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Frequency range and use
High Frequency (HF) is the term used to describe that part of the electromagnetic spectrum comprising the frequency range from 100 kHz to 300 GHz. At high frequency, the electric and the magnetic fields, which together make up the electromagnetic field, are interrelated and considered jointly for measurements. HF field exposure is usually measured in watts per square meter (W/m2).
HF fields are used in a variety of technologies, most widely for communication purposes (e.g. mobile phones, base stations, Wi-Fi, radio, TV, security devices), and also in medicine (e.g. Magnetic Resonance Imaging (MRI) equipment) and for heating purposes (e.g. microwave ovens).
HF effects on the body and health implications
The critical effect of HF exposure relevant to human health and safety is heating of exposed tissue. HF fields can penetrate into the body (the higher the frequency, the lower the penetration depth) and cause vibration of charged or polar molecules inside. This results in friction and thus heat.
The body can accommodate a small increase in heat, in a similar way that excess body heat is dissipated when performing sporting activity. This is because the human body has a strong ability to regulate its internal temperature. However, above a certain level (referred to as the threshold) depending on the duration of exposure, HF exposure and the accompanying temperature rise can provoke serious health effects, such as heatstroke and tissue damage (burns).
Acute and long-term effects of HF exposure below the thermal threshold have been studied extensively without showing any conclusive evidence of adverse health effects.
A considerable amount of research has been conducted on the relationship between HF fields and health outcomes such as headaches, concentration difficulty, sleep quality, cognitive function, cardiovascular effects, etc. This research to date has not shown any such health effects. The only consistently observed finding is a small effect on brain activity measured by electroencephalography (EEG). The biological implication of these small changes is, however, unclear. For example, they have not been shown to affect sleep quality or be associated with any other adverse effects.
Extensive research has been undertaken in relation to exposure to HF fields used specifically in mobile telephony. Among all of this research, the risk of tumors in close proximity to the ear where the phone is held, e.g. brain tumors, has been the focus of numerous epidemiological studies. A few of these epidemiological studies have reported a slight statistical increase in risk of some brain tumours for the small group of long-term and heavy mobile phone users (read more). Reporting biases and weaknesses of the studies may explain the observed findings. Several studies have not reported any increase in brain tumors with mobile phone use. Also, experimental studies on animals and cells have failed to confirm the findings of the epidemiological studies, and there is no biophysical mechanism that could explain carcinogenicity at such low exposure levels. In addition, the increased risk observed in some of the epidemiological studies is inconsistent with the stable frequency of occurrence of these cancers in the population. That is an important consideration, given the widespread and significant increase in the use of mobile phones in the general population during the last few decades.
The overall evaluation of all the research on HF fields leads to the conclusion that HF exposure below the thermal threshold is unlikely to be associated with adverse health effects.
To avoid hazards to health and prevent adverse interaction with high frequency fields (i.e. to prevent whole-body heat stress and excessive localized heating), ICNIRP recommends limiting the exposure to HF so that the threshold at which these interactions become detrimental is never reached. The exposure limits, called basic restrictions, are set in relation to the threshold known to show adverse effects, with an additional reduction factor to take care of scientific uncertainties pertaining to the determination of the threshold. The basic restrictions are generally expressed in terms of the specific absorption rate (SAR). Distinct SAR values are recommended for sources operated close to the body and those operating at a remote distance. Exposure levels outside the body, called reference levels, are derived from the basic restrictions for whole-body exposure using worst-case assumptions, in such a way that remaining below the reference levels (in air) implies that the basic restrictions will also be met (in the body).