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More about the Absorbed Dose of Radiation

Non-ionizing solar radiation.

Overview

Radiation signs

Radiation can be ionizing and non-ionizing. It is the former that causes damage to human and animal tissue. When this article refers to “radiation,” ionizing radiation is meant. The absorbed dose of radiation is different from the radiation exposure because it measures the amount absorbed by a given body, not the total amount of radiation in the environment.

The two values may be similar for highly absorbent materials, but this is often not the case, as absorbency differs greatly for materials. For example, a sheet of lead will absorb gamma radiation more readily than would a sheet of aluminum of the same thickness.

Gamma Sapiens gamma radiation detector for Android smartphones

Units for Measuring the Absorbed Dose of Radiation

One of the most common units to measure the amount of radiation absorbed by an object is a gray. One gray represents the amount of radiation present when one joule of energy is absorbed by one kilogram of material. A gray represents a large amount of radiation, much greater than a person would typically absorb. For example, 10 to 20 gray is usually lethal for humans. Therefore fractions of gray, such as centigray (0.01 gray), milligray (0.001 grays), and so forth are used. Rad is an obsolete unit proportional to gray. One gray is 100 rad, which makes one rad equal to one centigray. Although it is obsolete, it can still be seen often in publications.

The amount of radiation a body absorbs is not always equivalent to the amount of damage this radiation will cause. Additional units, such as radiation dose equivalent units, are used to describe radiation as relevant to the damage it can cause.

Dental radiographs

Radiation Dose Equivalent Units

While radiation absorbed dose units are commonly used in scientific literature, the general public may not be familiar with them. The media more commonly uses radiation dose equivalent units. They are used to determine the effect that the radiation has on the body as a whole and tissue in particular. It allows assessing biological damage more easily than with conventional radiation absorbed dose units because it takes into consideration the amount of damage different types of radiation can cause.

The severity of damage that a given type of ionizing radiation can cause to tissue is calculated using the relative biological effectiveness ratio. The values differ when a different type of radiation is absorbed by the body. If different body organs and tissues are affected by the same type of radiation, for example, beta, gamma, or x-ray radiation, then the severity of the damage is the same. Other radiation affects different cells to a different degree. For example, alpha particles, when absorbed (often through ingestion, since they do not penetrate matter easily), are 20 times more dangerous to living organisms than beta or gamma radiation.

To calculate the equivalent dose of radiation one has to multiply the absorbed dose by the relative biological effectiveness for the particles that cause this radiation. From the above example, this coefficient is 1 for the beta, gamma, and x-rays, but 20 — for alpha particles. Banana equivalent dose units and sieverts are examples of dose equivalent units.

Sieverts

Sieverts measure the amount of energy emitted by the radiation per a given amount of tissue mass. This is one of the most commonly used units when discussing the harmful effects of radiation on people and animals. For example, a generally fatal dose for people is about 4 sieverts (Sv). A person may still be saved if treated quickly, but a dose of 8 Sv is lethal. Generally, people absorb much smaller doses of radiation, therefore often millisieverts and microsieverts are used. 1 millisievert is 0.001 Sv, and 1 microsievert is 0.000001 Sv.

Banana Equivalent Dose

A banana equivalent dose is equal to 0.1 microsieverts

Banana equivalent dose (BED) units are used to measure the amount of radiation that the body absorbs after eating one banana. A banana equivalent dose can also be expressed in sieverts, it is equal to 0.1 microsieverts. Bananas are used because they contain potassium-40, a radioactive isotope that naturally occurs in some foods. Some examples in BED include: a dental X-ray is similar to eating 500 bananas; a mammogram is equivalent to eating 4000 bananas; and a fatal dose of radiation is like eating 80 million bananas.

There is debate about using banana equivalent dose units because the effect that the radiation has on the body is not equivalent for different radioactive materials. The amount of potassium-40 is also regulated by the body, so when it is taken in through food, it is then expelled, to keep the level uniform.

Effective Dose

The units above are used for radiation that is uniformly absorbed by the tissue, usually in a localized area. They help determine how much radiation affects a particular organ. To calculate the effect on the entire body when only some part of the body is absorbing radiation, an effective radiation dose is used. This unit is needed because the increase in the risk of cancer is different for different organs, even if the amount of radiation absorbed is the same.

Effective dose calculations account for that by multiplying the absorbed radiation by the coefficient of the seriousness of the impact of radiation on each type of tissue or organ. When determining values of coefficient for different organs, researchers weighed not only the overall cancer risk but also the duration and quality of life of the patient, once cancer is contracted.

An effective dose is also measured in sieverts. It is important to understand when reading about radiation measured in sieverts, whether the source refers to the effective dose, or the radiation dose equivalent. It is likely that when sieverts are mentioned in mass media in the general context of talking about radioactivity-related accidents and disasters, the source is referring to the radiation dose equivalent. Often there is not enough information about which body tissues are affected or may be affected by the radioactive contamination, therefore it is not possible to talk about the effective dose.

Ionizing radiation sign

Effects of Radiation on the Body

Sometimes it is possible to estimate what effect radiation will have on the body while looking at radiation absorption, measured in gray. This unit is spelled “gray” both in singular and plural forms. Gray is used when measuring the radiation prescribed for localized treatment of cancer. The amount of radiation in gray allows one to predict the effects of this treatment on the treated region and the body as a whole. During radiation therapy, the cumulative absorption rates through the duration of the treatment are generally high in the area being treated. This radiation absorption may permanently destroy the glands that produce saliva, sweat, and other moisture when the dose exceeds 30 grays (Gy). The result is dry mouth and similar side effects. Doses of 45 Gy or more destroy hair follicles and cause irreversible hair loss.

It is important to note that while the total absorption of radiation will result in biological damage, the extent of this damage is highly dependent on the duration of time, over which this absorption occurs. For example, a dose of 1,000 rad or 10 Gy is fatal if absorbed within several hours, but it may not even cause acute radiation sickness (ARS) if spread out over a longer duration of time.

Aero L-29 Delfín — the jet trainer for the air forces of Warsaw Pact nations from the 1960s. Toronto (Canada) Wings and Wheels Festival 2009.

Radiation in Air Travel

Radiation levels are higher at higher altitudes because cosmic radiation causes greater exposure and absorption than terrestrial radiation. Compared to the 0.06 microsieverts per hour on the ground it increases about 100 times to 6 microsieverts per hour at cruising altitudes.

The total yearly exposure can be calculated as follows. According to the information on the Air Canada website, a commercial pilot employed by this airline spends about 80 hours per month or 960 hours per year in flight. This gives a total exposure of 5760 microsieverts or 5.76 millisieverts per year. This is a little less than a chest CT scan (the scan is 7 millisieverts). It is one-tenth of the maximum allowed yearly dose that radiation workers in the USA can be exposed to.

It is important to note that the information above is an estimate based on cruising altitudes, but the actual exposure may be different because it depends on the altitude. Individual exposure will also depend on the airline and the work safety regulations in the countries of origin. Additional radiation is caused by the normal background radiation that each crew member is exposed to during daily activities not related to work. This additional radiation is about 4 millisieverts per year for people living in North America.

Such exposure increases the risk of cancer. There are also risks to unborn children if one or both parents have been exposed to radiation before the conception. Finally, there are risks if an unborn child was irradiated while the mother worked as a crew member during pregnancy. The risks range from childhood cancer to mental and structural abnormalities.

Radiation in Medicine

Radiation is used in the food industry and medicine. Its properties of destroying the DNA are useful for humans, as long as they are applied to organisms such as bacteria, but not people.

In addition to localized cancer treatments discussed above, radiation is used to kill bacteria and sterilize various instruments because it damages and destroys animal tissue and DNA molecules. For example, in medicine, it is used to sterilize instruments and rooms. The instruments are usually placed in air-tight bags, to ensure that they remain sterilized until it is time to use them. Too much radiation can break down materials such as metals, therefore it is important to use adequate amounts of radiation.

Irradiated poultry. The international Radura logo.

Radiation in Food Manufacturing

Radiation’s ability to destroy cells and DNA of living organisms is also used to de-contaminate food and prevent it from going bad quickly. It either makes microorganisms unable to reproduce or kills pathogens and bacteria such as E. coli. Some countries have legislation against irradiation of certain or all foods, while other countries have legal requirements for all imported foods of a given type to be irradiated. In the USA, for example, it is required that a range of imported produce, especially tropical fruit, are irradiated before import to prevent the spread of fruit flies.

When radiation is absorbed by food, it also slows down some of the biochemical reactions in the enzymes. This prevents spoilage by slowing down the ripening process and growth of plants. Such interventions prepare food for intercontinental travel by giving it a longer shelf life.

Process

Radioactive Cobalt-60 isotope is used to treat food products to kill bacteria. Researchers in the area are working on determining radiation levels that provide a balance between killing microorganisms and preserving the original taste of the food. Currently, most foods are processed with radiation under 10 kilograys (10,000 grays), but this dose may range from 1 to 30 kilograys depending on the product.

Radiation used in this process can be that of gamma rays or x-rays, as well as radiation of electrons. The food is usually moved through the radiation facility on a conveyor belt and can be pre-packaged. This is similar to the process of sterilizing medical equipment. Different types of radiation have a different range of penetration, thus the type of radiation is selected based on the food type. For example, irradiating hamburger patties may be done with electron irradiation, while deeper penetration of x-ray radiation is needed to irradiate bird carcasses.

Controversy

The radioactive isotopes do not stay inside the food itself, so this is not a concern in food irradiation. Nonetheless, food irradiation is a controversial subject because the radioactive materials need to be produced, transported safely to the food plants, and handled carefully. This does not always occur, and a wide range of accidents, leaks, malfunctions, and other problems is reported at various irradiation facilities across the globe.

Another concern is that irradiation will result in a decrease in sanitation and the use of proper safety handling techniques in the food processing industry. Some think that irradiation is becoming a cover-up for inappropriate handling of food at the plants and that it also encourages unsafe food handling among consumers. Irradiation can decrease the nutritional content of foods because it destroys or deteriorates some vitamins and microflora that is needed for digestion and other functions. Some researchers that oppose food irradiation also believe that it increases carcinogens and toxic elements in food.

Terra radiometer

Many countries currently only allow irradiation of spices and herbs. However, the nuclear industry, which is involved in producing the radioactive isotopes used in food irradiation, is lobbying in many countries to allow irradiation of other food products such as meat, grains, fruits, and vegetables.

Countries that do allow irradiation generally require either an explicit irradiation label logo, the radura, on the packaging, or to include the information about irradiated foods in the list of ingredients. This may not apply to products contained inside processed foods, and restaurants may not be required to inform consumers on whether or not they serve food made from irradiated ingredients. This is a problem because it revokes the choice from the consumers on whether to eat irradiated products. Finally, food irradiation is costly and it increases the cost for many of the foods that are irradiated.

Measuring Radiation

People who are exposed to radiation at work are often required to wear special devices, dosimeters, to determine whether the cumulative dose of radiation they receive is safe. Astronauts, workers at nuclear power plants, response and decontamination teams that work with hazardous materials, as well as doctors working in the area of nuclear medicine are some of the people who are required to wear these dosimeters. The dosimeters can sometimes inform the user when a particular set dose has been exceeded, for example with an alarm. This total dose is often measured in sieverts. Despite the rules in place, some countries do not enforce them or did not do so in the past. For example, during the Chernobyl cleanup efforts early in the disaster, doses recorded for the workers were not based on the actual measurements. According to the eyewitness accounts, instead, the doses were fabricated based on an estimate of the radiation in the area where one was assigned work for the day.

References

This article was written by Kateryna Yuri

Unit Converter articles were edited and illustrated by Anatoly Zolotkov