Why Are Kids More Sensitive to Radiation?
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Why Are Kids More Sensitive to Radiation?

By Dr. Zoomie

Hi, Dr. Zoomie – got a question for you. I’ve heard that kids are more sensitive to radiation than adults are but I never understood why. Can you explain this? And do we keep getting less and less sensitive to radiation as we age? Inquiring minds want to know!

Wow – this is really a fraught subject because of how much we worry about our kids, and we worry just as much – or even more – about the ones that haven’t quite arrived yet. When I was a medical radiation safety officer, for example, I had a patient who had received dental x-rays tell me that her dentist – and her OB-GYN – recommended she terminate her pregnancy after finding out that she’d been pregnant at the time of her latest dental exam (which included a set of x-rays). And our Radiology Department asked me to write a fact sheet about giving CT scans to pediatric patients because they’d had parents refusing to let their kids have CTs – in some cases preferring exploratory surgery – because of their concerns about the effects of radiation on their kids. In both cases, let me hasten to say, these fears were based on correctly understanding that young children and developing fetuses are more sensitive to radiation than are adults, but on failing to understand that the radiation dose from these medical examinations was much lower than what can cause harm. So – with that out of the way – let’s see how this radiation sensitivity thing works, starting with properties that makes some cells more or less sensitive to radiation than others.

More than a century ago French scientists Jean Bergonié and Louis Tribondeau came up with a set of observations that remain in use today, although they serve as a good first approximation more than as a “law” of nature. They noted that three properties of cells seemed to have a significant impact on cells’ sensitivity to radiation:

  • Rate of cell division – rapidly dividing cells have less time to repair DNA damage before the cell goes on to reproduce, increasing the amount of damage passed on to future generations of cells,
  • The cells’ reproductive lifetime – damaged cells that have a long reproductive lifetime will produce more damaged progeny cells which, themselves, will produce still more damaged progeny
  • Degree of specialization – while all of our cells have the same DNA and the same genes, highly specialized cells (cells that only do a few things, such as neurons) use fewer genes than do cells that undertake many tasks

So let’s apply this to our question and see where we end up.

“My how you’ve grown” is a cliché – one that I heard thousands of times as I was growing up and one I’ve used myself. The reason that kids grow so quickly is that their cells divide more rapidly than do the cells of adults. In addition, children (and their cells) are at the beginning of their lives, giving them the longest possible reproductive lifetime. These characteristics also apply to fetal cells, with two interesting twists.

One twist is that fetal cells divide even more quickly than the cells of kids…which is probably not much of a surprise when we consider that a single fertilized ovum grows into a newborn baby with about two trillion cells in only 9 months. Our cells divide more rapidly before we’re born than during any other time in our lives, with the rate of cell division dropping as we age. But this is all sort of mundane – looking at how the cells change as a single cell grows and differentiates into dozens of tissues and organs with hundreds of types of cells…that’s where things get really interesting.

Think about it – the ovum is going to turn into every type of cell to form every type of tissue and every organ in the human body, making it the least specialized cell in the body (next least-specialized are pluripotent stem cells). Unlike any other cell, every gene contained in the ovum will one day be used. As the cells continue to differentiate each type of cell becomes increasingly specialized – there’s no reason (for example) for a neuron to produce stomach acid, nor for a muscle cell to store information. What’s interesting is that as gestation progresses and the individual cells become increasingly specialized, the sensitivity to radiation changes – the fetus is most sensitive early in the pregnancy when the tissues and organs are forming; after that point the fetus is about as sensitive to radiation as any young child, growing increasingly less sensitive as we age.

Our different tissues and organs have different sensitivities to radiation also, depending in large part on the same factors I mentioned earlier. Our skin constantly replenishes itself, as does the lining of our digestive tract (and other organs), not to mention the organs that produce our blood cells. These are relatively unspecialized cells that are continually dividing and that do so over our entire lives…so it’s not surprising that these organs are the first to be affected by high doses of radiation (e.g. nausea, vomiting, skin burns, reduced blood cell counts) or that these organs are more likely to develop cancer after exposure to lower radiation doses.

Having said all this, I’d like to mention that the radiation dose that’s required to cause health problems, to cause cancer, and to cause birth defects is a lot higher than what most people think – if you’re worried about your radiation exposure or exposure to someone else, please talk with a radiation safety professional (e.g. at www.hps.org) or with a healthcare professional who has a good understanding of the health effects of radiation exposure (e.g. a medical physicist or radiation oncologist).