With the popularity and development of amateur radio activities, there are more and more opportunities for radio enthusiasts to face electromagnetic radiation every day. Although everyone is very concerned about the quality of QSO, it is often mentioned that the radio waves may cause harm to the human body. Some fans think that radio waves are harmful to the body. Since they started to like amateur radio stations, they have often been dizzy and have not lost their hair. Others think that it is no big deal. In fact, people's ability to withstand electromagnetic radiation varies, and the actual conditions of exposure vary, so it is impossible to draw conclusions about individual symptoms.
People realize that the harm of electromagnetic waves to humans can be traced back to the 1930s. At that time, many high-frequency heating workers showed symptoms such as fatigue; during World War II, radar was widely used, and many people were exposed to microwaves. The first case of microwave cataract was also reported after the war. Unfortunately, people in that era were more concerned about ionizing radiation. Obviously, these reports did not attract enough attention. Organized scientific research activities began in the 1950s. In 1954, the United States mobilized experts from all parties to conduct collaborative research. They conducted huge animal experiments, using radiation intensity greater than 100mW / cm2 (103W / m2). After long-term observation and calculation, the expert door recognized that 10mW / cm2 is the "safe" upper limit.
The former Soviet Union began specialized research in 1953. Unlike the United States, in addition to laboratory work, a large number of them are in-depth epidemiological investigations on the front line of production. It may be that the research methods are different. Almost all American experts believe that electromagnetic radiation first heats the body and then produces other effects (called thermal effects). The former Soviet experts generally believed that low-intensity long-term effects also have serious consequences (called non- Thermal effect). Based on this understanding, the former Soviet Union formulated a health standard that is 1,000 times stricter than that of the United States. China began organized research in 1963. After years of on-site investigations and experiments, the existence of non-thermal effects was determined, and more reasonable standards were formulated.
It is generally believed that the effect of electromagnetic radiation on the human body is directly related to frequency (f), power flow density (Pd), area and location of radiation, and radiation time (t). Due to wave impedance (air).
Power flow density
Equation (2) shows that for plane waves in free space, Pd can be obtained by measuring the field strength E. The unit of Pd is W / m2. For the sake of image, mW / cm2 and μW / cm2 are used in this paper. The conversion relationship is:
1W / m2 = 0.1mW / cm2 = 100μW / cm2 (3)
When it is not easy to test Pd, it can be estimated by the following formula:
(Applicable to far field) (4)
Where G is the transmit antenna gain, P0 is the transmit power, and d is the distance from the test point to the antenna. That is to say, using a handheld to launch, set PO = 5W, G = 2 (about 3dB), standing 3m away from the antenna, the exposure is about 13μw / cm2, this value is basically safe.
If d is small, formula (4) cannot be used. The actual test shows that using a hand-held device (5W) to transmit, when the antenna is 5cm away from the face, the exposure is more than 20mW / cm2, which is a dangerous indicator. For a rough test, another antenna can be used to connect the sensitive power meter. After reading the received power, divide by the equivalent area of ​​the receiving antenna.
Not all power is absorbed by people. For different frequencies, partial reflections will occur at different depths of the body. In contrast, at the low end of VHF and UHF, the higher the frequency, the greater the reflection. In addition, unless the ideal plane wave, the human body exposure is very uneven. As mentioned above, if the disturbance is not considered, only an area facing the antenna is actually radiated by more than 20mW / cm2, so the safety standards can be relaxed.
Strong electromagnetic radiation first causes thermal effects on the body. When using radio equipment to transmit at 30W and bringing your face close to the antenna, you will soon feel warm. At this time, if you quickly leave the irradiation, it may produce weak lens opacity (checked with a slit lamp microscope), and some people will have short-term dizziness and irritability later. All symptoms can disappear within a short time without leaving any sequelae. If you stay in the above environment for a long time, most people will feel dizzy and irritated. Afterwards he felt fatigued, extremely tired, dry mouth and insomnia. Due to temporary edema of the lens, it may be accompanied by vision loss. Unless such radiation is frequent, all symptoms can recover in a short time.
Strong electric waves can also cause changes in body temperature and heart rate. It is generally believed that the reason is that radio waves disrupt the function of the body's temperature regulation center and autonomic nerves. Very strong radio waves can even cause local tissue necrosis. This situation is more likely to occur in the higher frequency microwave section.
Weak electromagnetic radiation (<1mW / cm2) is difficult to cause tissue heating, and the effect cannot be manifested quickly. The time and severity of symptoms are closely related to frequency, intensity, modulation method, and personal tolerance. There are few health studies on amateur bands. In addition to several experimental reports of 144MHz and 470MHz, the documents I have seen are mostly long-wavelength, 900MHz, 1.8GHz and 2450MHz experiments. The epidemiological investigations are especially in the long wave band (industrial heating) and the microwave band. Therefore, the situation mentioned here can only be the situation of the adjacent frequency band and the condition described by some fans.
A lot of research shows that the radiation around 3GHz has the most obvious biological effect. It is also suspected that there are biological effect peaks near 70MHz and 430MHz. However, some people believe that short-wavelength radio waves do not penetrate deep into the body through the skin, mainly acting on the skin's sensation area, causing vagal nerve tension, causing a series of other reactions; longer wavelength decimeter waves can change the function of the central nervous system. Generally speaking, weak signals radiate for a long time, which mainly damages the regulatory function of the midbrain area, and causes hypothalamic dysfunction, which eventually leads to disorder of neuro-humoral regulation. Many facts have proved that radiation can cause pituitary dysfunction and many other nerve stimuli. These changes ultimately affect the entire body and cause a series of symptoms. Fortunately, there is not enough evidence to prove that exposure to radiation for too long (within a few years) can cause organic lesions.
Another study suggests that weak radiation (<1mW / cm2) can cause changes in the blood-brain barrier. This phenomenon is particularly evident in the UHF section. The experiment also found that the biological effect of unmodulated radio waves was significantly smaller than that of audio and sub-audio modulated radio waves. But the report did not specify the modulation method. Changes in the blood-brain barrier are reflected in the increased permeability during irradiation, and many impermeable molecules and drugs can pass through at this time. Although this effect is expected to be used for medical purposes, more people believe that some substances that should not pass through enter the brain, which will affect brain function and cause a series of small reactions in the body. For a long time, radio wave syndrome occurs.
There is still disagreement about the teratogenic and carcinogenic effects of electromagnetic radiation on humans. Beginning in the 1970s, there have been reports of pregnant women having teratogenicity after receiving microwave physiotherapy. However, there are few cases. It is an indisputable fact that men receive medium and strong radio frequency radiation (1 ～ 10mW / cm2, UHF high-end) for a period of time, and their sperm susceptibility is low. It will return to normal weeks to months after the radiation is stopped. Animal experiments confirmed that microwave has both teratogenic and mutagenic effects. Scientists in China have discovered in the 1980s that 2450MHz, short-term and intermittent radiation exceeding 20 mW / cm2 can cause the rat embryo malformation rate to increase significantly, and also induce adult rat somatic cell mutations. At the same time, it was found that the symptoms and radiation showed a dose relationship. In recent years, mobile phones have been popularized on a large scale, and the frequency has also increased. There have been reports of "brain tumors caused by the use of mobile phones" abroad. Also because there are too few cases, it is not enough to conclude. Moreover, the situation of use varies from person to person, and many statements are ambiguous. However, whether you use a mobile phone or a walkie-talkie, it can cause a power flow density of more than 10mW / cm2 in the head. It is dangerous if such exposure is frequent (fever-grade ham and busy business people). Oncologists point out that tissues are exposed to abnormal stimulation for a long time and repeatedly, and the probability of onset is much higher than that without stimulation. Because electromagnetic radiation can cause disturbances in the regulation of neurohumoral fluids, it can also cause organic lesions, including tumors, after more than ten years.
The average person occasionally receives weak electromagnetic radiation (tens of μW / cm2 ～ 1mW / cm2) without causing any symptoms. Long-term continuous exposure, there will be a reaction after a certain time, the threshold is about 20 ~ 50μW / cm2. Of course, there will be changes in EEG and many biochemical indicators before conscious symptoms appear. Patients often present with neurasthenia, including general weakness, insomnia or drowsiness, and memory loss. Among them, the incidence of fatigue and memory loss is high, and individual reports even reach more than 60%. Investigations show that VHF and UHF low-end are more likely to cause symptoms than short-wave band. The threshold for the short wave band may be as high as 100 μW / cm2. Long-term but discontinuous exposure is a common situation among wireless enthusiasts and mobile phone users. This situation is better than continuous irradiation. It is generally believed that the intensity of intermittent irradiation is below 500 μW / cm2, which is safe in the short term (corresponding to the UHF section). For fever-grade ham, the safety value should be lower because of the longer launch time. Deduced from the existing investigation report, it is recommended that the radiation intensity should not exceed 300μW / cm2.
There are very few epidemiological investigations in the amateur radio community. There is no more information here, and I hope readers will understand. It has been reported that exposure to UHF section below 100μw / cm2 intensity can cause abnormal blood pressure. Symptoms usually appear after exposure for 2 to 3 years or longer. Depending on the individual, approximately a similar proportion of people develop hypertension and hypotension, and some individuals have large fluctuations in blood pressure. Therefore, under relatively weak radiation, it can manifest other hazards besides divine decline, so attention must be paid.
There is currently no special amateur hygiene standard. The national standard (GB8702-88) stipulates that the allowable intensity of occupational radiation (an average intensity of any 6min within 24h) is within the range of 3 to 30MHz and is not greater than 60 / f (W / m2) (ie, where f is the frequency) , In the range of 30 ～ 3000MHz, not more than 2W / m2 (); and for non-professional workers such as residents, Pd (3 ～ 30MHz) ≤12 / f (W / m2), Pd (30 ～ 3000MHz) ≤0.4W / m2 (ie 40μW / cm2). Relatively speaking, Czech and Russian standards are dozens of times stricter, while Japanese standards are slightly looser. Considering that most of the symptoms are reversible, and that the improvement of the level of protection requires a process, China has formulated looser standards. In fact, living in 40μW / cm2 irradiation for several years, there must be a lot of people suffering from dizziness. Amateur radio stations work intermittently, and the standard can be relaxed. The author believes that HAM should try to avoid the radiation of more than 50mW / cm2 (equivalent to 30W of the vehicle platform, 15 ~ 35cm of the antenna with G = 3dB distance in the open area), and the radiation of 10mW / cm2 (equivalent to 5W of the hand platform, 6 ~ 15cm distance) In radiation, the time of each operation should not exceed 1min, try not to exceed 20min every day, if it is difficult to "swing" for a long time, and every day, it is recommended to control the radiation to 100 ~ 300μW / cm2 (equivalent to 5W hand table, From 30 to 80cm or more). The intensity of the short band can be greater. From the analysis of the existing reports, the strength of not more than 1mW / cm2 is acceptable, but unless the antenna is placed indoors or on the balcony, it is generally not easy to reach this value. Of course, the values ​​given above are not absolutely safe. Take 100μW / cm2 (300mW for hand, about 22cm). In the 70cm band, if you work 2 hours a day, for 3 consecutive years, more than half of the HAM will feel the situation. It should also be noted that irradiation-induced pituitary and thyroid dysfunction is very harmful to children and adolescents, so there should be more stringent standards.
When we play amateur radio, we must pay attention to environmental protection. Many friends like to put the antenna on the balcony or window sill (for example, on the outdoor unit of the air conditioner). If 25W is used for transmission at this time, the radiation in the next room may already be 100μW / cm2. If the neighbors wake up one day, it may lead to disputes. For short-wave radios with a power of more than 300W, it is best to register with the environmental protection department according to GB regulations, otherwise, if someone reports, "accepting criticism" is inevitable.
When it comes to avoiding adverse effects, that is, protection, I think first of all, we should deepen our understanding and get rid of the luck. Perhaps some readers believe that the symptoms caused by radio waves are generally not organic changes. It is easy to recover after stopping contact, and it is not important to prevent or not protect. Or do you think it ’s neurasthenia? It ’s not a big deal. Besides, many friends have been playing amateur radio for a few years and it ’s no problem. It is true that direct harm does not seem serious at the current level of awareness. When Madame Curie refined radium, there was no protection, and in the end it was nothing. From another perspective, if the "sub-health" symptoms cause other problems, it is obviously not worth it.
Protection should start from the source. The first is to control the power. In the U / V section, the 59+ and 51 signals will not affect the language transmission. So don't blindly pursue good signal reports. The power can be larger during the call. After the report is exchanged, it should be reduced to the minimum power that can complete the communication. Especially for the HAM using a handheld, try to find a better location when connecting, so as to reduce the power. Some portable devices have a minimum power above 0.3W, and those with conditions can adjust the low power below 0.1W. The second is to make a good design for the installation of the antenna feed system, and increase the distance between the antenna and the operator (Pdâˆd-2), especially the vertical distance. The antenna of the vehicle platform can be mounted on the roof of the building, so do not install it on the window sill. It must be installed on the wall on the side of the window and shielded by the wall (depending on the conditions, it can provide 5-25dB attenuation). The feeder should use coaxial cable and be well grounded to reduce leakage energy. When using the handheld, you should develop good habits, ensure that the antenna is more than 20cm away from the body (corresponding to Pd <1mW / cm2 at 5W power), and use an external microphone when possible. Friends who drive must suck the antenna on the roof of the car, not on the front and rear machine covers. It must not be sucked back into the car (due to the shielding of the car body, the radiation may be higher at this time). There are reports that a certain amount of ultraviolet light can enhance the body's tolerance to electromagnetic radiation, so it can appropriately increase outdoor activities and spend more time in the sun.
Finally, I hope that the majority of HAMs will participate in the communication activities in a healthy and happy way. At the same time, it is hoped that qualified members can conduct more amateur radiation radiation studies to make amateur radio more perfect.
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