Microwave burn
Microwave burns are burn injuries caused by thermal effects of microwave radiation absorbed in a living organism.
In comparison with
Microwave damage can manifest with a delay; pain or signs of skin damage can show some time after microwave exposure.[1]
Frequency vs depth
The depth of penetration depends on the frequency of the microwaves and the tissue type. The
As lower frequencies penetrate deeper into the tissue, and as there are fewer nerve endings in deeper-located parts of the body, the effects of the radio frequency waves (and the damage caused) may not be immediately noticeable. The lower frequencies at high power densities present a significant risk.
The microwave absorption is directed by the
The layers of the body can be approximated as a thin layer of epidermis, dermis, adipose tissue (subcutaneous fat), and muscle tissue. At dozens of gigahertz, the radiation is absorbed in the top fraction to top few millimeters of skin. Muscle tissue is a much more efficient absorber than fat, so at lower frequencies that can penetrate sufficiently deep, most energy gets deposited there. In a homogeneous medium, the energy/depth dependence is an exponential curve with the exponent depending on the frequency and tissue. For 2.5 GHz, the first millimeter of muscle tissue absorbs 11% of the heat energy, the first two millimeters together absorb 20%. For lower frequencies, the attenuation factors are much lower, the achievable heating depths are higher, and the temperature gradient within the tissue is lower.[2][4]
Tissue damage
The tissue damage depends primarily on the absorbed energy and the tissue sensitivity; it is a function of the microwave power density (which depends on the distance from the source and its power output), frequency, absorption rate in the given tissue, and the tissue sensitivity. Tissues with high water (resp. electrolyte) content show higher microwave absorption.
The degree of the tissue damage depends on both the achieved temperature and the length of exposure. For short times, higher temperatures can be tolerated.
The damage can be spread over a large area, when the source is a relatively distant energy radiator, or a very small (though possibly deep) area, when the body comes to a direct contact with the source (e.g. a wire or a connector pin).[5]
The
Skin
Exposure to frequencies common in domestic and industrial sources rarely leads to significant skin damage; in such cases, the damage tends to be limited to
Muscle and fat tissue
Microwave burns show some similarities with electrical burns, as the tissue damage is deep rather than superficial. Adipose tissue shows less degree of damage than muscles and other water-rich tissues. (In contrast, radiant heat, contact burns and chemical burns damage subcutaneous adipose tissue to higher extent than deeper muscle tissue.) Full-thickness biopsy of the area between burned and unburned skin shows layers of more and less damaged tissue ("tissue sparing"), layers of undamaged fat between damaged muscles; a pattern that is not present in conventional thermal or chemical burns. Cells subjected to electrical burns show microscopic nuclear streaming on histology examination; this feature is not present with microwave burns. Microwaves also deposit more energy to areas with low blood supply and to tissue interfaces.[1][8]
Hot spots may be formed in the tissue, with a consequent higher absorption of microwave energy and even higher temperature achieved, with localized necrosis of the affected tissue following.[9] Sometimes, the affected tissue can even be charred.[10]
Muscle tissue destruction can lead to
Eyes
Cases of severe conjunctivitis were reported after technicians looked into powered waveguides.[4]
Microwave-induced
For the near field 2.45 GHz frequency, the minimum power density to cause cataracts in rabbits was found to be 150 mW/cm2 for 100 minutes; a retrolental temperature of 41 °C was necessary to be achieved. When the eye temperature was kept low by external cooling, cataracts were not produced by higher field intensities; that supports the hypothesis of a thermal mechanism being involved.[15]
Nerves
When the temperature of the brain is raised to or above 42 °C, the blood–brain barrier permeability increases.[15]
A
Other tissues
The thermal effects of microwaves can cause
Pulmonary burn can be present when lungs are exposed; chest x-ray is used for diagnosing.[11]
Exposure of abdomen may lead to bowel obstruction due to stenosis of the affected bowel; flat and upright abdominal x-ray is used to check for this condition.[11]
Injury cases
Household microwave ovens have shielding around the inside of the oven that prevents microwaves from leaking out, as well as safety interlocks that prevent the oven from operating when the door is open. Therefore, burns due to direct exposure to microwave energy (as opposed to touching hot food) should not occur under normal circumstances.
Infants and microwave ovens
There are several cases of child abuse where an infant or child has been placed in a microwave oven. The typical feature of such injuries are well-defined burns on the skin nearest to the microwave emitter, and histology examination shows higher damage extent in tissues with high content of water (e.g., muscles) than in tissues with less water (e.g., adipose tissue).[17]
One such case involved a teenage babysitter who admitted to having placed a child in the microwave oven for approximately sixty seconds. The child developed a
Another case involved a five-week-old female infant that had multiple full-thickness burns totaling 11% of the body surface area. The mother claimed the infant had been near a microwave oven, but not inside it. The infant survived but required amputations of parts of one leg and one hand.[1]
Additionally, there have been two alleged infant deaths caused by microwave ovens.[18][19][20] In all these cases, the babies were placed within microwaves and died of subsequent injuries.
Adults and microwave ovens
A case of nerve damage by an exposure to radiation from a malfunctioning 600 watt microwave oven, operated for five seconds with the door open, with both arms and hands exposed, was reported. During exposure, there was a pulsating, burning sensation in all fingers. Erythema appeared on the back sides of both hands and arms. Four years later, denervation of median nerve, ulnar nerve, and radial nerve in both arms was shown on an electromyography test.[1][21]
The first microwave oven injury was reported in 1973. Two women operated a microwave oven in a department store snack bar. After several years, the oven showed a malfunction manifesting by burning the food. The first woman noticed burning sensations in her fingers and very little pain or tenderness when nearby to the operating oven. A small lesion appeared on her left index finger, near the base of the fingernail. In the next four weeks, three fingers of her right hand became affected as well. Transverse ridging and deformations close to the nail base appeared on her fingernails. After five months since the initial symptoms, she visited a doctor; the examination found no abnormalities other than the nails. Topical steroid cream used over six weeks led to gradual improvement. The second woman experienced nail deformation at the same time as the first one, with the same clinical findings. The oven was returned to the manufacturer before the involvement of the doctor, and the amount of leakage could not be assessed.[21]
On July 29, 1977, H.F., a 51-year-old teacher, was attempting to remove a
In 1983, a 35-year-old male was heating a sandwich in a microwave oven at work. After opening the door, the magnetron did not shut off and his right hand was exposed to microwave radiation as he retrieved the sandwich. After exposure, his hand was pale and cold; 30 minutes later the man presented himself to a doctor, with paresthesia in all fingers and the hand still pale and cold. An Allen's test showed a return to normal color after 60 seconds (normal is 5 seconds). By 60 minutes after exposure the hand was normal again, and the patient was discharged without treatment. A week later there was no paresthesia, motor weakness nor sensory deficit.[21]
Other
An engineer replaced a
Medical uses
Microwave damage to tissues can be intentionally exploited as a therapeutic technique, e.g.
Microwave heating seems to cause more damage to bacteria than equivalent thermal-only heating.[27] However food reheated in a microwave oven typically reaches lower temperature than classically reheated, therefore pathogens are more likely to survive.
Microwave heating of blood, e.g. for transfusion, is contraindicated, as it can cause hemolysis and hyperkalemia.[8]
Microwave heating is one of the methods for inducing hyperthermia for hyperthermia therapy.
High-energy microwaves are used in
Perception thresholds
Safety limits exist for microwave exposure. The U.S. Occupational Safety and Health Administration defines energy density limit for exposure periods of 0.1 hours or more to 10 mW/cm2; for shorter periods the limit is 1 mW-hr/cm2 with limited excursions above 10 mW/cm2. The U.S. Food and Drug Administration (FDA) standard for microwave oven leakage puts limit to 5 mW/cm2 at 2 inches from the oven's surface.[24]
For 5.8 GHz, exposure to 30 mW/cm2 causes increase of facial skin temperature by 0.48 °C, corneal surface heats by 0.7 °C, and the temperature of retina is estimated to increase by 0.08–0.03 °C.[9]
Exposure of skin to microwaves can be perceived as a sensation of heat or pain. Due to lower penetration of higher frequencies, perception threshold is lower for higher frequencies as more energy is dissipated closer to the body surface. When the entire face is exposed to 10 GHz microwaves, the feeling of heat is evoked at energy densities of 4–6 mW/cm2 for 5 or more seconds, or about 10 mW/cm2 for a half second. Experiments on six volunteers exposed to 2.45 GHz microwaves shown perception thresholds on forearm skin to be at the average of 25–29 mW/cm2, ranging from 15.40 to 44.25 mW/cm2. The sensation was indistinguishable from heat delivered by infrared radiation, though the infrared radiation required about five times lower energy density. Pain threshold for 3 GHz was demonstrated to range from 0.83 to 3.1 W/cm2 for 9.5 cm2 of exposed area, depending on length of the exposure; other source says the dependence is not directly on the power density and exposure length, but primarily on the critical skin temperature.[9]
Microwave energy can be focused by metal objects in the vicinity of the body or when implanted. Such focusing and resultant increased heating can significantly lower the perception, pain and damage thresholds. Metal-framed glasses perturb microwave fields between 2–12 GHz; individual components were found to be resonant between 1.4 and 3.75 GHz.[9]
A security guard with a metal plate in his leg experienced heating of the plate when patrolling near tropospheric scatter transmitter antennas; he had to be removed from their vicinity.
In the 30–300 GHz band, dry clothing may serve as an
Pulsed microwave radiation can be perceived by some workers as a phenomenon called "
Other concerns
Some
Low-level exposure
As the energy of radio frequency waves and microwaves is insufficient to directly disrupt individual chemical bonds in small or stable molecules, the effects are considered limited to thermal. Energy densities that are not sufficient to overheat the tissues are not shown to cause lasting damage[
Myths
A common myth among radar and microwave communication workers is that the exposure of the genital area to microwaves renders a man sterile for about a day. The power density necessary for this effect is however sufficient to also cause permanent damage.[23]
References
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