sample What happens when we are exposed to radiation? 1.1 Natural radiation Cosmic radiation

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1 1.1 Natural radiation 3 1 What happens when we are exposed to radiation? 1.1 Natural radiation For as long as humans have walked the earth, we have continually been exposed to naturally-occurring radiation. Even today, an average person is exposed to 2.4 millisieverts (msv) of radiation each year. This dose consists of external exposure in the form of both cosmic radiation (0.39 msv) and terrestrial radiation (0.48 msv), and internal irradiation from food (0.29 msv) and inhalation mainly of radon gas (1.26 msv) 1). A person who lives past the age of 42 years is exposed to more than 100 msv of natural radiation. This 100 msv dose is a recurring theme in the book so I would ask the reader to commit it to memory as the average natural radiation dose of a middle-aged adult. It is also important to acknowledge that radiation exposure before conception can reportedly cause adverse hereditary effects in offspring, but to date these effects have never been seen in humans exposed to radiation Cosmic radiation If we were to take a dosimeter on board a jet aircraft, we would see that the amount of radiation encountered at high altitudes is hundred times greater than that on the ground. A round trip between Tokyo and New York exposes a passenger to 0.2 msv of radiation 2). Assuming a 24-hour flight time, this equates to msv/h or 8 microsieverts (μsv)/h. This figure appears very high compared to the 0.03 to 0.08 μsv/h of radiation measured on a typical day in Tokyo. The discrepancy can be attributed to the earth s atmosphere and magnetic field which shield us from most cosmic rays. Incidentally, astronauts

2 4 1 What happens when we are exposed to radiation? aboard the International Space Station receive around 1 msv/day, although no demonstrable effects of this exposure have been reported Terrestrial radiation Taking our dosimeter aboard a bullet train, we would see that radiation rises when traveling inside a tunnel but falls when crossing a bridge. This is because the earth contains various radioactive materials which completely surround us while inside a tunnel, thus giving a higher reading. Conversely, when crossing a bridge the river water would partially shield us from radiation emanating from the riverbed, resulting in a lower reading. The quantity of radioactive matter in the ground also varies from place to place. In Japan, it generally tends to be higher in the west and lower in the east. Moreover, some regions of the world emit doses of terrestrial radiation that are several to several dozen times greater than in Japan Radiation from hot springs Radium onsen or hot springs are popular among the Japanese for their perceived health benefits, with many people bathing in and even drinking the water. In order for a hot spring to be recognized as a radium onsen in Japan it must contain at least 111 becquerels/liter (Bq/L), and renowned hot springs often contain in excess of 1,000 Bq/L. After emitting alpha (α) particles, this radium decays and turns into radon gas. It then transforms into the element polonium and finally becomes stable lead, during which time it releases beta (β) particles and gamma (γ) rays. The release of radon from the ground means that considerable radiation is present not only in the spring water of radium onsen but also in the surrounding air, although no adverse health effects have been reported as a consequence of this radiation Effects on the human body Radiation from food We ingest various radioactive substances when breathing and eating. Radon is a typical example of inhaled radiation. A single kilogram (kg) of food usually contains several dozen to several hundred Bq of radioactive potassium (potassium-40) so the body of an average 60 kg adult male would contain around 4,000 Bq of this isotope. Potassium-40 has been present since the earth s

3 1.2 Radiation and radioactivity 5 formation and has a half-life of 1.28 billion years. Carbon, a key element of the human body, contains a minute amount of radioactive carbon (carbon-14) which has a half-life of 5,730 years. On average, the body of an adult male contains approximately 2,500 Bq of radioactive carbon, or radiocarbon. Taking into account the amount of potassium and carbon as well as other trace elements, the body of an adult male contains around 7,000 Bq of radioactive matter Impact of natural radiation on the human body To investigate the effects of natural radiation on the human body, we must compare people who have been exposed to this radiation with those who have not been exposed (i.e., controls). However, such a control does not exist on earth so it is impossible to accurately assess the effects of natural radiation. The level of terrestrial radiation may vary from several to several dozen times according to the geographical region but this variability has not given rise to any observable adverse health effects. 1.2 Radiation and radioactivity Becquerels & sieverts I will now explain some of the terms required to understand the abovementioned concept of natural radiation as well as the nuclear accidents described in subsequent chapters. Radiation is typically divided into two types; ionizing radiation and nonionizing radiation. Ionizing radiation (such α and β particles, γ and X-rays, and neutrons) is capable of producing ions in the matter it passes through, while non-ionizing radiation (such as radio- or microwaves and visible infrared light) does not have enough energy to ionize molecules or atoms in the material it interacts with. A material that emits radiation is called a radioactive material and its ability to emit radiation is referred to as its radioactivity. Likening this relationship to that of a light bulb and its luminescence, radioactivity is emitted from a radioactive material just as luminescence is emitted from a light bulb. Radioactivity tells us how rapidly radiation is emitted from a substance so the higher the material s radioactivity, the greater the amount of radiation released. Radioactivity is measured in units called becquerels (Bq), wherein 1 Bq is defined as the quantity of a radioactive material that will have one

4 6 1 What happens when we are exposed to radiation? transformation in one second. Another concept is radiation dose (also known as absorbed dose ) which is a measure of the energy that ionizing radiation imparts to a given mass of matter. Where luminescence is measured in lux and indicates the brightness of light perceived by the human eye, radiation dose is expressed in gray (Gy) wherein a dose of 1 Gy represents the absorption of 1 joule of energy by 1 kg of matter. However, different kinds of ionizing radiation possess very different traits. For instance, α particles cannot penetrate a single piece of paper and β particles are stopped by a thin sheet of aluminum but γ rays easily pass through the human body and can only be blocked by a thick piece of lead. The impact of radiation on the human body therefore differs significantly according to the type of radiation to which a person is exposed. The sievert (Sv) is a unit that takes these differences into account by measuring the amount of biological damage to living tissue as a result of radiation exposure (i.e., the effective dose ), and 1 Gy of β, γ or X-ray radiation is equal to 1 Sv whereas 1 Gy of neutron radiation equals 2.5 to 20 Sv. In previous scientific literature, assessment of radiation risk from the atomic bombings of Japan, the Chernobyl disaster and medical exposure has typically been based on the absorbed dose of each bodily organ. Radiation from medical exposure or nuclear reactor accidents mainly consists of β, γ or X-rays so 1 Gy is equivalent to 1 Sv. Radiation from the atomic bombings generally comprised γ rays and neutrons so risk assessment has been performed using the weighted Gy which takes the effects of neutrons into account, and 1 weighted Gy is roughly equal to 1 Sv of equivalent dose. For the sake of clarity, this book uses Sv as much as possible when referring to radiation risk assessment. In radiation protection applications, the Sv is used to measure both the abovementioned equivalent dose and the effective dose. The probability of cancer or genetic damage following radiation exposure varies according to the tissue and organ type so the effective dose was conceived as a way of determining the impact of radiation on the body regardless of the type of radiation or the site of exposure. The effective dose is found by multiplying the equivalent dose of each tissue or organ by the respective tissue weighting factor (an indicator of the relative radiation sensitivity of each type of tissue/organ) and summing the products. Effective dose has been criticized because it involves conversion from a physical measurement into a unit of biological damage, but it is generally

5 Table 1.1 Key units used to measure radioactive materials Units measuring radioactivity Becquerel (Bq) International System of Units (SI)-derived unit defined on pages 3 4. Curie (Ci) Non-SI unit. 1 Ci = 3.7 billion Bq. The radioactivity of 1 g of radium is roughly equal to 1 Ci. Units measuring total absorbed radiation energy (absorbed dose) Gray (Gy) SI unit. A dose of 1 Gy is the amount of radiation required to deposit 1 joule (J) of energy in 1 kg of matter. Rad (rad) Non-SI unit. 100 rad = 1 Gy Units measuring biological effects of absorbed radiation Sievert (Sv) SI unit. Defined on page 6. Roentgen equivalent in man (rem) Non-SI unit. 100 rem = 1 Sv Units measuring X-ray or γ-ray exposure Roentgen (R) Roentgen is the amount of radiation that causes g of air (1 cm 3 of dry air at standard atmospheric pressure and 0 ) to produce one electrostatic unit of positive or negative charge (esu). In most cases, 1 R is about the same as 1 rad. regarded as a useful way of expressing and calculating the effects of both internal and external radiation exposure on the body. This more or less concludes our explanation of Bq as a unit of radioactivity and Sv as a unit of the effect of radiation on the human body. An explanation of the other units used in relation to radioactive materials can be found in Table Radiation from nuclear disasters 1.3 Radiation from nuclear disasters 7 The radiation from nuclear disasters described in this book was primarily from man-made radioactive materials that did not exist prior to the creation of the atomic bomb. However, the effects of radiation on the human body expressed in Sv are the same regardless of whether the source is natural or man-made. The adverse health effects of natural radioactive materials have previously been reported, for instance, in uranium mining and fluorescent dye industry workers, and there have also been cases of radium-induced skin ulcers and even murder by poisoning with polonium. I will now briefly explain some of the terms used when describing the effects of radiation on the human body.