Read this article to learn about the precautionary measures with radioisotopes.
The greatest practical disadvantages of using radioisotopes are their toxicity; they produce ionizing radiations. When absorbed, radiation causes ionisation and free radicals from that interact with the cell’s macromolecules, causing mutation of DNA and hydrolysis of proteins.
The toxicity of radiation is dependent not simply on the amount present but on the amount of absorbed radiation and its biological effect.
There are, therefore, a series of additional units used to describe these parameters. Originally, radiation hazard was measured in terms of exposure, i.e., a quantity expressing the amount of ionization in air. The unit of exposure is the roentgen (R), which is the amount of radiation that produces 1.61 × 1015 ion-pairs (kg air) (or 2.58 × 10-4 coulombs (kg air)-1).
The amount of energy required to produce an ion-pair in air is 5.4 × 1-18 joules (J) and so the amount of energy absorbed by air with an exposure of 1 R is;
1.61 × 1015 × 5.4 x 10-18 = 0.00869 J (kg air)-1.
Although the roentgen has been used as a unit of radiation hazard, it is now considered in adequate tor two reasons: first, it is defined with reference to X-rays (or у-rays) only; and, secondly, the amount of ionization or energy absorption in different types of material, including living tissue, is likely to be different from that in air.
The concept of radiation absorbed dose (rad) was introduced to overcome these restrictions. The rad is defined as die dose of radiation that gives an energy absorption of 0.01 J (kg absorber)-1; this has now been changed to the gray, an SI unit, representing absorption of 1 J kg-1 (i.e., 100 rads).
The gray (Gy) is a useful unit, but it still does not adequately describe the hazard to living organisms. This is because different types of radiation are associated with differing degrees of biological hazard. It is, therefore, necessary to introduce a correction factor, known as the weighting factor (W), which is calculated by comparing the biological effects of any type of radiation with that of X-rays. The unit of absorbed dose, which takes into account the weighting factor is the sievert (Sv) and is known as the equivalent dose.
Thus:
equivalent dose (Sv) = Gy × W.
The majority of isotopes used in biological research emit β-radiation. This is considered to have a biological effect that is very similar to X-rays and has a weighting factor of 1. Therefore, for β-radiation, Gy = Sv. Alpha particles, with their stronger ionizing power, are much more toxic and have a weighting factor of 20.
Therefore, for α-radiation, 1 Gy = 20 Sv. It is likely that, as our knowledge of the biological effectiveness of different forms of radiation progresses, so the quality actor for different types of radiation may change in the future. Absorbed dose from known sources can lie calculated from knowledge of the rate of decay of the source, the energy of radiation, the penetrating power of the radiation and the distance between the source and the laboratory -worker.
As the radiation is emitted from a source in all directions, the level of irradiation is related to the area of sphere, 4πr2. Thus the absorbed dose is inversely related to the square of the distance from the source (r); or, if put another way, when the distance is doubled, the dose is quartered.
A useful formula is
dose1 x distance12 = dose2 x distance22
The rate at which dose is delivered is referred to as the dose rate, expressed in Sv h-1. It can be used to calculate your total dose. For example, a source may be delivering 10 µSv h-1. If you work with the source for
6 h, your total dose would be 60 µSv.
Currently the dose limit for workers exposed to radiation is 15 mSv in a year to the whole body, but thus is rarely ever approached by biologists because the levels of radiation used are so low. Limits are set for individual organs. The most important of these to know are for hands (500 mSv year-1) and for lens of the eye (150 mSv year-1).
Dose limits are constantly under review and, although dose limits are set, it is against internationally agreed guidelines to work such a limit, i.e., to assume that all is satisfactory if the limit is not exceeded. Instead, the ALARA principle is applied, to work always to a dose limit that is as low as reasonably achievable.
Work that may cause a worker to exceed three-tenths or one-tenth of the dose limit must be carried out in a controlled area or a supervised area, respectively. In practice, work in the biosciences rarely involves a worker receiving a measurable dose. Supervised areas are common but not always required (e.g., for 3H or 14C experiment).
Controlled areas are required in only certain circumstances, for example, for isotopes or radio iodination work. A major problem, however, in biosciences is the internal radiation hazard. This is caused by radiation entering the body, for example by inhalation, ingestion, absorption or puncture. This is a likely source of hazard where work involves manipulations of radioactive liquids and gases; most works in biology involves manipulations of radioactive liquids.
Control of contamination is assisted by:
i. Complying with local rules, written by an employer,
ii. Conscientious personal conduct in the laboratory,
iii. Regular monitoring,
iv. Carrying out work in some kind of contaminant.
Calculating the dose received following the ingestion of a radioisotope is complex. Detailed information is published by the International Commission on Radiological Protection and assessments, for example, for experiments on human volunteers, can be obtained from the National Radiological Protection Board.
However, one relatively simple concept is the annual limit on intake (ALI). The ingestion of one ALI results in a person receiving a dose limit to the whole body or to a particular organ. Some ALIs are shown in Table 13.7. Management of radiation protection is similar in most countries. In the USA, there is a Code of Federal Regulations.
In the UK there is the Radioactive Substances Act (1993) and the Ionizing Radiations Regulations (1999). Every institution requires certification (monitored by the Environmental Protection Agency in the USA or the Environmental Agency in the UK) and employs a Radiation Protection Advisor.
When handling radioisotopes the rule is to:
i. Maximize the distance between yourself and the source,
ii. Minimize the time of exposure and
iii. Maintain shielding at all times.
