Contents Citing support from the U.S. Food and Drug Administration’s Center for Devices and Radiological Health, the U.S. Federal Communications Commission (FCC) announced in December 2019 that it reaffirms the RF radiation exposure limits it first adopted in 1996 [1]. The action was undertaken in the face of appeals from some to tighten, and others to loosen, the existing limits.
In the process, the FCC also resolved and terminated a 2013 Notice of Inquiry that sought public input on whether it should modify its existing RF exposure rules considering recent scientific opinions and authoritative expert views, among other issues [2]. Apparently, six years since the Notice of Inquiry, the FCC deems it appropriate to maintain the existing RF exposure limits. It is interesting to observe that the FCC declined to make changes that would stiffen the current rules or to make any changes that would effectively relax the current rules.
Note that the FCC’s exposure limits are currently specified up to 100 GHz. These limits could, in principle, be applied to the millimeter-wave (mm-wave) bands used for 5G services and to future uses of wireless technologies at even higher frequencies; in fact, the FCC has signified such.
In the recently released Notice of Proposed Rulemaking and Memorandum Opinion and Order [2], the FCC proposes to formalize additional limits on localized RF exposure from devices operating at higher gigahertz frequencies and extend this to terahertz frequencies. It further proposes to extend the same constant exposure limits that presently apply from 6 to 100 GHz up to a maximum frequency of 3,000 GHz (3 THz), which is commonly regarded as the upper bound of the RF bands.
Newer technologies that employ techniques like adaptive array antennas and beamforming create complex electromagnetic fields that present challenges for current RF measurement methods. The FCC’s RF exposure rules do not yet specify a metric or spatial maximum power density limit for localized exposure at higher frequencies. As wireless devices and systems are being developed to operate at higher frequencies for future 5G services in the mm-wave bands, the FCC appears ready to propose a general localized power density exposure limit above 6 GHz of 40 W/m2, averaged over 1 cm2 and applicable up to the upper frequency boundary of 3 THz, for the general population or in cases of uncontrolled exposure. The FCC is currently inviting comments on this proposal [2].
The RF and microwave exposure rules established by the FCC are based on specific absorption rate (SAR) and maximum permissible exposure (MPE) limits [3], [4]. SAR is the accepted metric or quantity corresponding to the relative amount of RF and microwave power deposition/energy absorption in a portion of or in the whole body (i.e., any part of a wireless-device or cell phone-handset user but the entire body when a user is in the radiation domain of a Wi-Fi antenna or base station). The basic restrictions for human exposure are defined by SAR limits. MPE limits are derived from the SAR limits, in terms of free-space field strength and power density.
For exposures from cell-phone-related operations, the FCC specifies a quantity of local tissue SAR of 1.6 W/kg, as determined in any 1 g of body tissue. Also, an average value of 0.08 W/kg in any 1 g of body tissue was set for whole-body exposures.
The FCC rules impose basic restrictions on SAR limits for general public and occupational exposures to avoid whole-body heat stress and excessive localized tissue heating, specifically to prevent biological and health effects in response to an induced body temperature rise of 1°C or more for an average period of 6 min [3], [4]. This level of temperature increase results from individuals’ exposure under moderate environmental conditions to a whole-body SAR of approximately 4 W/kg for about 30 min. A whole-body average SAR of 0.4 W/kg was chosen as the restriction in order to provide protection for occupational exposure. An additional reduction factor of five was introduced for public exposure, giving an average whole-body SAR limit of 0.08 W/kg. This value was purposefully relaxed by a factor of 20 to permit a maximum local tissue SAR of 1.6 W/kg. The power density limits or MPE applicable to general population and occupational exposure for 1.5–100 GHz are 10 W/m2 and 50 W/m2, respectively, for whole-body continuous exposure.
According to the FCC, more than 1,000 comments and representations were filed in response during the six years since the 2013 Notice of Inquiry. It is not surprising to learn that some of the filings urged the FCC to tighten RF exposure limits, whereas others asked for less restrictive limits.
Supporters for stricter RF guidelines include the American Academy of Pediatrics, American Academy of Environmental Medicine, California Brain Tumor Association, Center for Family and Community Health at the University of California Berkeley, and International EMF Scientist Appeal, among others. They have called on the FCC to adopt stronger exposure limits on RF radiation exposure. Many also implored the FCC to impose a moratorium on the wireless industry to pause its deployment of 5G services. The stated reason is that more research is needed due to the paucity of scientific knowledge regarding the effects on human health of much higher RF frequencies and the impact of the ubiquitous small-cell base stations dictated by 5G deployment.
Among those advocating for the FCC to adopt weaker regulatory limits on RF radiation are CTIA–The Wireless Association, the Mobile Manufacturers Forum, the Telecommunications Industry Association, and consultants for the wireless industry. Many of these petitions also contended that the scientific evidence to date suggests that, in terms of health effects, 5G is no different from any other cellular mobile technology and systems deployed to date. Arguments were presented for weakening cell-phone RF exposure limits to peak local SARs at 2 W/kg, averaged over 10 g of tissue (the FCC limit is 1.6 W/kg over 1 g). This larger averaging mass would make the limits less stringent by a factor of two or more.
More specifically, some submissions also expressed opposition to the requirement that cell-phone retailers warn customers about the possible radiation dangers of holding phones close to their bodies. In this regard, it is noteworthy that recently [5] the U.S. Supreme Court rejected a challenge filed by CTIA–The Wireless Association against the “cell-phone right to know” law adopted by the City of Berkeley, California, in May 2015 (see http://bit.ly/berkeleymedia).
The city’s ordinance took effect in 2016. It requires dealers to notify customers of the FCC’s RF radiation standards for cell phones and, specifically, that RF exposure “may exceed the federal guidelines” if users carry a phone in a shirt or pants pocket or tucked into a bra while they’re connected to a wireless network. Furthermore, retailers must display the warning on a poster or in a handout flyer, as attributed to the City of Berkeley.
However, the FCC did accede to treat the pinnae (outer ears) like other extremities of the body for purposes of determining compliance with the FCC’s RF exposure limits, irrespective of petitions that appealed otherwise. As extremities, the pinnae, along with the hands, wrists, feet, and ankles, are subject to less stringent localized RF exposure limits than the rest of the body. For these parts of the human body, the peak spatial-average SAR limit for general population exposure is set at 4 W/kg, averaged over any 10 g of tissue.
It is significant to note that, in affirming treatment of the pinnae as extremities and through associated comments, the FCC acknowledged that its RF radiation exposure limits are based solely on localized thermal effects. Also, the FCC refused to recognize that, unlike the hands, wrists, feet, and ankles, the pinnae are contiguous to the head: any RF-induced field will impact the head and brain directly.
More importantly, as noted previously, the larger averaging mass renders the exposure limits less stringent by a factor of two or more for local SARs averaged over 10 g of tissue. Thus, the 4-W/kg SAR averaged over 10 g is equivalent to a 1-g SAR of 8–12 W/kg in the pinnae or external ear, causing excessive local tissue temperature elevation that is easily masked by a 10-g SAR. Moreover, the mass of pinnae is about 10 g and is geometrically jagged and uneven, which would further accentuate SAR and temperature disparity in causing localized thermal effects.
Recent scientific results on the correlation of SAR with induced tissue temperature elevation, and the dependence on mass of averaging tissue and exposure duration, show that, in general, SAR provides a better correlation with temperature elevation for exposure durations between 1 and 2 min (short durations) at most frequencies used for current wireless technologies [6], [7]. In this case, a mass of 1 g is optimal, but the correlation coefficient remains above 0.9 at 2 min for a 2-g mass.
For longer exposures, the maximum correlation coefficient is reduced, and the correlation favors a larger averaging mass. At steady state (30 min), the correlation of temperature increase with SAR is maximum for a mass of 5–9 g at frequencies below 6 GHz.
However, for exposures at higher gigahertz frequencies (mm-waves and 5G), RF energy absorption tends to be more superficial and concentrated. Energy deposition could occur quickly in a smaller tissue area or mass, causing intense temperature elevation within a very short exposure time period.