Radiation and Health: Cosmic Radiation, Astronauts, and … Extinctions?

By Andrew Karam, Ph.D., CHP — Apr 12, 2021
Space is a harsh environment. We’re all familiar with the big risks from television – asteroids, solar flares, lack of air, extreme temperatures, and the occasional testy alien ships. What we don’t think about quite as much is the radiation. With trips to the Moon and Mars, in the planning stages, we are once again about to send people into that harsh universe. In addition to the engineering, logistics, and everything else that goes into such trips, NASA is looking at revising the limits for radiation exposure its astronauts can receive. 
Ionizing Radiation

Radiation was a concern during the Apollo space program, where we barely crossed the street in cosmic terms – the trajectory of the craft traveling to the Moon was partially based on minimizing the amount of time the astronauts spent traversing the van Allen radiation belts that surround the Earth. Since the end of the Apollo program, human beings have remained safely ensconced within the protection of Earth’s magnetic field – but that half-century is about to end. 

This is the first in a series discussing the subject of radiation exposure in space. This article will dwell on the biological effects of radiation exposure, including the sort of radiation to which our astronauts will be exposed. Then we will look at how radiation safety is practiced in space and finally consider how the magnetic field around the earth protects us every day. Let’s get started!

Radiation and its health effects

Radiation – or more specifically, ionizing radiation (that’s radiation with enough energy to strip electrons from atoms, creating charged particles, or ions, within a cell) – can harm us. Most people don’t really understand how radiation causes harm and how much radiation is needed before it starts to become dangerous. And it all begins with ionizations. 

Picture a cell sitting there and doing whatever it is that cells do, producing proteins, metabolizing nutrients, disposing of waste, communicating with other cells. And then a high-speed, very high energy particle or a gamma-ray, passes through the cell. It is absorbed by a carbon atom and transfers too much energy, tearing an electron from the atom. Now, we have a charged carbon ion and an electron. Often they will be attracted to one another and will recombine. Sometimes they won’t – sometimes, they’ll go on to react with other atoms in the cell, causing some of them to form chemically reactive molecules known as free radicals. These free radicals and those released from a host of other cellular sources can go on to attack and alter other molecules in the cell…including the DNA molecule.  

DNA Damage

Our cells are impressively adept and quick at repairing damage to DNA no matter how it occurs. Radiation-induced damage is no different from other forms of DNA damage. But sometimes the damage isn’t repaired correctly; sometimes it’s not repaired at all.  

If a person is blasted with a massive radiation dose in a short time, an acute radiation exposure, DNA damage accumulates more rapidly than the body can repair it. This results in radiation “damage” such as burns, radiation sickness, and the like. But this doesn’t often happen on Earth outside of major accidents. In space, we don’t expect to see that level of radiation exposure very often either – likely only when a significant solar flare or a coronal mass ejection flings massive amounts of hot ionized gas into space. Most of the time, on Earth and in space, radiation exposure is somewhat more sedate.

Even with the low levels of exposure we find in nature, we can still accumulate radiation damage to our genome; it’s just that the effects are neither as immediate nor as dreadful as those acute radiation injuries. Chronic radiation exposure can’t cause skin burns, and it certainly won’t make anybody into a Hulk – but it can cause cancer and possibly other health problems over years or decades. 

Even though our DNA repair mechanisms are very effective, they’re not perfect. Now and again, a mistake will slip through – these mistakes happen all the time, regardless of radiation exposure. Over the years, these mistakes accumulate, and sometimes they’ll affect a critical gene. For example, damage to a particular gene that helps the cell repair a very specific type of damage (called a mismatch) has been linked to a hereditary colon cancer [1] 

How much radiation is too much?

How much radiation does it take to produce skin burns, radiation sickness, death, Hulkification, congenital disabilities, or cancer? [2] The biological effects of radiation are measured in rem; a CT scan usually involves one or two rem, a chest x-ray 0.01 rems.  Below a dose of 25 rem (250 mSv), we see no clinically significant effects while, above this level, we start to see a depression in blood cell counts, nausea and vomiting, and other effects as the dose grows. 


Dose in rem

Equivalent Chest X-Rays

Chromosome damage



Possible congenital disabilities



Blood cell reduction



Radiation sickness



Lethal to 50% of individuals


40,000 – 80,000

Lethal to 100% of individuals




This figure shows the associated risk of cancer. It is important to note that, while there is little disagreement about the effects of acute radiation exposure at high doses, there is a great deal of controversy over the effects of chronic radiation exposure. The linear relationship that passes through a dose of zero shown in this figure is only one of several possibilities, including the possibility that below about 0.1 on that scale of Sv [3], there is no effect whatsoever, or even the possibility that low doses of radiation might help to protect against cancer. This controversy is far from resolved, although each faction believes their case to be conclusive.

Both the table and graph show that, while radiation can cause health problems – even death – in the short term and can cause cancer over the years and decades following exposure, it takes a far greater dose than most of us would think. The five rems that can cause congenital disabilities, for example, require five CT scans of the uterus. The amount of radiation necessary to be fatal is so high that there have been fewer fatalities from radiation sickness or “poisoning” than coal mining or crab fishing – even when we include reactor accident victims. 

All of the bad things we associate with radiation exposure can happen (well, except for the Hulkification) but require far more radiation exposure than most would realize. I won’t try to make a case that radiation is “safe” because “safe” is laden with our individual values and concerns. But I will state frankly that radiation is less risky than most feel it to be.


[1] A DNA mismatch allows the persistence of mismatch mutations.

[2] Cancer is fundamentally different from these other effects because it’s a random process.

[3] Sv or the sievert is another measure of radiation 1 Sv = 100 rem or 10,000 Chest X-rays or 100 CT scans.


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