Types of Radiation and their Adverse Biological Impact on Humans

FROM THE LECTURE SERIES: Understanding the Misconceptions of Science

The non-ionizing types of radiation are microwaves, radio waves, and cell phone radiation. These are very low energy and can only hurt if the intensity is turned up high, and things like cell phones are completely safe. Understand the actual intensity of all radiations to avoid the adverse effects of the high energy ones.

Vector diagram showing different types of UV radiation.
UVA, UVB, and UVC ultraviolet radiation each has a different intensity and may harm a person’s skin causing burns, cancer, or wrinkles. (Image: Siberian Art/Shutterstock)

Effects of Different Types of Ultraviolet Radiation

Ultraviolet radiation, often called UV radiation, is high enough energy to do damage to atomic bonds. There are UVA and UVB as types of UV. There is also UVC, but that radiation does not make it through the Earth’s ozone layer. UVA, UVB, and UVC are different colors of light that are not visible to humans.

UVA penetrates more deeply into the skin and causes tans, while UVB is responsible for burns. But both of them cause skin cancer, although this is truer for UVB. They also cause wrinkles, and the best thing to do is to use sunscreen that comes labeled with an SPF factor. The number associated with the SPF factor tells how effective it is in stopping UV light. Wearing SPF 15 means staying in the sunlight 15 times as long to get the same exposure as wearing no sunscreen. An SPF of 50 means staying in the sun 50 times as long. How much is needed depends on how fair the skin is, where the person is, how long they are in the sun.

Ever-Changing Radioactive Nuclei

While the different types of radiation are important, so is to remember that the amount of radiation changes over time. For example, suppose that there are 100 radioactive nuclei. The nuclei are not really radioactive at the beginning but just sitting there, doing nothing. The actual act of being radioactive is the moment when they shoot off a particle and then the nucleus changes its identity into a different nucleus.

Diagram explaining radioactive decay.
Nuclei are not active at the start but become so once they shoot a particle to change their identity into a different nucleus. (Image: Inductiveload/Public domain)

Each nucleus has a certain probability to decay in a particular amount of time. Some do and some do not. It is an entirely random process. Waiting for long enough, there will be a time that half of them will be decayed, leaving 50 non-decayed nuclei. The amount of time it takes for half of the nuclei to decay is called the half-life and the amount of time it takes for that to happen depends on the substance. Some elements decay over fractions of a second, while others can take minutes, days, weeks, months, years, or even longer.

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Mathematics of Decreasing Radioactivity

After one half-life, there are still 50 nuclei that have not decayed yet. Waiting for another half-life, half of the remainder have decayed and the other 25 have not. Waiting for another half-life, 12 1/2 or 13 remain. The bottom line is that the amount left drops by half for each half-life.

Looking at this, in the first half-life, 50 nuclei decayed, while in the second, only 25 decayed. Which means there was more radioactivity in the first half-life. So, the best response to encountering radioactivity is to shield from it long enough for it to decay away. After 5 half-lives, the radioactivity is about 2 percent, which is what it was to begin with.

Iodine 131 and Cesium 137

In a radioactive situation, there are some worrisome substances. One is iodine 131 and the other is cesium 137. Iodine 131 has a half-life of about eight days. So, waiting for almost a month, the danger will be completely past. In contrast, cesium 137 has a half-life of 30 years which means waiting for 180 years for it to decay away.

Act of Decaying

If there is an identical amount of radioactive iodine and cesium, that means both of them will have an identical number of nuclei to decay. Taking the example of 100 nuclei, 50 iodine nuclei would decay in only eight days, while it would take 30 years for the cesium to decay. Since radioactivity is just the act of decaying, this means that iodine would be more dangerous, since the decays happen in a shorter time. With equal amounts of radioactive material, it is the one with the short half-life that has more decays in a short amount of time that will be more dangerous. If there are very different amounts of radioactive material, that changes things. But half-life is an important parameter. A short half-life usually means more dangerous, but easier to wait out.

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Energies of Different Radiation

The energy of different types of radiation all have their biological consequences. If someone restricts themselves to a certain amount of radiation, not all elements will emit the same energy. For instance, in gamma rays, thallium 208 emits a gamma ray with 2.6 million electron volts of energy. In contrast, uranium 235 emits a gamma ray with 0.2 million electron volts. So, gamma radiation from thallium is 13 times more energetic than from uranium.

Rough illustration explaining different wavelengths of electromagnetic radiation of Earth's atmospheric absorption.
Radiation energies create biological threats. With restrictions to a certain amount of radiation, not all elements will emit the same energy. (Image: NASA (original); SVG by Mysid./Public domain)

To calculate the radiation dose received in a certain time, it is important to know how big the chunk of radioactive material was because a bigger chunk has more atoms to decay. It is also important to know what the half-life of the element is, because if it was just a few minutes, there are more decays per second than if the half-life were 5,000 years. Finally, it is important to know what the energy was, where it was deposited, and what kind it was, with alpha particles stopping on the surface and neutrons penetrating deeply.

Units of Gray and Rads

When knowing about energy deposited, it is important to know the units. For absorbed energy, there are two units commonly used. Gray, which is equal to one joule of absorbed energy per kilogram, is the proper metric unit. There is an older unit, called the rad, which is short for Radiation Absorbed Dose. The rad is equal to 0.01 grays or, equivalently, 1 gray is 100 rads. Rads are commonly used, but strongly discouraged.

This is a transcript from the video series Understanding the Misconceptions of Science. Watch it now, on The Great Courses Plus.

Differing Biological Damage of Radiation

Not all types of radiation are the same when it comes to biological damage. Some do a great deal of damage and have a dangerous effect on people’s health. Beta and gamma radiation are less dangerous than alpha radiation. Alpha particles are stopped by the skin, but if someone breathes a radioactive alpha-emitting source, those particles hit the inner surface of their lung and drop all of their energy in one spot. The gammas and betas penetrate more deeply, to spread their energy loss over more cells, thus no one cell gets hit as hard as it does by an alpha particle. Neutrons do intermediate damage between the relatively-benign gammas and betas and the more damaging alphas.

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Damaging Ability of Radiation

Neutrons hit nuclei, while others lose their energy in the region occupied by an atom’s electrons. Each type of radiation has a different biological impact which means some different units taking into account each type of radiation’s relative damaging ability. For example, if the relative damage is called Q, for quality factor, the rad or gray number can be multiplied by Q to give the effective biological impact. But different types of radiation do different amounts of damage. For example, put a plastic sheet over someone’s hand and they get a little sweaty. On the other hand, put that same plastic over their face and they will be in real trouble. It shows that sometimes actions that sound the same in the abstract can have very different impacts on biology, and radiation is one of them.

Units of Rem and Sieverts

The radiation dose unit that corresponds to rads is the rem, an acronym for Roentgen Equivalent Man. While the unit corresponding to grays is the sievert where 100 rems equal one sievert. A radiation dose of one sievert makes a person sick and can be deadly. In contrast, the amount of radiation encountered over the course of a year is a lot smaller. Accordingly, the handiest unit of biologically relevant radiation dose is the millisievert or a 1000th of a sievert.

Common Questions About Types of Radiation and their Adverse Biological Impact on Humans

Q: What are the different types of ultraviolet rays?

Ultraviolet radiation, often called UV radiation, is high enough energy to do damage to atomic bonds. Different types of ultraviolet rays are UVA, UVB, and UVC. UVA, UVB, and UVC are different colors of light that are not visible to humans.

Q: Which is more harmful UVA or UVB?

UVA penetrates more deeply into skin and causes tans, while UVB is responsible for burns. But both of them cause skin cancer, although this is truer for UVB.

Q: Why is cesium 137 so dangerous?

Cesium 137 is dangerous because it has a half-life of 30 years which means waiting 180 years for it to decay away.

Q: What is the half-life of iodine 131?

Iodine 131 has a half-life of about eight days. So, by waiting for almost a month, the danger will be completely past.

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