Analysis of recommended localized human exposure limits for radiofrequency fields in the frequency range from 6 GHz to 300 GHz

Health Canada has developed an Executive Summary of its analysis of recommended localized human exposure limits for radiofrequency fields in the frequency range from 6 GHz to 300 GHz. The Executive Summary describes the methodology and findings of Health Canada's approach for developing the recommendations, which were published in January 2021.

Executive summary

To help protect the health and safety of Canadians from radiofrequency electromagnetic fields (RFEMF), Health Canada continuously monitors and assesses the scientific literature, conducts research and provides recommendations on safe human exposure levels in its publication Safety Code 6 - Limits of Human Exposure to Radiofrequency Electromagnetic Energy in the Frequency Range from 3 kHz to 300 GHz. The Code is periodically revised to reflect new knowledge in the scientific literature. The current version of Safety Code 6 includes exposure limits applicable to radiofrequency fields above 6 GHz which are based on whole body exposure scenarios (ie. the RF field exposes the entire body). Since forthcoming technologies will begin to utilize frequencies above 6 GHz for devices which may be held close to the body, Health Canada deemed it necessary to provide recommendations for localized human exposure limits in this frequency range. Localized human exposure limits are based on exposure scenarios where the radiofrequency source is held close to the body and only a small area is exposed. Two international organizations, the International Commission on Non-Ionizing Radiation Protection (ICNIRP) and the Institute of Electrical and Electronics Engineers (IEEE) have recently published guidelines with new localized exposure limits in the 6 to 300 GHz frequency range (ICNIRP, 2020; IEEE, 2019). Health Canada has evaluated these new safety limits as part of its process to develop an official Health Canada recommendation for localized exposures in this frequency range.

In this report, the scientific basis supporting the ICNIRP (2020) guidelines for localized exposure limits in the 6 to 300 GHz frequency range has been summarized to provide context. The guidelines include new quantities for evaluating localized exposures to frequencies above 6 GHz for basic restrictions, namely the absorbed power density applicable to continuous exposures and absorbed energy density for pulsed exposures. ICNIRP (2020) also specifies a new spatial averaging area of 4 cm2 for localized exposures above 6 GHz and an additional spatial averaging area of 1 cm2 for frequencies above 30 GHz to account for smaller beam diameters that can be produced by devices emitting higher frequencies to address emerging technologies. The IEEE (2019) standard introduced a new quantity, the "epithelial power density", for evaluation against dosimetric reference levels (akin to basic restrictions) and specifies the same spatial averaging scheme as ICNIRP (2020) and the same localized exposure limits for continuous exposures. However, the IEEE (2019) standard specifies different requirements than ICNIRP (2020) for pulsed exposures. IEEE (2019) specifies a peak power density limit for a reference window of 100 ms and a fluence limit (energy density limit per pulse) that is applicable above 30 GHz. The latter appears to be excessively restrictive and has been rejected because it does not behave according to the accepted Pennes heat exchange model. Aside from the fluence limit, ICNIRP (2020) provides safety limits that are more conservative than IEEE (2019). Therefore, the detailed analysis in this report focuses on the ICNIRP (2020) guidelines.

Based upon a systematic review approach, Health Canada has identified two adverse health outcomes that are relevant to localized exposure to millimeter-wave RFEMF. These are a heat-pain sensation, which demonstrates an absolute threshold temperature of ~42-43 °C, and tissue damage which can occur when skin or cornea are heated and maintained at temperatures at or above 43 °C. Other adverse health effects are theoretically possible if localized exposure to millimeter-wave RFEMF heats the core body temperature by more than 1 °C, however such effects are unlikely to occur from millimeter-wave RFEMF without first exceeding the heat-pain sensation temperature threshold in Type 1 tissues as most energy will be deposited in superficial tissues due to the limited penetration depth of millimeter-wave RFEMF. Therefore, Health Canada is in agreement with ICNIRP that the primary adverse health effects to be avoided as millimeter-wave RFEMF intensity increases are a heat-pain sensation and thermal tissue damage to Type 1 tissues (e.g. skin/cornea).

Based upon an analysis of millimeter-wave RFEMF studies and complementary research (e.g. hyperthermia and non-RFEMF tissue heating) on temperature thresholds for the occurrence of heat-pain sensation and thermal tissue damage (Section 4.2), Health Canada concludes that the ICNIRP 'Operational Adverse Health Effect Threshold' (OAHET) of 41 °C is a conservative estimate of the minimum temperature where adverse health effects (heat-pain sensation or thermal tissue damage) may occur. Based upon an analysis of complementary evidence of resting (normothermal) Type 1 tissue temperatures, Health Canada considers the normothermal temperature of skin and cornea to range between 33-36 °C. Therefore, Health Canada considers the ICNIRP OAHET value of 41 °C to be at least 5 °C above normothermal Type 1 tissue temperatures. With the application of 10-fold and 2-fold safety margins in the ICNIRP (2020) localized exposure limits for the uncontrolled and controlled environments, ICNIRP intends to limit the maximum localized tissue temperature increase at the maximum allowable exposure limits to 0.5 °C and 2.5 °C, respectively. These temperature increases are well below the threshold for all known adverse health effects from millimeter-wave RFEMF. It is important to note that modest temperature increases (1-2 °C) to Type 1 tissues are routinely experienced in our daily lives from a variety of heat sources. Small deviations from the targeted tissue temperature elevations under unique/exotic exposures scenarios will still maintain Type 1 tissue temperature elevations below the threshold for adverse effects.

To assess if the recommended localized exposure limits above 6 GHz specified in ICNIRP (2020) result in localized tissue temperatures below the OAHET and respect the intended conservativeness (i.e. provide the intended reduction factors), numerical modelling was employed using an approximate Gaussian beam model. This model can be used to estimate temperature increases in human tissues by: i) determining the wave propagation across multiple tissue layers by assuming the incident EMF is a plane wave, from which the absorbed EMF is calculated as a function of depth in the tissue, ii) accounting for the effect of a finite beam diameter by multiplying the depth distribution of absorbed EMF by a Gaussian transverse distribution, and iii) solving the Pennes Bio Heat Transfer Equation in all layers by considering the effect of heat diffusion, heat transport by blood perfusion and convective heat loss at the air-skin boundary. As a representation of human superficial tissues: a 3-layer model composed of skin, subcutaneous adipose tissue (SAT) and muscle was considered for frequencies between 6 GHz and 60 GHz and a 4-layer model composed of epidermis, dermis, SAT, and muscle was considered for frequencies above 60 GHz up to 200 GHz. The numerical model allowed an evaluation of the impact of time- and spatial-averaging, for both continuous and pulsed exposures.

Through application of the numerical model, temperature estimates indicating a temperature increase higher than ICNIRP's OAHET were considered non-conservative, otherwise the limits were considered conservative. This analysis was done for a variety of beam diameters, frequencies and exposure durations. The results demonstrated that ICNIRP's localized exposure limits in the 6 to 300 GHz frequency range were not sufficiently conservative for all exposure scenarios. The level of non-conservatism was especially pronounced for short pulse exposures at frequencies of 30 GHz or higher and with small beam diameters. Under certain worst-case exposure conditions, tissue temperature elevations were estimated to be as much as 3.57 times higher than ICNIRP's target temperature increases of 0.5 °C and 2.5 °C for uncontrolled and controlled environments, respectively, translating to a localized tissue temperature increase of ~1.79 °C for uncontrolled environment exposures and ~8.93 °C for controlled environment exposures. In addition, for the controlled environment (e.g. occupational), temperatures could exceed the OAHET of 41 °C and could possibly lead to a heat-pain sensation or tissue damage. It is important to note that the worst-case exposure conditions that resulted in these temperatures elevations can be addressed by modifications to the ICNIRP (2020) localized exposure limits.

Based upon the observed non conservatism in the ICNIRP (2020) localized exposure limits under certain exposure scenarios, Health Canada recommends the application of the ICNIRP (2020) localized exposure limits with some modifications. These modifications are intended to help ensure that localized tissue temperature increases from exposures to RFEMF are kept well below the scientifically established thresholds of health effects. If Health Canada's recommended modifications to the ICNIRP localized exposure limits are applied, the associated maximum (worst-case) tissue temperature increases would be ~0.77 °C for uncontrolled environment and ~3.85 °C for controlled environment, which are below the threshold of all established adverse health effects for localized exposure to radiofrequency fields in the 6 to 300 GHz frequency range.

Recommendations:

For localized exposure to RFEMF at frequencies in the 6 to 300 GHz frequency range, Health Canada recommends using the:

The recommendations above can be reproduced in a simplified table format where the information in ICNIRP pertaining to frequencies below 6 GHz can be removed because it was not subject to this evaluation:

Table 1: Basic restrictions for local electromagnetic field exposure above 6 GHz up to 300 GHz
Exposure Scenario Exposure Duration
(t)
Local Absorbed
Energy Density
[ kJ/m2]
Local Absorbed
Power Density
[ W/m2]
Controlled Environment 0 sec < t < 360 sec 36 [0.05+0.95(t/360)0.5] n/a
t ≥ 6 min n/a 100
Uncontrolled Environment 0 sec < t < 360 sec 7.2 [0.05+0.95(t/360)0.5] n/a
t ≥ 6 min n/a 20

Notes:

  1. "n/a" signifies "not applicable" and does not need to be taken into account when determining compliance.
  2. "t" is time in seconds, and restrictions must be satisfied for all values of t between >0 s and <360 s, regardless of the temporal characteristics of the exposure itself.
  3. Local absorbed power density exposures are to be averaged over 6 min.
  4. Local absorbed power density is to be averaged over a square 4-cm2 surface area of the body. Above 30 GHz, an additional constraint is imposed, such that the spatial peak exposure is restricted to two times that of the 4-cm2 restriction.
  5. Local absorbed energy density is to be averaged over a square 4-cm2 surface area of the body. Above 30 GHz, an additional constraint is imposed, such that the spatial peak exposure is restricted to 72[0.025+0.975(t/360)0.5] kJ/m2 for controlled environment and 14.4[0.025+0.975(t/360)0.5] kJ/m2 for uncontrolled environment exposure.
  6. Exposure from any pulse, group of pulses, or subgroup of pulses in a train, as well as from the summation of exposures (including non-pulsed EMF), delivered in t s, where t is the sum of all periods in which there is non-zero exposure, must not exceed these levels.
Table 2: Reference levels for local electromagnetic field exposure above 6 GHz up to 300 GHz
Exposure Scenario Exposure Duration
(t)
Local Incident
Energy Density
[ kJ/m2]
Local Incident
Power Density
[ W/m2]
Controlled Environment 0 sec < t < 360 sec 275/fG0.177 X 0.36[0.05+0.95(t/360)0.5] n/a
t ≥ 6 min n/a 275/fG0.177
Uncontrolled Environment 0 sec < t < 360 sec 55/fG0.177 X 0.36[0.05+0.95(t/360)0.5] n/a
t ≥ 6 min n/a 55/fG0.177

Notes:

  1. "n/a" signifies "not applicable" and does not need to be taken into account when determining compliance.
  2. fG is frequency in GHz; t is time interval in seconds, such that exposure from any pulse, group of pulses, or subgroup of pulses in a train, as well as from the summation of exposures (including non-pulsed EMF), delivered in t seconds, where t is the sum of all periods in which there is non-zero exposure, must not exceed these levels.
  3. Incident energy density is to be calculated over time t.
  4. Incident power density is to be averaged over 6 min.
  5. For frequencies of >6 GHz to 300 GHz: (a) within the far-field zone, compliance is demonstrated if the incident power density, averaged over a square 4-cm2 projected body surface space, does not exceed the above reference level values; plane-wave equivalent incident power density may be substituted for the incident power density; (b) within the radiative near-field zone, compliance is demonstrated if the incident power density, averaged over a square 4-cm2 projected body surface space, does not exceed the above reference level values; and (c) within the reactive near-field zone reference levels cannot be used to determine compliance, and so basic restrictions must be assessed.
  6. For frequencies of >6 GHz to 300 GHz: (a) within the far-field or radiative near-field zone, compliance is demonstrated if the incident energy density, averaged over a square 4-cm2 projected body surface space, does not exceed the above reference level values; (b) within the reactive near-field zone, reference levels cannot be used to determine compliance, and so basic restrictions must be assessed.
  7. For frequencies of >30 GHz to 300 GHz, the spatial peak incident power density exposure must not exceed twice that of the square 4-cm2 restrictions.
  8. For frequencies of >30 GHz to 300 GHz, the spatial peak incident energy density exposure must not exceed 275/fG0.177 X 0.72[0.025+0.975(t/360)0.5] kJ/m2 for controlled environment and 55/fG0.177 X 0.72[0.025+0.975(t/360)0.5] kJ/m2 for uncontrolled environment exposure.
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