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Electric current while passing through human body influences on it in thermal, electrolytic and biological manner. Thermal action is characterised by heating of living tissue, up to their burns; electrolytic - by decomposition of organic liquids, in that number blood, changing their chemical content; biological - damaging bio-electrical processes and, so, the functions of nervous system and tissue.
Varieties of outcomes of electric current influence on living organism are divided conditionally into two basic types: local electric traumas, at which the local strike may arise - electric burn, electric signs (marks), skin metallization, and general striking, when whole man organism may be damaged because of violation of normal activity of vitally important organs and systems, - it is an electric blow.
The Electric burn is the most widespread electric trauma. Current burn is contra-distinguished in networks with voltage not above 2 kWt, which caused by passing through the body tissue current with strength not more than 1 A, and arc burn, which arises from electric arc temperature, that may reach more than 3500 °С. Arc arises at accidental short current in plants with voltage not more than 6 kV, and also in networks higher than 10 kV at man approach to current conducting parts, which are found them under voltage.
The Electric signs are the blots of grey or daffodil colour on skin surface in place of contact with current-conducting parts. They, as a rule, are painless and in process of time must ascend.
Skin metallization is worming of metal steams into skin depth, when body part is found itself in immediate contact from place of electric arc formation. Struck part of the skin has a rough surface, colour of which is determined by colour of metal compounds that wormed into skin. A sick feeling of burn and presence of foreign body disappear on dying off degree of damaged skin. A big hazard is eye metallization.
Electric blow is excitation of living organism tissue by electric current that is accompanied by spontaneous convulsive muscles shortening, which frequently may cause a mortal outcome.
Electric current operates on organism reflexly through nervous system. At the beginning an excitation phase takes place: a reaction on pain appears, arterial pressure rises and other. Then a braking phase treads: nervous system becomes weak, arterial pressure falls down, breathing weakens, a pulse falls and becomes more frequent, a depression state arises. By Result of excitation and further braking of work of nervous system can be bringing of cardiac activity and breathing - an electric shock.
Cardiac activity may cease at straight action of electric current, that passes over domain of the heart (primary fibrillation), and by reason of reflex arteries spasm (second fibrillation). The heart fibrillation causes violation of blood circulation and, if not to take measures, which proceed cardiac activity, then death may happen.
Suppression of breathing, its paralysis is outcome of electric current influence on muscles of thorax that provide a breathing process. If such influence is long, then asphyxia may happens - asphyxiation from lack of oxygen and carbonic acid surplus in human body.
Electric shock is a difficult nervously reflex reaction on irritation by electric current. With shock may arise deep discords of breathing, circulation of the blood, nervous and other systems of organism. Shock may continue from several minutes till ten days and if not to take measures on guiding out of such state, then biological death may happen.
Hazard degree from influence of electric current depends on row of factors: strength of current, that passes through man; type and frequency of electric current; human body electric resistance; current paths in the body; duration of current influence; human individual peculiarities; conditions of external environment; factor of attention.
Strength of current is a basic factor determining a human striking outcome. Voltage in electric chain, in which man found himself, and also resistance of his body, influence on striking outcome by electric current only in that degree, in which they determine the current strength, that passes through human body.
Current with different strength acts differently on man. Following reference values of current are picked out: perceptible current (passing through organism current bringing perceptible irritation) is equal from 0.6 to 1.5 mA for variable current with frequency 50 Hz and 5 - 7 mA for constant current; a limit for unletting current (passing through human body current bringing to insuperable convulsive muscles shortening of the hand, in which conductor is clutched) is equal from 10 to 15 mA for variable current with frequency 50 Hz and 50 - 80 mA for constant current;
A limit for fibrillating current (current causing heart fibrillation at passing through human organism) is equal to 100 mA for variable current with frequency 50 Hz and 300 mA for constant current.
Type and frequency of electric current also may influence on outcome of man striking. Constant current is less dangerous and its limits 3-4 times higher than for variable current with frequency 50 Hz. However this may happens only at voltage 400 V. In diapason 400 - 600 V hazard of constant current and variable current with frequency 50 Hz are practically identical, and with further voltage increasing a relative danger of constant current increases too. Physiological processes on living cells cause this.
With increasing frequency of electric current from 0 to 50 Hz thresholds of irritation reaction by current fall down. With further frequency growth thresholds of current begin rise. Noticeable lowering of danger of human striking is observed at frequencies higher than 1000 Hz. Experiments over animals have proved that impressive voltage with frequency 50 Hz causes death in 100 % of the cases and at frequency 200 Hz animals perish. Fibrillation current under this effect increases twice, and at frequency 400 Hz - more than three times. At frequencies near to 10 kHz danger of electric blow fully disappears and only burn danger omits at touch to current-conducting parts.
Electric resistance of human body is different not only for different peoples, but for different body tissue also, that conditioned by physical, biochemical and biophysical phenomena. That's why it is non-linear and unstable value. Volumetric resistivity for current with frequency 50 Hz composes [Ohm×m]:
| 1. horny layer of dry skin |
| 2. bone |
| 3. fatty tissue |
| 4. muscle tissue |
| 5. blood |
So-and-so, skin, especially its horny top layer is, which consists of died horny cells, has large resistance. In figure 1, the human body resistance to constant current is higher than to variable one with any frequency. Moistened skin has in 1.5...2 times lesser resistance, than dry skin because moisture dissolves on skin surface the salts and acids, that exude from organism with sweat, and that adds to its conductivity.
Figure 1. Electric resistance of human body
where: a) -is schematic inclusion of human body into electric chain; b) and c) – are equivalent schemes of electric human body: Ze is a full electric resistance of epidermis; Rвн is a resistance of internal tissue; Rе- is an active resistance of epidermis; Се - electric capacitance of epidermis of healthy man; 1- electrodes; 2 - horny layer of the skin; 3 - an external layer of the skin - is a epidermis; 4 - internal skin layers and hypodermic tissue; 5 - an internal skin layer is a dermis; 6 – underskin tissue; 7 - skin layer.
Resistance of internal organs in general does not depend on enclosed voltage and composes 300 - 600 Ohm.
General human body resistance depends on value of enclosed voltage. With its augmentation skin resistance decreases because of clamp of its top layer, that may sometimes happen even at voltage near to 50 V. Further voltage increase still decreases general resistance, and at 300 V it comes around resistance of internal organs.
Long passing of current contributes to lowering of total human body resistance as a result of raised blood circulation in under-skin tissue and, so, to rise of sweat. At voltage 20...30 V during 1 - 2 minutes resistance can fall down on a par on 25%.
Because human body resistance to electric current is non-linear and unstable and it is difficult to conduct the computations with it, with sufficient accuracy for practice, man body resistance conditionally is taken to be active, linear and equal to 1000 Ohm.
A current path in human body influences on striking outcomes. Danger is especially large, if current, passing through vitals organs (heart, lungs, cerebrum), acts immediately on them. If current does not pass over these organs, then its action on them only reflex and striking probability is lesser.
Frequently met current path through human body is called by "current loop". In majority of cases (55%) the electric blows take place for two basic loops: а) from one palm or both palms to legs; б) from one palm to another palm. This collision is most probable. But for these two most probable paths only 28% of all of mortal striking occurs, and other 72% occur for outstanding loops, for example, from rear part of hand to legs (25% of cases), from palm to rear part of hand (28%). This brings us over to our point of view that rear part of the hand is a most vulnerable place for current on human body. Refereeing from this, a danger is determined not only by current paths through vital organs, but also how body touches to the current-conducting parts of equipment, what is a closeness of nervous outflows on this part of the body and is there a acupuncture zone.
Duration of current action on man influences on striking outcome, because with increasing of time of current action protective function of human organism falls down, and striking danger enhances. Next acceptable norms of currents are adopted in dependence to time of passing:
| Passing time, с | 0.7 | 0.5 | 0.2 | |
| Current, mA |
Also likelihood of heart fibrillation beginnings is fixed, in dependence on current passing time. It is known fact that cardiac muscle in different phases of its activity is not identically sensible to electric current. It may be believed that most sensitive to current, sufficient for heart fibrillation, is a phase Т, with duration 0.2 s, period, when expires shortening of ventricles and a cardiac muscle gets across weakened state. What lesser the duration of current passing through organism, that lesser likelihood of its influence on heart in vulnerable phase, and so for striking danger.
Individual human organism peculiarities considerably influence on striking result at electric trauma. For example, effects of current of the same strength depend on man mass, its physical development. An unletting current for one person may be only perceptible for other. For women thresholds of current are approximately 1.5 times lower than for men.
Degree of current effects depends also on state of nervous system and whole organism. Consequently, in excitation state of nervous system, depression, illness (specially illness of skin, cardiac-vascular system, nervous system and other), at intoxication man is more sensible to passing current.
Conditions of external environment determine the striking outcomes in that degree, in which they are capable to change value of current, that passes through man, changing electric organism resistance. For example, with temperature rise of external environment number of difficult striking increases, and with pressure rise - diminishes. In connection with that all production workplaces, in accordance to maintenance regulations of electric equipment, are subdivided on three categories in dependence with conditions like external phenomena.
Factor of human attention also largely influences on striking outcomes. It is known fact, that for protection from dangerous irritant the internal man resources may mobilise. If man is prepared to electric blow, then danger degree sharply falls down, and unexpected blow gives rise to more difficult outcomes. Eloquent assertion: "By chance to be killed from electric current" is a surplus. To put man to death by current is extremely heavily.
At first help assignment for victim from influence of electric current it is necessary to execute such operations: to absolve from source of current; immediately to bring on ambulance; quickly to define a victim state and to provide rest to him, flow of crisp air, heat; in lack of breathing and pulse - to do artificial breathing and external heart massage.
When a number of persons, who gives help, is equal to two or more, then one of them must make a heart massage to victim (to press by open palm on thorax), second must make artificial breathing (it is necessary to make 10-12 inhales in a minute). Pressing must be taken once in a second with interruptions on 2 s after each four - six pressings. If help is lend by one man, then after two - three inhales it must be taken four - six pressings for heart massage.
Альфа-излучение представляет собой поток альфа-частиц — ядер гелия-4. Альфа-частицы, рождающиеся при радиоактивном распаде, могут быть легко остановлены листом бумаги. Бета-излучение — это поток электронов, возникающих при бета-распаде; для защиты от бета-частиц энергией до 1 МэВ достаточно алюминиевой пластины толщиной в несколько миллиметров. Гамма-излучение обладает гораздо большей проникающей способностью, поскольку состоит из высокоэнергичных фотонов, не обладающих зарядом; для защиты эффективны тяжёлые элементы (свинец и т. д.), поглощающие МэВ-ные фотоны в слое толщиной несколько см. Проникающая способность всех видов ионизирующего излучения зависит от энергии.
| Возможные последствия для человека различных доз облучения за короткий промежуток времени.Доза (миллирентген) | Последствия |
| 50 - 200 | Уменьшение белых кровяных клеток, тошнота, рвота; около 10 % погибают в течение нескольких месяцев при 200 мР |
| 200 - 400 | Потеря кровяных клеток, высокая температура, кровотечение, выпадение волос, тошнота, рвота, кожные нарывы; погибает до 20 % |
| 500 - 1000 | Тяжёлые расстройства желудочно–кишечного тракта, острая сердечно–сосудистая недостаточ-ность, поражение централь-ной нервной системы. Гибель в течение нескольких недель. |
| Смерть в течение нескольких часов |
Воздействие ионизирующего излучения может повреждать клетки человеческого организма двумя способами. Один из них – генетические повреждения, которые изменяют гены и хромосомы. Они могут проявиться в виде генетических дефектов у потомков. Другой способ – соматические повреждения, которые наносят вред жертве в течение её жизни. Примерами служат ожоги, некоторые виды лейкемии, выкидыши, глазные катаракты, а также раковые заболевания костей, щитовидной железы, молочной железы и лёгких.
Га́мма-излуче́ние (гамма-лучи, γ-лучи) — вид электромагнитного излучения с чрезвычайно малой длиной волны — < 5•10−3 нм и, вследствие этого, ярко выраженными корпускулярными и слабо выраженными волновыми свойствами.
Principles of radiation protection
Radiation protection can be divided into occupational radiation protection, which is the protection of workers, medical radiation protection, which is the protection of patients and the radiographer, and public radiation protection, which is protection of individual members of the public, and of the population as a whole. The types of exposure, as well as government regulations and legal exposure limits are different for each of these groups, so they must be considered separately.
There are three factors that control the amount, or dose, of radiation received from a source. Radiation exposure can be managed by a combination of these factors:
1. Time: Reducing the time of an exposure reduces the effective dose proportionally. An example of reducing radiation doses by reducing the time of exposures might be improving operator training to reduce the time they take to handle a source.
2. Distance: Increasing distance reduces dose due to the inverse square law. Distance can be as simple as handling a source with forceps rather than fingers.
3. Shielding: The term 'biological shield' refers to a mass of absorbing material placed around a reactor, or other radioactive source, to reduce the radiation to a level safe for humans.[1] The effectiveness of a material as a biological shield is related to its cross-section for scattering and absorption, and to a first approximation is proportional to the total mass of material per unit area interposed along the line of sight between the radiation source and the region to be protected. Hence, shielding strength or "thickness" is conventionally measured in units of g/cm2. The radiation that manages to get through falls exponentially with the thickness of the shield. In x-ray facilities, the plaster on the rooms with the x-ray generator contains barium sulfate and the operators stay behind a leaded glass screen and wear lead aprons. Almost any material can act as a shield from gammaor x-rays if used in sufficient amounts.
Practical radiation protection tends to be a job of juggling the three factors to identify the most cost effective solution.
In most countries a national regulatory authority works towards ensuring a secure radiation environment in society by setting requirements that are also based on the international recommendations for ionizing radiation (ICRP - International Commission on Radiological Protection):
§ Justification: No unnecessary use of radiation is permitted, which means that the advantages must outweigh the disadvantages.
§ Limitation: Each individual must be protected against risks that are far too large through individual radiation dose limits.
§ Optimization: Radiation doses should all be kept as low as reasonably achievable. This means that it is not enough to remain under the radiation dose limits. As permit holder, you are responsible for ensuring that radiation doses are as low as reasonably achievable, which means that the actual radiation doses are often much lower than the permitted limit
Types of radiation
Different types of ionizing radiation behave in different ways, so different shielding techniques are used.
§ Particle radiation consists of a stream of charged or neutral particles, both charged ions and subatomic elementary particles. This includes solar wind, cosmic radiation, and neutron flux in nuclear reactors.
§ Alpha particles (helium nuclei) are the least penetrating. Even very energetic alpha particles can be stopped by a single sheet of paper.
§ Beta particles (electrons) are more penetrating, but still can be absorbed by a few millimeters of aluminum. However, in cases where high energy beta particles are emitted shielding must be accomplished with low density materials, e.g. plastic, wood, water or acrylic glass (Plexiglas, Lucite) [1]. This is to reduce generation of Bremsstrahlung X-rays. In the case of beta+ radiation (positrons), the gamma radiation from the electron-positron annihilation reaction poses additional concern.
§ Neutron radiation is not as readily absorbed as charged particle radiation, which makes this type highly penetrating. Neutrons are absorbed by nuclei of atoms in a nuclear reaction. This most-often creates a secondary radiation hazard, as the absorbing nuclei transmute to the next-heavier isotope, many of which are unstable.
§ Cosmic radiation is not a common concern, as the Earth's atmosphere absorbs it and the magnetosphere acts as a shield, but it poses a problem for satellites and astronauts and frequent fliers are also at a slight risk. Cosmic radiation is extremely high energy, and is very penetrating.
§ Electromagnetic radiation consists of emissions of electromagnetic waves, the properties of which depend on the wavelength.
§ X-ray and gamma radiation are best absorbed by atoms with heavy nuclei; the heavier the nucleus, the better the absorption. In some special applications, depleted uranium is used, but lead is much more common; several centimeters are often required. Barium sulfate is used in some applications too. However, when cost is important, almost any material can be used, but it must be far thicker. Most nuclear reactors use thick concrete shields to create a bioshield with a thin water cooled layer of lead on the inside to protect the porous concrete from the coolant inside. The concrete is also made with heavy aggregates, such as Baryte, to aid in the shielding properties of the concrete.
§ Ultraviolet (UV) radiation is ionizing but it is not penetrating, so it can be shielded by thin opaque layers such as sunscreen, clothing, and protective eyewear. Protection from UV is simpler than for the other forms of radiation above, so it is often considered separately.
In some cases, improper shielding can actually make the situation worse, when the radiation interacts with the shielding material and creates secondary radiation that absorbs in the organisms more readily.
Shielding design
Shielding reduces the intensity of radiation exponentially depending on the thickness.
This means when added thicknesses are used, the shielding multiplies. For example, a practical shield in a fallout shelter is ten halving-thicknesses of packed dirt, which is 90 cm (3 ft) of dirt. This reduces gamma rays to 1/1,024 of their original intensity (1/2 multiplied by itself ten times). Halving thicknesses of some materials, that reduce gamma ray intensity by 50% (1/2) include[2]:
| Material | Halving Thickness, inches | Halving Thickness, cm | Density, g/cm³ | Halving Mass, g/cm² |
| lead | 0.4 | 1.0 | 11.3 | |
| steel | 0.99 | 2.5 | 7.86 | |
| concrete | 2.4 | 6.1 | 3.33 | |
| packed soil | 3.6 | 9.1 | 1.99 | |
| water | 7.2 | 1.00 | ||
| lumber or other wood | 0.56 | |||
| air | 0.0012 |
Column Halving Mass in the chart above indicates mass of material, required to cut radiation by 50%, in grams per square centimetre of protected area.
The effectiveness of a shielding material in general increases with its density.
Signs and symptoms
Classically acute radiation syndrome is divided into three main presentations: hematopoietic, gastrointestinal and neurological/vascular. These symptoms may or may not be preceded by a prodrome.[1] The speed of onset of symptoms is related to radiation exposure, with greater doses resulting in a shorter delay in symptom onset.[1]
1. Hematopoietic. This syndrome is marked by a drop in the number of blood cells. This may result in infections due to low white blood cells, bleeding due to low platelets, andanemia due to low red blood cells.[1]
2. Gastrointestinal. This syndrome typically occurs at exposure doses of 600–1000 rad (6–10 Gy).[1] Nausea, vomiting, loss of appetite, and abdominal pain are usually seen within one to two hours.[1]
3. Neurovascular. This syndrome typically occurs at exposure doses greater than 1000 rad (10 Gy).[1] It presents with neurological symptoms such as dizziness, headache, ordecreased level of consciousness with an absence of vomiting.[1]
The prodrome associated with ARS typically includes nausea and vomiting, headaches, fatigue, fever and short period of skin reddening.[1] These symptoms may occur at radiation doses as low as 35 rad (0.35 Gy). Thus, they may not be followed by acute radiation sickness.[1]
Symptoms:
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