THE IMPORTANCE OF ELECTROMAGNETIC RADIATION SAFETY MEASUREMENTS FOR PUBLIC AREAS AND PLACES OF WORK
THE IMPORTANCE OF ELECTROMAGNETIC RADIATION SAFETY MEASUREMENTS
FOR PUBLIC AREAS AND PLACES OF WORK
By: Vaughan Taylor Pr. Tech Eng.
Transmission Specialist, LS of South Africa (Pty) Ltd
1. INTRODUCTION
It is difficult to even begin to imagine what the world would be like without ubiquitous mobile and smartphone devices which make extensive use of the Radio Frequency (RF) spectrum (and in turn, generate electromagnetic waves) for establishing protocols for voice and data communication channels. According to Statista[1] – the current number of smartphone users in the world today is around 6.6 billion – which translates to about 84% of the world’s population therefore own a smartphone!
Electromagnetic radiation (or EMR) consists primarily of waves of an electromagnetic field (EMF) propagating through space, carrying electromagnetic energy. Whilst the subject of electromagnetic radiation comprises of a broad ‘spectrum’ (pun unintended) of radio waves, microwave, infrared, visible light, ultraviolet, X and gamma rays – our specific interest is therefore in the application of non-ionising EMR being in the form of radio waves. Traditionally RF spectrum has been largely used for radio and television broadcast/strategic communications services but with the sharp increase of mobile and other devices requiring access to the Internet, there has been a proliferation in the rollout of base stations for these technologies in both urban and rural locations.
These are unlike high-power terrestrial FM/TV transmitter facilities, mainly as they provide coverage within ‘cellular’ areas often targeting these with the use of high-gain sector-type antenna systems. The level of EMF emission from a particular base station is non-constant and in essence depends on the level of ‘traffic’ being handled, along with the subscriber/s distance from the base station.
Whilst it is accepted that the base stations may add little to the exposure that we are already receiving from existing TV and radio transmitting station towers, there is considerable merit in conducting such analyses within a given environment (both within public and occupational workspaces) to ensure compliance with the applicable standards.
The coupling of electromagnetic energy into the human body occurs primarily through absorption at frequencies above 100 kHz. Heating of the human body occurs at frequencies of 10 MHz to 300 GHz and temperature rises in excess of 1 to 2°C can have adverse effects such as heat exhaustion and heat stroke. As the human body is comprised of at least 70% water, the molecules have a dipole characteristic and therefore can align in an electric field. However in an RF field, these ‘dipoles’ are permanently realigned and absorb energy which gives rise to the heating effect.
2. REVIEW OF STANDARDS
Countries set their own national standards for exposure to EM fields. However – it has been observed that these standards draw upon the guidelines as set by the International Commission on Non-Ionizing Radiation Protection (or ICNIRP).
ICNIRP is a non-governmental, scientifically independent body (recognised by the World Health Organisation) that produces guidelines for limiting the exposure of persons to electromagnetic fields, based upon factual and scientific evidence from its membership. The limit value recommendations as published by ICNIRP serve for the protection against the detrimental effects on health as a result of electric, magnetic, and electromagnetic fields (EMF).
The ICNIRP (2020) version may be downloaded through following the link as provided https://www.icnirp.org/cms/upload/publications/ICNIRPrfgdl2020.pdf. [3]
ICNIRP therefore defines two types of limit values (i.e. for general public and occupational exposure/s), basic restrictions and reference levels.
Specific absorption rate (SAR) is a measure of the rate at which energy is absorbed per unit mass by a human body when exposed to a radio frequency (RF) electromagnetic field which in this case, is a smartphone or other wireless device. It is defined as power absorbed per mass of tissue, with a value expressed in Watts/kg. SAR is thus the physical quantity for defining the basic restrictions within the high frequency (i.e. 10MHz and above) range.
SAR therefore provides a means for measuring the RF exposure characteristics of a mobile phone under controlled conditions to ensure that the devices are within the safety guidelines set by the applicable regulatory body. The SAR values of type-approved mobile devices are published by the relevant manufacturers. A whole body SAR of 1-4 W/kg over a period of 30 minutes leads to a temperature increase of less than 1°C
As it is a complicated process to measure SAR outside of controlled laboratory conditions, it is a given that ICNIRP defines ‘reference values’, i.e. which are for fields (quantified as the electric/magnetic field strength [E/H] and power density as [S]) outside the body. These reference values (or levels) are therefore specified as the maximum interaction between the external field and the person/s exposed to the EMR.
It is important at this juncture to differentiate between the terms ‘emission’ (source of exposure) and ‘immission’ (the actual exposure) respectively. Therefore ‘emission’ monitoring measures the power being fed into a source and thus the EMF as is being developed (in close proximity to the source). Immission (or ambient exposure), records the resultant fields (E/H) as are produced in the wider area of the source. Typically these ambient exposures are comprised of more than just of a single source. It is appropriate for all sources as identified using frequency selective measurement process to be combined so as to produce the total immission value.
Table one (below) provides the basic restrictions as specified by ICNIRP for occupational and general public exposure levels, for the whole body, for the frequency ranges 100 kHz to 10 GHz respectively. Note that the limit values for the general public are considerably lower than those provided for occupational exposure – the reason for this is through age and health differences between the two groups.
As the ICNIRP guide concentrates on emissions of RF transmissions operating from 10MHz and above, and thus the recommendations apply only to the ranges 10 MHz to 10 GHz unless otherwise stated.
It is important to note that parts of the human body can receive EM exposure levels as dependent on particular situations, i.e. the head is mostly affected by such a field, though the eyes may react with higher sensitivity to such exposures. Of importance is that ICNIRP specifies that the localised exposure of body parts (i.e. trunk, head and limbs) is greater than that of the overall body exposure. Note that for the purposes of this exercise – the determination of such values are done only for entire body exposures.
In Tables two and three (below) – the ICNIRP-expressed reference levels are provided for general public and occupational exposure/s for the frequency range 10 MHz to 10 GHz.
For an in depth discussion of the above reference levels, the reader is advised to consult ICNIRP 2020 guide. However – the following pointers are considered to be most pertinent;
· The measured field strength units (i.e. E, H and S) are stipulated as RMS values.
· Reference levels are to be stated as unperturbed values (meaning that these are to be determined with the absence of the person at the location).
· For 100 kHz to 10 GHz - The values of E2, H2 and S are to be averaged over any six minute period
· In the far field – the evaluation of E, or H or S are considered to be sufficient
· Reference levels are intended to be spatially averaged over the entire body of an individual (however the local SAR cannot be exceeded).
The graphic below shows the applicable ICNIRP V/m reference levels over the stated frequency range of 100 kHz to 10 GHz.
3. RECOMMENDED PRACTICES FOR SITE SURVEYS
It follows that the planning and preparations of conducting such measurements are directly proportional to the outcome of the results that are achieved. Of equal importance is that the measurement points as selected must be representative of the site under investigation. It is highly desirable to capture the maximum power values though this is not always practically achievable. In such cases, interpolation is required for determination that which is measured instantaneously to reflect the maximum immission value. As these measurements have health and safety ramifications and if these are not treated with the appropriate care – there can be both financial and legal consequences should such measurements be not correctly characterised.
4. RECOMMENDED MEASUREMENT PRACTICES
The measurements as are conducted must offer sufficient selectivity between high and low level sources of emissions. One must therefore discriminate between the emissions as generated for example by base stations, localised radio/television transmitting stations and that generated by end-user (terminal equipment) such as a mobile phone. In carrying out a preliminary measurement of the spectrum environment, it is most important to identify the frequencies (particularly those which are in the range 400 MHz to 2 GHz) as their limit values are frequency dependent and these will need correlation as per the information provided in Tables 2 and 3 (shown earlier in the text). It is impossible to simply carry out a broadband measurement with the understanding that this will provide a measure of the degree of the immissions which are present.
Moreover – it is expected that the measurement engineer must understand the nature of the signals that are being evaluated, e.g. requiring of specialised ‘code-selective’ techniques to be applied for example to UMTS (3G), LTE (4G) and 5G technologies.
For the most accurate measurements of EMF to occur – it is recommended that the ‘sweeping method’ be used to establish the maximum values of emission from the source of the EMF.
Whilst the approach for measurement of EMF is typically through use of a calibrated antenna and spectrum analyser, there are various commercially available instruments (such as those from vendors being Narda STS, Rohde & Schwarz and Anritsu) which fulfil the requirements and allow the measurements to be undertaken with accuracy and repeatability.
BIBLIOGRAPHY
1. RF Electromagnetic Radiation – Theory and Measurement Practices. Course presented at LS Telcom AG, Lichtenau. Christian Bornkessel and Prof Dr. Ing. Matthias Wuschek. February 2019
REFERENCES
[1] https://www.statista.com/statistics/330695/number-of-smartphone-users-worldwide/
[2] https://www.icnirp.org/cms/upload/publications/ICNIRPrfgdl2020.pdf