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Electromagnetic radiation can be classified into two types: ionizing radiation and non-ionizing radiation, based on its capability of ionizing atoms and breaking chemical bonds. Ultraviolet and higher frequencies, such as X-rays or gamma rays are ionizing, and these pose their own special hazards: see radiation and radiation poisoning. By far the most common health hazard of radiation is sunburn, which causes over one million new skin cancers annually.
Very strong radiation can induce current capable of delivering an electric shock to persons or animals. It can also overload and destroy electrical equipment. The induction of currents by oscillating magnetic fields is also the way in which solar storms disrupt the operation of electrical and electronic systems, causing damage to and even the explosion of power distribution transformers, blackouts (as occurred in 1989), and interference with electromagnetic signals (e.g. radio, TV, and telephone signals).
Extremely high power electromagnetic radiation can cause electric currents strong enough to create sparks (electrical arcs) when an induced voltage exceeds the breakdown voltage of the surrounding medium (e.g. air). These sparks can then ignite flammable materials or gases, possibly leading to an explosion.
This can be a particular hazard in the vicinity of explosives or pyrotechnics, since an electrical overload might ignite them. This risk is commonly referred to as HERO (Hazards of Electromagnetic Radiation to Ordnance). MIL-STD-464A mandates assessment of HERO in a system, but Navy document OD 30393 provides design principles and practices for controlling electromagnetic hazards to ordnance.
On the other hand, the risk related to fueling is known as HERF (Hazards of Electromagnetic Radiation to Fuel). NAVSEA OP 3565 Vol. 1 could be used to evaluate HERF, which states a maximum power density of 0.09 W/m² for frequencies under 225 MHz (i.e. 4.2 meters for a 40 W emitter).
The best understood biological effect of electromagnetic fields is to cause dielectric heating. For example, touching or standing around an antenna while a high-power transmitter is in operation can cause severe burns. These are exactly the kind of burns that would be caused inside a microwave oven.
This heating effect varies with the power and the frequency of the electromagnetic energy. A measure of the heating effect is the specific absorption rate or SAR, which has units of watts per kilogram (W/kg). The IEEE and many national governments have established safety limits for exposure to various frequencies of electromagnetic energy based on SAR, mainly based on ICNIRP Guidelines, which guard against thermal damage.
There are publications which support the existence of complex biological effects of weaker non-thermal electromagnetic fields (see Bioelectromagnetics), including weak ELF magnetic fields and modulated RF and microwave fields. Fundamental mechanisms of the interaction between biological material and electromagnetic fields at non-thermal levels are not fully understood.
A 2009 study at the University of Basel in Switzerland found that intermittent (but not continuous) exposure of human cells to a 50 Hz electromagnetic field at a flux density of 1 mT (or 10 G) induced a slight but significant increase of DNA fragmentation in the Comet assay. However that level of exposure is already above current established safety exposure limits.
In May 2011, the WHO's International Agency for Research on Cancer published a review of the evidence on health risks of electromagnetic fields (EMFs), concluding that there was limited evidence that cellphone users might be at increased risk of glioma and acoustic neuroma, and that there was inadequate evidence of any other health risks posed by EMF. This designation as "possibly carcinogenic" has often been misinterpreted as indicating that of some measure of risk has been observed; the designation indicates that the possibility could not be conclusively ruled out using the available data.
"There is no conclusive evidence of any harm caused by exposures [to electric and magnetic fields] at levels found in Canadian homes and schools, including those located just outside the boundaries of power line corridors."
From the Belgian government's website for "Federal Public Service: Health, Food Chain Safety and Environment":
"As of 1 March 2014, new regulations will apply to the sale of mobile phones. On the one hand the sale of mobile phones that have been specially manufactured for young children (under 7s) will be prohibited. On the other hand the SAR (Specific_absorption_rate) value will have to be listed everywhere where mobile phones are sold: in stores as well as for distance sales over the Internet." ... "Why these measures? As a precaution. According to the International Agency for Research on Cancer (IARC, 2011) there may be an increased risk of brain cancer due to the intensive use of a mobile phone. The IARC has therefore classified radio frequency as “possibly carcinogenic”. Measures are being taken pending clearer scientific conclusions. The intention is to raise awareness among mobile phone users." 
The preponderance of evidence suggests that the low-power, low-frequency, electromagnetic radiation associated with household current does not constitute a short or long term health hazard, and although some biophysical mechanisms for the promotion of cancer have been proposed (such as the electric fields around power lines attracting aerosol pollutants), none have been substantiated. Nevertheless, some research has reported correlation with a number of adverse health effects, although controversy can include whether observed correlation implies causation. These include, but are not limited to, childhood leukemia, adult leukemia, neurodegenerative diseases (such as amyotrophic lateral sclerosis), miscarriage, and Alzheimer's disease. Some research has found no relationship with amyotrophic lateral sclerosis, Parkinson's disease, or multiple sclerosis.
One response to the potential dangers of overhead power lines is to place them underground. The earth and enclosures surrounding underground cables prevent the electric field from radiating significantly beyond the power lines, and greatly reduce the magnetic field strength radiating from the power lines, into the surrounding area. However, the cost of burying and maintaining cables at transmission voltages is several times greater than overhead power lines.
In 1997 the National Cancer Institute (NCI) released a report published in the New England Journal of Medicine, the result of a seven-year epidemiological investigation. The study investigated 638 children with acute lymphoblastic leukemia (ALL) and 620 controls and concluded that their study provided "little evidence that living in homes characterized by high measured time-weighted average magnetic-field levels or by the highest wire-code category increases the risk of ALL in children." Following the report, the US Department of Energy disbanded the EMF Research and Public Information Dissemination (RAPID) Program, saying that its services were no longer needed.
In 2005, the Canadian Federal-Provincial-Territorial Radiation Protection Committee said, "The outcome of a recently conducted pooled analysis of several epidemiological studies shows a two-fold increase in the risk of leukemia in children living in homes, where the average magnetic field levels are greater than 0.4 microtesla (4 milligauss). [However,] it is the opinion of [this committee] that the epidemiological evidence to date is not strong enough to justify a conclusion that EMFs in Canadian homes, regardless of locations from power lines, cause leukemia in children."
The World Health Organization issued a fact sheet, No. 322, in June, 2007 based on the findings of a WHO work group (2007), the IARC (2002) and the ICNIRP (2003), which reviewed research conducted since the earlier publication. The fact sheet says "that there are no substantive health issues related to ELF electric fields at levels generally encountered by members of the public." For ELF magnetic fields, the fact sheet says, "the evidence related to childhood leukaemia is not strong enough to be considered causal", and "[as regards] other childhood cancers, cancers in adults, ... The WHO Task Group concluded that scientific evidence supporting an association between ELF magnetic field exposure and all of these health effects is much weaker than for childhood leukaemia. In some instances (i.e., for ... breast cancer) the evidence suggests that these fields do not cause them."
According to Dr. Lakshmikumar at the National Physical Laboratory, India, a direct, causal, link between RF radiation and cancer (including leukemia) would require one to be "willing to discard Planck's Law… and the entire body of quantum physics."
In 2010, Maslanyj et al., applying the Bradford-Hill criteria to available evidence, considered the application of low-cost exposure reduction measures as appropriate precautionary responses to "small and uncertain public health risks". Even after pooling all the data, they found it fell short of establishing "strength of association, dose-response relationship, biological plausibility and coherence, and analogy". They recognised that controversy would continue so long as other interpretations of the data were possible.
In 2001, Ahlbom et al. conducted a review into EMFs and Health, and found that there was a doubling in childhood leukemia for magnetic fields of over 0.4 µT, but said that it "... may be partly due to bias. This is difficult to interpret in the absence of a known mechanism or reproducible experimental support."
In 2002 a study by Michelozzi et al. found a relationship between leukemia and proximity to the Vatican Radio station transmitters although "the study has limitations because of the small number of cases and the lack of exposure data."
In 2005 Draper et al. found a 70% increase in childhood leukemia for those living within 200 metres (656 ft) of an overhead transmission line, and a 23% increase for those living between 200 and 600 metres (656 and 1,969 ft). The authors concluded that "the relation may be due to chance or confounding." The authors considered it unlikely that the increase from 200 m to 600 m is related to magnetic fields as they are well below 0.4 µT at this distance. Bristol University (UK) has published work on a theory that could account for this increase, and would also provide a potential mechanism, being that the electric fields around power lines attract aerosol pollutants.
The World Health Organisation issued Factsheet No. 263 in October 2001 on ELF (Extremely low frequency) EMFs and cancer. It said that they were "possibly carcinogenic", based primarily on IARC's similar evaluation with respect to childhood leukemia. It also said that there was "insufficient" data to draw any conclusions on other cancers. The WHO later noted that result had been based on evidence which was "weakened by methodological problems" and that "on balance, the evidence related to childhood leukaemia is not strong enough to be considered causal."
The UK Department of Health set up the Stakeholder Advisory Group on ELF EMFs (SAGE) to explore the implications and to make recommendations for a precautionary approach to power frequency electric and magnetic fields in light of any evidence of a link between EMF and childhood leukemia. The first interim assessment of this group was released in April 2007 , and found that the link between proximity to power lines and childhood leukemia was sufficient to warrant a precautionary recommendation, including an option to lay new power lines underground where possible and to prevent the building of new residential buildings within 60 m (197 ft) of existing power lines. The latter of these options was not an official recommendation to government as the cost-benefit analysis based on the increased risk for childhood leukemia alone was considered insufficient to warrant it. The option was considered necessary for inclusion as, if found to be real, the weaker association with other health effects would make it worth implementing.
Mobile phone radiation and health concerns have been raised, especially following the enormous increase in the use of wireless mobile telephony throughout the world (as of August 2005[update], there were more than 2 billion users worldwide). Mobile phones use electromagnetic radiation in the microwave range, and some[unreliable medical source?] believe this may be harmful to human health. These concerns have induced a large body of research (both epidemiological and experimental, in non-human animals as well as in humans). Concerns about effects on health have also been raised regarding other digital wireless systems, such as data communication networks.
The World Health Organization, based upon the consensus view of the scientific and medical communities, states that health effects (e.g. headaches or promotion of cancer) are unlikely to be caused by cellular phones or their base stations, Still, the "IARC has classified radiofrequency electromagnetic fields as possibly carcinogenic to humans (Group 2B), a category used when a causal association is considered credible, but when chance, bias or confounding cannot be ruled out with reasonable confidence."
Physicist Vasant Natarajan claimed "We should indeed worry that our modern industrialized world is full of carcinogens — from pesticides in the food we eat, to industrial pollutants in our air and water. But cell-phone radiation is not one of them." He explained why such radiation cannot cause cancer in humans, due to its energy being below any carcinogenic threshold.
While the most acute exposures to harmful levels of electromagnetic radiation are immediately realized as burns, the health effects due to chronic or occupational exposure may not manifest effects for months or years.
High-power extremely-low-frequency RF with electric field levels in the low kV/m range are known to induce perceivable currents within the human body that create an annoying tingling sensation. These currents will typically flow to ground through a body contact surface such as the feet, or arc to ground where the body is well insulated.
Microwave exposure at low-power levels below the specific absorption rate set by government regulatory bodies are considered harmless non-ionizing radiation and have no effect on the human body. However, levels above the specific absorption rate set by the FCC at considered potentially harmful. ANSI standards for safe exposure levels to RF and microwave radiation are set to a SAR level of 4 W/kg, the threshold before hazardous biological effects occur due to energy absorption in the body. A safety factor of ten was then incorporated to arrive at the final recommended protection guidelines of a SAR exposure threshold of 0.4 W/kg for RF and microwave radiation. There is disagreement over exactly what levels of RF radiation are "safe," particularly with regard to low levels of exposure. For instance, Russia and eastern European countries in particular set SAR thresholds for Microwaves and RF much lower than western countries.
Two areas of the body, the eyes and the testes, can be particularly susceptible to heating by RF energy because of the relative lack of available blood flow to dissipate the excessive heat load. Laboratory experiments have shown that short-term exposure to high levels of RF radiation (100-200 mW/cm²) can cause cataracts in rabbits. Temporary sterility, caused by such effects as changes in sperm count and in sperm motility, is possible after exposure of the testes to high-level RF radiation
Long-term exposure to high-levels of microwaves, is recognized, from experimental animal studies and epidemiological studies in humans, to cause cataracts. The mechanism is unclear but may include changes in heat sensitive enzymes that normally protect cell proteins in the lens. Another mechanism that has been advanced is direct damage to the lens from pressure waves induced in the aqueous humor.
Exposure to high-power microwave RF is known to create effects ranging from a burning sensation on the skin and microwave auditory effect, to extreme pain at the mid-range, to physical microwave burns and blistering of skin and internals at high power levels.
Recent technology advances in the developments of Millimeter wave scanners for airport security and WiGig for Personal area networks have opened the 60 Ghz and above Microwave band to SAR exposure regulations. Previously, microwave applications in these bands were for point-to-point satellite communication with minimal human exposure. Radiation levels in the millimeter wavelength represent the high microwave band or close to Infrared wavelengths. 
Infrared wavelengths longer than 750 nm can produce lens changes. La "cataracte des verriers"( glassblower's cataract ) is an example of a heat injury that damages the anterior lens capsule among unprotected glass and iron workers. cataract-like changes can occur in unprotected workers who observe glowing masses of glass or iron for many hours a day.
Another important factor is the distance between the worker and the source of radiation. In the case of arc welding, infrared radiation decreases rapidly as a function of distance, so that farther than 3 feet away from where welding takes place, it does not pose an ocular hazard anymore but, ultraviolet radiation still does. That is why welders wear tinted glasses and surrounding workers only have to wear clear ones.
Moderate and high-power lasers are potentially hazardous because they can burn the retina of the eye, or even the skin. To control the risk of injury, various specifications, for example ANSI Z136 in the US and IEC 60825 internationally, define "classes" of laser depending on their power and wavelength. These regulations also prescribe required safety measures, such as labeling lasers with specific warnings, and wearing laser safety goggles when operating lasers. See, Laser safety.
Short-term exposure to strong ultraviolet sunlight causes sunburn within hours of exposure.
Ultraviolet light, specifically UV-B, has been shown to cause cataracts and there is some evidence that sunglasses worn at an early age can slow its development in later life. Most UV light from the sun is filtered out by the atmosphere and consequently airline pilots often have high rates of cataracts because of the increased levels of UV radiation in the upper atmosphere. It is hypothesised that depletion of the ozone layer and a consequent increase in levels of UV light on the ground may increase future rates of cataracts. Note that the lens filters UV light, so once that is removed via surgery, one may be able to see UV light.
Prolonged exposure to ultraviolet radiation from the sun can lead to melanoma and other skin malignancies. Clear evidence establishes ultraviolet radiation, especially the non-ionizing medium wave UVB, as the cause of most non-melanoma skin cancers, which are the most common forms of cancer in the world.
Ultraviolet radiation of wave lengths shorter than 300 nm. (actinic rays) can damage the corneal epithelium. This is most commonly the result of exposure to the sun at high altitude and in areas where shorter wave lengths are readily reflected from bright surfaces such as snow, water, and sand.
Exposure to ionizing radiation such as X-rays are known to cause cataracts. X-ray radiation in a dose of 500-800 Rads directed toward the lens surface can cause cataracts, sometimes with a delay of several months to a year before the opacities appear
The principal hazard associated with use of x-ray equipment is localized skin burns following exposure to the primary beam. Experience with exposure of relatively large areas of skin to radiation has shown that it requires doses of approximately 300 Rads (3 Gray) to produce a visible reddening of the skin. Doses of approximately 1500 rad (15 gray) are required in order to produce serious burns with blistering. When doses reach 3000 Rads (30 Gray) very serious burns requiring skin grafts or amputation may result. The bum symptoms may require from one to several weeks to develop, depending on the dose.Ionizing radiation can cause. See, radiation-induced cancer, radiation burns, and radiation poisoning.
Note: As of 2014, there are currently no commercially available soft x-ray detectors that operate in the 50 to 10,000 electron volt range.
Large doses of gamma ray and other ionizing radiation can cause extensive cellular damage and death. Epidemiological data on survivors of the atomic bombs, dropped during World War II on Hiroshima and Nagasaki, comprise the largest body of evidence on the effects of high levels of ionizing radiation exposure. These data demonstrate a higher incidence of cancer among exposed individuals and an increased probability of cancer as the level of exposure increases.
A whole-body exposure to 5 or more gray of high-energy radiation at one time usually leads to death within 14 days. This dosage represents 375 joules for a 75 kg adult (equivalent to the chemical energy in 20 mg of sugar). Since gray are such large amounts of radiation, medical use of radiation is typically measured in milligray (mGy).
As experienced from follow-up after radiation therapy, epilation may occur on any hair-bearing skin exposed to doses above 1 Gy. Hair loss may be permanent with a single dose of 10 Gy, but if the dose is fractionated permanent hair loss may not occur until dose exceeds 45 Gy. The salivary glands and tear glands have a radiation tolerance of about 30 Gy in 2 Gy fractions, a dose which is exceeded by most radical head and neck cancer treatments, potentially causing dryness. Dry mouth (xerostomia) and dry eyes (xerophthalmia) can become irritating long-term problems and severely reduce the patient's quality of life. Similarly, sweat glands in treated skin (such as the armpit) tend to stop working, and the naturally moist vaginal mucosa is often dry following pelvic irradiation.
Canada Safety Code 6 recommends electric field limits of 100 kV/m for pulsed EMF to prevent air breakdown and spark discharges. Additional rational for EMF restrictions is to avoid auditory effect and energy-induced unconsciousness in rats.