Time-temperature Thresholds and Safety Factors for Thermal Hazards from Radiofrequency Energy above 6 GHz

Abstract

Two major sets of exposure limits for radiofrequency (RF) radiation, those of the International Commission on Nonionizing Radiation Protection (ICNIRP 2020) and the Institute of Electrical and Electronics Engineers (IEEE C95.1-2019), have recently been revised and updated with significant changes in limits above 6 GHz through the millimeter wave (mm-wave) band (30-300 GHz). This review compares available data on thermal damage and pain from exposure to RF energy above 6 GHz with corresponding data from infrared energy and other heat sources and estimates safety factors that are incorporated in the IEEE and ICNIRP RF exposure limits. The benchmarks for damage are the same as used in ICNIRP IR limits: minimal epithelial damage to cornea and first-degree burn (erythema in skin observable within 48 h after exposure). The data suggest that limiting thermal hazard to skin is cutaneous pain for exposure durations less than ≈20 min and thermal damage for longer exposures. Limitations on available data and thermal models are noted. However, data on RF and IR thermal damage and pain thresholds show that exposures far above current ICNIRP and IEEE limits would be required to produce thermally hazardous effects. This review focuses exclusively on thermal hazards from RF exposures above 6 GHz to skin and the cornea, which are the most exposed tissues in the considered frequency range.

Fig. 1

Temperature-time relations for reversible damage to ocular tissues from RF energy. Studies are Guy et al. (1975; 2.45 GHz, visible cataract, single observations), Chalfin et al. (2002), corneal surface temperatures from single 3-s exposure at 94 GHz; ">Kojima et al. (2009,, , , ED₅₀ for minimal corneal epithelial damage from 6-min exposures at 40, 75, 95, 162 GHz and from 30-min exposures at 75 GHz. Also shown are thresholds for corneal thermal pain measured in four human subjects by Beuerman and Tanelian (1979). Dotted/dashed lines represent exposures in Arrhenius models for ED₅₀ for minimal visible damage to retina and cornea from infrared energy (Jean and Schulmeister, 2017, Jean et al., 2021). The upper exposure durations considered in developing the Arrhenius models are indicated by the short vertical bars on the Arrhenius curves.

Fig. 2

Thresholds for minimal thermal damage to skin and for cutaneous thermal pain. Parker et al. (2016) (single measurements on each of two subjects exposed to 94 GHz RF energy). The thresholds for cutaneous thermal pain sensations are from studies listed in Table 4. Thermal damage data from Moritz and Henriques (1947) for thresholds for erythema in humans from hot water applied to skin, and Suzuki et al. (1991) for superficial skin burns from heat applied via a 7 cm² heated surface to shaved abdomen of rats. The upper exposure duration considered in developing the Arrhenius model for skin is indicated by the short bar.

Fig. 3

Increases in tissue temperature from computer simulations vs. thresholds for thermal hazards. Symbols: maximum fluence pulses allowed by occupational exposure limits in IEEE C95.1–2019 (30–300 GHz), ICNIRP (2020; 6–300 GHz) and FCC (2010) using upper temperatures from exposures to the face from Laakso et al. (2017). Pulse widths are (from left to right) 0.2, 0.5, 1, 2, 5, 10 s at 100 GHz. Horizontal shaded line: approximate steady-state temperature increases at ICNIRP and IEEE limits for occupational exposure (from 1-D model by Sasaki et al. 2017).