide, breaks up, a nitrogen atom is formed along with an electron and an antineutrino. Here, the electron carries away most of the energy while the anti-neutrino will also have some energy. However, neutrinos rarely interact with other particles and hence don’t have any damaging effects. In fact, as you are reading this sentence, over 100 billion neutrinos from the sun passed through each of your fingertips and you didn’t even notice.

You are probably thinking what does all of this have to do with getting anesthesia at the dentist and that’s a great question. It turns out that medicine in the body works similar to the decay of a radioactive atom. There is no set timescale before a radioactive atom decays. Instead, there is only some probability. It’s like rolling two dice and only decaying if you roll two “1”s. Maybe you will only need to roll once but maybe you will need to roll 30 times. However, if you have thousands or millions of radioactive atoms, the group of atoms will decay along a known timescale. The time for half of the atoms to decay is known as the half-life. For example, half of the carbon-14 in some object will break up in 5,730 years and after 11,460 years only a quarter of the original carbon-14 will remain, which is why we can use carbon dating to determine how long ago a woolly mammoth died.

While medicine in the body is more complicated, the idea is still the same. After some set amount of time, around half of the medicine in your body is absorbed or dissolved in the bloodstream or in your fatty issue. Therefore, when the dentist doubled the amount of numbing agent to work on the left side of my mouth, the amount of time needed before I regained feeling would only increase by the half-life of the medicine in the anesthesia. So while determining how long my mouth would stay numb wasn’t rocket science, it could be considered nuclear science.

Header image by Gerd Altman from Pixabay.

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