by Neil Bauman, Ph.D.
February 28, 2017
An audiologist wrote,
I am an Italian audiologist. Whenever I can, I encourage the use of hearing loops. Recently, the registry office of an important Italian city was interested in installing a counter hearing loop. They wanted me to guarantee that the employees working at the looped counter would not have health problems from being exposed to the hearing loop’s magnetic field all day long. Where can I find information proving that this technology is harmless?
It is hard to prove that something is harmless because harm may be caused at levels of exposure lower than instruments can currently detect and measure. As the World Health Organization (WHO) points out, “Science cannot provide a guarantee of absolute safety.” (1)
That is why they set standards for what they feel is a safe level of exposure. However, as research and advances in measuring techniques reveals new information, the standards that were once considered “safe” may need to be revised. This has happened with exposure to noise levels and exposure to toxic chemicals. So it should be no surprise that it also happens in the standards set for exposure to electromagnetic fields.
Thus, rather than proving that exposure to magnetic fields is harmless, the best we can do at any given time, is show that exposure to magnetic fields does not exceed the best standards (or guidelines) available according to current scientific knowledge.
How do they set these guidelines? Since it is not ethical to use humans as guinea pigs, scientists use animals in their studies and apply the results to humans. They carefully watch for any abnormal behavior. This is because “abnormal behavior in animals is a very sensitive indicator of a biological response and has been selected as the lowest observable adverse health effect. Guidelines recommend the prevention of electromagnetic field exposure levels, at which behavioral changes become noticeable.” (2)
Exposure to low-frequency electromagnetic fields induces currents in the human body. At the same time, various biochemical reactions within the body itself generate currents as well. The cells or tissues will not be able to detect any induced currents below this background level. Therefore, at low frequencies, exposure guidelines ensure that the level of currents induced by an electromagnetic field is below that of natural body currents. (2)
Current electromagnetic field guidelines ensure that within the given exposure limit, no known adverse health effects will occur. Thus, current guidelines indicate that below a given threshold, electromagnetic field exposure is presumed to be safe. (2)
Note that the threshold level for behavior is not equal to the guideline limit. The International Commission on Non-Ionizing Radiation Protection (ICNIRP) applies a large safety factor to the level known to cause a health consequence. For example, “they apply a safety factor of 10 to derive occupational exposure limits, and a safety factor of 50 to obtain the guideline value for the general public.” (2)
Therefore, even if you experience a field strength several times higher than the guidelines permit, your exposure would still be within the safety margin. (2)
Now let’s see how this all applies to the magnetic fields produced by hearing loops.
First some technical details.
Magnetic induction (the strength of the magnetic field or flux density) is often measured in units of Tesla (T), milliTesla (mT) and microTesla (µT). 1 T = 1,000 mT = 1,000,000 µT.
You can also measure magnetic field strength in decibels, Gauss and Amperes/meter. There is a mathematical relationship between these four units so you can readily convert between them. For example, hearing loops are set to have a peak level of 400 mA/m which is another way of saying they are set to 0 dB, 5.030 mG (milligauss) or 0.503 µT.
Incidentally, in the USA, the strength of hearing loops is measured in decibels (dB) whereas in Europe they may use microTesla (µT).
How strong is a hearing loop’s magnetic field? Compared to the earth’s magnetic field which varies between 35 µT and 70 µT (36.8 dB and 42.8 dB) (3) the maximum (peak) strength of a properly-calibrated hearing loop (0 dB or 0.503 µT) would be 69.6 to 139.2 times weaker.
The average strength of a hearing loop is -12 dB (0.126 µT). This is 277.8 to 555.5 times weaker than the earth’s magnetic field. Thus the magnetic field in a hearing loop is always much less than the earth’s magnetic field strength which doesn’t bother us.
The guideline for continuous exposure to static magnetic fields for the general public is a maximum of 40 mT. (3) Note that 40 milliTesla is equal to 40,000 microTesla. Thus the maximum loop magnetic field strength of 0.503 µT is almost 80,000 times less than the static exposure standard.
Magnetic fields are created whenever current flows. The greater the current (measured in Amperes [A] or milliamperes [mA]), the stronger the resulting magnetic field is. (4)
Magnetic fields can be static (constant like the earth’s magnetic field or in a permanent magnet) or pulsing (alternating or constantly changing). The magnetic field caused by the standard 60 Hertz (50 Hz in Europe) current in our houses constantly changes from a positive peak to a negative peak 60 times each second.
Human ears are capable of hearing frequencies between 20 and 20,000 Hz. Since hearing loops operate within this range, they are considered low-frequency (audio-frequency) devices.
Fields of different frequencies interact with the body in different ways. (4) The good news is that lower-frequency magnetic fields are less harmful to the body than high-frequency fields such as radio waves and microwaves.
Tiny electrical currents exist in our bodies due to the chemical reactions that take place as part of our normal body functions. (5) Low-frequency magnetic fields induce circulating currents within our bodies. If they are sufficiently large, they can affect biological processes in our bodies.(5)
As you might expect, magnetic fields are strongest close to their origins (in this case, the hearing loop wire) and rapidly decrease in strength as the distance from the wire increases. (4)
We cannot get away from the varying magnetic fields around us. They are everywhere there is electricity. Since a magnetic field is generated whenever current flows, and all our homes and businesses are wired for electricity, we are exposed to magnetic fields whether we are using home appliances or are using wireless devices such as cell phones and radios. Thus, we are all exposed to a complex mix of magnetic fields whether we are at home, at work or at play.
For example, in homes not located near power lines, the background magnetic field may be up to about 0.2 µT. (6)
Since most household appliances are not operated very close to the body, the magnetic field strength, even at a distance of just 1 foot (30 cm), is more than 100 times lower than the current exposure guideline limit of 100 µT at 50 Hz (83 µT at 60 Hz) for the general public. (6)
As we have seen, the maximum exposure to a properly-calibrated hearing loop, when you are inside the loop, is just 0.503 µT. Outside the loop, the strength of the magnetic field quickly drops with increasing distance.
For a person standing 18 inches (50 cm) from a counter loop, the magnetic field strength drops from a peak of 0.503 µT (0 dB) and an average of 0.126 µT (-12 dB) to around a peak of 0.159 µT (-10 dB) and an average of around 0.02 µT (-28 dB).
For computer operators sitting at a distance of 1 to 1½ feet (30 to 50 cm) from the screen, alternating magnetic fields are typically below 0.7 µT. (6)
Thus, a person working at a counter containing a hearing loop is exposed to far less of a magnetic field from the hearing loop than he is being exposed to by his computer monitor (that is considered safe to operate all day).
Therefore, being around a counter loop is no more hazardous to your health that being around any of the many electrical devices you are exposed to all day long every day.
Based on their recent in-depth review of the scientific literature, the World Health Organization (WHO) concluded that current evidence does not confirm the existence of any health consequences from exposure to low-level electromagnetic fields. (5) This includes hearing loops.
(1) World Health Organization (WHO) – Electromagnetic Fields (EMF). Progress in Research. 2002.
(2) World Health Organization (WHO) – Electromagnetic Fields (EMF). Current Standards. 2002.
(3) World Health Organization (WHO) – Electromagnetic Fields and Public Health. 2006.
(4) World Health Organization (WHO) – Electromagnetic Fields (EMF). Definitions and Sources. 2002.
(5) World Health Organization (WHO) – Electromagnetic Fields (EMF). Summary of Health Effects. 2002.
(6) World Health Organization (WHO) – Electromagnetic Fields (EMF). Typical Exposure Levels at Home and in the Environment. 2002.