Radon gas, which is produced by the earth, is present in the air we breathe. The earth’s crust also contributes to our levels of exposure. In Canada, the estimated highest annual dose for terrestrial radiation is approximately 1.4 mSv measured in the Northwest Territories. However, the dose varies depending on location and geology, with doses reaching as high as 260 mSv in Northern Iran or 90 mSv in Nigeria. The average effective dose from the radiation emitted from the soil (and the construction materials that come from the ground) is approximately 0.5 mSv a year. For example, some regions receive more terrestrial radiation from soils that are enriched with uranium. There are also natural sources of radiation in the ground. Air travel also increases exposure to more cosmic radiation, for a further average dose of 0.01 mSv per Canadian per year. This compares to the top of Mount Lorne, Yukon, where at 2,000 m, a person would receive an annual dose of about 0.84 mSv. According to a study by Health Canada, the annual effective dose of radiation from cosmic rays in Vancouver, British Columbia, which is at sea level, is about 0.30 mSv. Regions at higher altitudes receive more cosmic radiation. The doses due to natural background radiation vary depending on location and habits. It comes from space, the ground, and even within our own bodies. In fact, life has evolved in a world containing significant levels of ionizing radiation. Radiation has always been present all around us. The unit used for effective dose is also the sievert. The result is the effective dose absorbed by the body. To obtain an indication of how exposure can affect overall health, the equivalent dose is multiplied by a tissue weighting factor (wT) related to the risk for a particular tissue or organ.

For example, bone marrow is much more radiosensitive than muscle or nerve tissue. In other words, the equivalent dose is expressed as a single unit that accounts for the degree of harm that different types of radiation would cause to the same tissue.ĭifferent tissues and organs have different radiation sensitivities (see Figure 13). This means that 1 Sv of alpha radiation will have the same biological effect as 1 Sv of beta radiation. The equivalent dose is expressed in a measure called the sievert (Sv). To obtain the equivalent dose, the absorbed dose is multiplied by a specified radiation weighting factor (wR) to reflect the relative biological effects of the different types of radiation. For example, 1 Gy of alpha radiation is more harmful to a given tissue than 1 Gy of beta radiation. The effect depends on the type of radiation (such as alpha, beta, gamma). However, equal absorbed doses will not necessarily produce equal biological effects. When radiation is absorbed in living matter, a biological effect may be observed. A dose of 1 gray is equivalent to 1 joule of energy deposited in a kilogram of a substance. The absorbed dose is measured in a unit called the gray (Gy). The energy absorbed in the human body from exposure to radiation is called an absorbed dose. Radiation dose quantities are described in three ways: absorbed, equivalent, and effective. The energy absorbed from exposure to radiation is called a dose. When ionizing radiation penetrates the human body or an object, it deposits energy.

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