Limits of Human Exposure to Radiofrequency Electromagnetic Fields in the Frequency Range from 3 KHZ to 300 GHZ - Safety Code 6

Appendix VII Rationale for Changes to the 1991 Version of Safety Code 6

1. Portable Transmitters and Exemption Clause

These devices are normally used in close proximity to the human body (within 2.5 cm), and therefore direct measurements of electric and magnetic fields do not give a reliable exposure level. In the 1991 version of Safety Code 6, portable transmitters operating below 1 GHz with radiated power of 7 W were exempt from compliance with the SAR requirements specified in Sections 2.1 and 2.2, regardless of the distance.

Recent scientific information ⁽⁴²⁾ indicates that certain devices (e.g., cellular telephones), which were previously exempt, produced exposures in excess of the SAR limits specified in Section 2.2. Thus, the exemption clause stated in the previous Safety Code 6 has been removed.

2. Time Averaging

The time factor of 0.1 h (6 min) may be considered as a thermal time constant. An exposure time of 0.1 h or longer will allow such mechanisms as heat conduction, blood circulation and thermoregulation to influence the temperature rise in a given body region ⁽⁴³⁾.

At frequencies above approximately 15 000 MHz, the time period over which exposure is averaged needs to be reduced to prevent skin burns. As the frequency increases, the power from RF fields is deposited within an increasingly thin layer of skin and subcutaneous tissue. The new limits bring the maximum allowed level at 300 000 MHz (wavelength 1 mm) into agreement with that specified in the ANSI laser standard at the same frequency⁽⁴⁴⁾, in accordance with the CENELEC proposal⁽⁴⁵⁾. Since at frequencies above 15 000 MHz all power is superficially deposited in the human body, considerations used in the derivation of exposure limits at lower frequencies (see Section 2) become less relevant. At the same time, skin burns become the limiting factor for both occupational as well as other exposures.

The 0.1 h time period over which exposure is averaged is not valid for the case of induced and contact currents at frequencies lower than about 100 kHz, where RF shock or burn may be possible. For the purpose of determining compliance with the limits specified in Sections 2.1.3 and 2.2.3, the currents should be measured with an instrument having an averaging time no greater than 1 second.

3. Peak Field Strength Limit

The electric field limit of 100 kV/m for single and multiple pulses is recommended for prevention of air breakdown and spark discharges. Additional clauses with respect to pulsed RF fields, similar to those in IEEE Std C95.1-1991 - 1999 Edition, have been added in Appendix III. The rationale for these clauses refers to auditory effects and energy-induced unconsciousness in rats. The basis for this limit is to reduce the energy absorption for each of the pulses to 20% of the energy absorption allowed under continuous wave exposures.

4. Contact Current Limits

The revised limits in this Code provide the same health and safety protection as the previous limits. The differences in numerical values reflect a different protocol for measurements in the establishment of conformity to the Code. These revised limits are identical to those in the IEEE Std C95.1-1991 - 1999 Edition standard. The selection of a specific formulation of the clauses and test procedures in the ANSI standard have resulted in the development of commercial instruments suitable for conformity testing. The circuits recommended in Appendix IV Part C of the previous Safety Code 6 have not been produced commercially and, therefore, are removed from this document. Instead, the clauses and test procedures in IEEE Std C95.1-1991 - 1999 Edition are recommended to facilitate conformity evaluation.

5. Induced Current Limits

Currents which are induced in the body by RF fields may flow through the body to ground. These limits have therefore been introduced in order to minimize shocks and burns due to RF field exposure, even though the measured fields are within the exposure limits specified in Sections 2.1.1 and 2.2.1⁽⁸⁾.

The perception of currents at frequencies lower than 70-100 kHz is quite different than that from currents at higher frequencies. While the perception is one of tingling or pricking sensation at frequencies lower than about 100 kHz, the sensory perception is that of warmth at the higher frequencies⁽⁴⁶⁾. Different current averaging times are, therefore, suggested for the induced and contact current limits given in Tables 3 and 7.

6. SAR Limits

In the 1991 version of Safety Code 6, Section 2.1.2, the limit for the SAR at the body surface and in the limbs (averaged over 10 g of tissue) was 25 W/kg. In response to the latest scientific information^{( 4,5)}, the limit has been reduced to 20 W/kg. Also in the 1991 version of Safety Code 6, the power density limits for the general public were set at one-fifth of the values for RF and microwave exposed workers. For the sake of consistency, the SAR limits for the general public (Section 2.2.2) have been adjusted to one-fifth of the values given in Section 2.1.2. The SAR limits for the eye, specified in the 1991 version of Safety Code 6, were based on the experimental data reported by Kues et al.⁽⁴⁷⁾. This report described that ocular exposure of anesthetized primates to 100 W/m2 pulsed fields at 2.45 GHz (SAR = 2.6 W/kg) and 200 W/m2 continuous-wave field (SAR = 6.3 W/kg) for 4 h/d resulted in endothelial cell damage in the primate corneas. Although the changes appeared reversible, some endothelial cell loss was reported. Further investigations by Kues et al.⁽⁴⁸⁾ indicated that 2.45-GHz pulsed microwaves, if administered after pretreatment with ophthalmic drugs, could produce ocular effects at a significantly lower SAR level. However, Kamimura et al.⁽⁴⁹⁾ reported that ocular exposure of anesthetized primates to 300 W/m2 continuous-wave field at 2.45 GHz did not result in ocular abnormalities. Since these studies have not been independently replicated and no further research has been conducted in this area, it is considered scientifically unjustified to continue imposing SAR limits for the eye based on such limited information. However, the authors of this document consider these limited findings sufficient to encourage the reduction of ocular exposures whenever possible. This recommendation shall remain valid until sufficient scientific information is available to accurately assess the health effects of RF exposure on the eye.

7. Field Strength Limits

In Tables 1 and 4, the coefficients used in determining the maximum field strength limits between 300 MHz and 1500 MHz have been slightly adjusted in order to satisfy equation III.5.

8. 3-kHz Low Frequency Boundary

The low frequency portion of this Code has been extended to 3 kHz to cover the entire very -low-frequency (VLF) band. At the time of writing, information on the health effects of electromagnetic fields at frequencies below 3 kHz was insufficient to establish exposure limits. The rationale for any parts or sections of this Code that remain unchanged from the 1991 version is given elsewhere⁽¹⁾.