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

2. Maximum Exposure Limits

In the following sections, the maximum exposure levels for both RF and microwave exposed workers (including occupationally exposed persons) and other individuals (including the general public) are specified. These levels shall not be exceeded.

The basic limits which shall not be exceeded are given in terms of the currents in the body, either by induction or contact with energized metallic objects, or in terms of the rate at which RF electromagnetic energy is absorbed in the body. The latter is expressed, more precisely, as the specific absorption rate (SAR), i.e., the rate of RF energy absorption per unit mass in the body. SAR has units of joules per second per kilogram or watts per kilogram (W/kg). In practice, direct measurements of SAR are feasible only under laboratory conditions. Recommended maximum exposure levels in terms of unperturbed electric and magnetic field strength as well as power density are therefore given in addition to the SAR limits. These maximum field intensities are at levels which would generate a SAR or induced body current no greater than the basic limit.

For exposure limits of RF and microwave exposed workers, a safety factor of approximately 10 was incorporated with reference to the scientific-consensus threshold for adverse health effects. For other persons including the general public, an additional safety factor (2 to 5) was included to arrive at lower limits. The incorporation of these safety factors is to provide for all possible conditions under which the exposure might occur. The rationale for lower general public limits is as follows:

1. Exposure to the public is potentially 24 hours a day for 7 days a week, compared with 8 hours a day, 5 days a week for RF and microwave exposed workers.
2. Certain members of the general public may be more susceptible to harm from RF and microwave exposure. To determine whether the maximum exposure levels and durations are exceeded, full consideration shall be given to such factors as:
   
   
   1. occupancy of areas;
   2. actual duration of exposure and time averaging (including ON/OFF times of the RF generators, direction of the beam, duty factors, sweep times, etc.);
   3. spatial characteristics of exposure, i.e., whole body or parts thereof;
   4. uniformity of the exposure field, i.e., spatial averaging. In certain instances and over a specific frequency range, higher exposure levels are permitted for short durations. If this is the case, the field strengths and power densities should be averaged over one-tenth hour period (0.1 h or 6 min). Graphs are provided in Appendix I for easy identification of maximum exposure levels at various frequencies.

2.1 RF and Microwave Exposed Workers (Including Occupationally Exposed Persons)

In the far-field zone, electric field strength, magnetic field strength and power density are interrelated by simple mathematical expressions (Appendix III). Therefore, any one of these parameters defines the remaining two. In the near-field zone, both the electric and magnetic field strengths shall be measured, since there is no simple relationship between these two quantities. Instrumentation for the measurement of magnetic fields at certain frequencies may not be commercially available. In this case, the electric field strength shall be measured. Field strengthmeasurements are described in Appendix V.

2.1.1 Field Strength Limits

1. An RF and microwave exposed worker shall not be exposed to electromagnetic radiation in a frequency band listed in Column 1 of Table 1 if the field strength exceeds the value given in Column 2 or 3 of Table 1, when averaged spatially and over time, or if the power density exceeds the value given in Column 4 of Table 1, when averaged spatially and over time. The spatial averaging is carried out over an area equivalent to the vertical cross-section of the human body (projected area). A time-averaging period of 0.1 h (6 min) should be employed for frequencies up to 15 000 MHz. Above these frequencies, a different averaging time is used and is described in Section 2.3.2.
2. Where the electromagnetic radiation consists of a number of frequencies in the same or different frequency bands shown in Column 1 of Table 1, then the ratio of the measured value at each frequency to the limit at that given frequency shown in Column 2, 3, or 4 shall be determined and the sum of all ratios thus obtained for all frequencies shall not exceed unity when averaged spatially and over time. For field strength measurements, the measured values and the limits shall be squared before determining the ratios. The limit, as applied to multiple frequencies, can be expressed as:

where f is the frequency for which measurements were taken and, where the electric or magnetic field strength is measured,

or where the power density is measured,

* Power density limit is applicable at frequencies greater than 100 MHz.

Notes:

1. Frequency, f, is in MHz.
2. A power density of 10 W/m² is equivalent to 1 mW/cm².
3. A magnetic field strength of 1 A/m corresponds to 1.257 microtesla (µT) or 12.57 milligauss (mG).

Example 2.1:

After time and spatially averaged measurements, the electric fields to which an RF worker is exposed are found to be 30 V/m, 40 V/m, 50 V/m and 60 V/m at 20 MHz, 90 MHz, 150 MHz and 1300 MHz, respectively. The relative values with respect to the exposure limits in the frequency bands of concern are given as follows:

• R₁ = (30/60)² = 0.25 for 20 MHz (in the frequency band 10-30 MHz)
• R₂ = (40/60)² = 0.44 for 90 MHz (in the frequency band 30-300 MHz)
• R₃ = (50/60)² = 0.69 for 150 MHz (in the frequency band 30-300 MHz)
• R₄ = (60/127.6)² = 0.22 for 1300 MHz (in the frequency band 300-1 500 MHz)

From which R₁ + R₂ + R₃ + R₄ = 1.6, which exceeds unity and therefore the combined field strength does not conform with the Safety Code limit. Note that 127.6 V/m comes from substituting f = 1300 in the exposure limit term 3.54f ^0.5 which is found in Column 2 of Table 1.

Example 2.2:

Assume that a worker is exposed to RF fields at three different frequencies. Exposure measurements were performed, which were time and spatially averaged, producing the following conditions:

• 0.1 A/m at 27 MHz
• 70 V/m at 915 MHz
• 25 W/m² at 10 000 MHz

The relative values with respect to the exposure limits in the frequency bands of concern are given as follows:

• R₁ = (0.1/0.18)² for 27 MHz (in the frequency band 10-30 MHz)
• R₂ = (70/107.1)² for 915 MHz (in the frequency band 300-1 500 MHz)
• R₃ = 25/50 for 10 000 MHz (in the frequency band 1 500-15 000 MHz)

From which R₁ + R₂ + R₃ = 0.99, which is less than unity and therefore the combined field strengths and power density conform with the Safety Code limit.

2.1.2 Basic Restrictions - Specific Absorption Rate (SAR) Limits

SAR, as defined in Appendix VIII, is a measure of the rate at which electromagnetic energy is absorbed in the body. Methods for SAR determination are described in Appendix V. At frequencies between 100 kHz and 10 GHz, SAR limits take precedence over field strength and power density limits and shall not be exceeded.

The SAR should be determined for cases where exposures take place at 0.2 m or less from the source. For conditions where SAR determination is impractical, field strength or power density measurements shall be carried out. In cases where SAR determination is appropriate, the values in Table 2 shall not be exceeded:

*Defined as a tissue volume in the shape of a cube.

Although not a requirement of the Code, it is suggested that whenever possible, the organ-averaged SAR for the eye not exceed 0.4 W/kg. As stated in Appendix VII, this suggestion shall remain until sufficient scientific information is available to accurately assess the health effects of RF exposure on the eye.

2.1.3 Induced and Contact Current Limits

Limits for induced and contact currents are intended to reduce the potential for RF shock or burns as follows:

1. For free standing RF and microwave exposed workers (no contact with metallic objects), current induced in the human body by electromagnetic radiation in the frequency bands listed in Column 1 of Table 3 shall not exceed the value given in Column 2 of Table 3. Induced current measurements are described in Appendix V.
2. No object with which an RF and microwave exposed worker may come into contact by hand grip shall be energized by electromagnetic radiation in the frequency bands listed in Column 1 of Table 3 to such an extent that the current flow through an electrical circuit, having an impedance equivalent to that of the human body, exceeds the value given in Column 3 of Table 3, as measured with a contact current meter. Contact current measurements are described in Appendix V.

Note: The maximum permitted currents may be perceptible (such as a tingling or warming sensation), but are not sufficient to cause any pain or damage such as burns.

3. Where the electromagnetic radiation consists of a number of frequencies in the same or different frequency bands, shown in Column 1 of Table 3, the ratio of the square of the measured current in each frequency to the square of the limit at that given frequency, shown in Column 2 or 3 (depending on whether it is induced or contact current) shall be determined, and the sum of all ratios thus obtained for all frequencies shall not exceed unity when time averaged. The limit, as applied to multiple frequencies, can be expressed as

where f is the frequency for which measurements were taken and

Notes:

1. Frequency, f, is in MHz.
2. The above limits may not adequately protect against startle reactions and burns caused by transient spark discharges for intermittent contact with energized objects.
   
   
   4. For frequencies between 3 kHz and 100 kHz, the averaging time to be applied to the induced and contact currents shall be 1 second(s). For frequency between 100 kHz and 110 MHz, time averaging shall be applied to the square of the induced and contact currents and shall be consistent with the averaging time in Table 1 (0.1 h or 6 min), provided that the time-averaged square of the current in any 0.1 h (6 min) period does not exceed the limit given in the following relation:

where I_av is the maximum allowable time-averaged current for exposure times less than 0.1 h (6 min), I_lm is the current limit through each foot (100 mA) as specified in Table 3, and T_exp is the exposure time in minutes during any 0.1 h (6 min) period. Shown in Table 4 are the higher values of I_av that may be allowed for exposure times less than 0.1 h (6 min).

* Maximum instantaneous current for exposure times less than 0.5 min. is 350 mA.

Note: The above limits may not adequately protect against startle reactions and burns caused by transient spark discharges for intermittent contact with energized objects.

To limit the temperature rise in the extremities such as ankles or wrists of the exposed individuals for exposure times less than 0.5 min, a maximum instantaneous current of 350 mA shall not be exceeded(7).

Example 2.3:

Assume that the time-averaged induced currents through both feet of an RF worker were found to be 5 mA, 80 mA and 120 mA at 0.005 MHz, 0.06 MHz and 1 MHz, respectively. The relative values with respect to the current limits in the frequency bands of concern are given as:

• r₁ = (5/10)² = 0.25 for 0.005 MHz (in the frequency band 0.003-0.1 MHz)
• r₂ = (80/120)² = 0.44 for 0.06 MHz (in the frequency band 0.003-0.1 MHz)
• r₃ = (120/200)² = 0.36 for 1 MHz (in the frequency band 0.1-110 MHz)

From which r₁ + r₂ + r₃ = 1.05, which exceeds unity and therefore the total current through both feet does not conform with the Safety Code limit. Note that 10 mA and 120 mA come from substituting f = 0.005 and 0.06, respectively, in the exposure limit term 2000f which is found in Column 2 of Table 3.

2.1.4 Peak Field Strength Limit for Pulsed Fields

For exposures to pulsed electromagnetic fields in the frequency range of 0.1 to 300 000 MHz, the peak value of the instantaneous field (temporal peak), in terms of electric field strength, shall not exceed 100 kV/m. For pulse duration less than 100 ms, and in the preceding frequency range, peak limits shall be derived using the formula given in Appendix IV. If the derived peak limits are greater than 100 kV/m, the exposure limit shall be taken as 100 kV/m.

2.2 Exposure of Persons Not Classed as RF and Microwave Exposed Workers (Including the General Public)

2.2.1 Field Strength Limits

1. A person other than an RF and microwave exposed worker shall not be exposed to electromagnetic radiation in a frequency band listed in Column 1 of Table 5, if the field strength exceeds the value given in Column 2 or 3 of Table 5, when averaged spatially and over time, or if the power density exceeds the value given in Column 4 of Table 5, when averaged spatially and over time. The spatial averaging is carried out over an area equivalent to the vertical cross-section of the human body (projected area). A time-averaging period of 0.1 h (6 min) should be employed for frequencies up to 15 000 MHz. Above these frequencies, a different averaging time is used and is described in Section 2.3.2.
2. Where the electromagnetic radiation consists of a number of frequencies in the same or different frequency bands, shown in Column 1 of Table 5, then the ratio of the measured value at each frequency to the limit at that given frequency shown in Column 2, 3, or 4 shall be determined, and the sum of all ratios thus obtained for all frequencies shall not exceed unity when averaged spatially and over time. For field strength measurements, the measured values and the limits shall be squared before determining the ratios. See Section 2.1.1 for more details on calculating the sum.

* Power density limit is applicable at frequencies greater than 100 MHz.

Notes:

1. Frequency, f, is in MHz.
2. A power density of 10 W/m² is equivalent to 1 mW/cm².
3. A magnetic field strength of 1 A/m corresponds to 1.257 microtesla (µT) or 12.57 milligauss (mG).

2.2.2 Basic Restrictions - Specific Absorption Rate (SAR) Limits

SAR, as defined in Appendix VIII, is a measure of the rate at which electromagnetic energy is absorbed into the body. Methods for SAR determination are described in Appendix V. At frequencies between 100 kHz and 10 GHz, SAR limits take precedence over field strength and power density limits and shall not be exceeded. The SAR should be determined for cases where exposures take place at 20 cm or less from the source. For conditions where SAR determination is impractical, field strength or power density measurements shall be carried out. In cases where SAR determination is appropriate, the values in Table 6 shall not be exceeded:

* Defined as a tissue volume in the shape of a cube.

Although not a requirement of the Code, it is suggested that whenever possible, the organ-averaged SAR for the eye not exceed 0.2 W/kg. As stated in Appendix VII, this suggestion shall remain until sufficient scientific information is available to accurately assess the health effects of RF exposure on the eye.

2.2.3 Induced and Contact Current Limits

Limits for induced and contact currents exist to reduce the potential for RF shock or burns as follows:

1. For free standing individuals (no contact with metallic objects), current induced in the human body by electromagnetic radiation in the frequency bands listed in Column 1 of Table 7 shall not exceed the value given in Column 2 of Table 7. Induced current measurements are described in Appendix V.
2. No object, with which a person may come into contact by hand grip, shall be energized by electromagnetic radiation in the frequency bands listed in Column 1 of Table 7 to such an extent that the current flow through an electrical circuit, having an impedance equivalent to that of the human body, exceeds the value given in Column 3 of Table 7, as measured with a contact current meter. Contact current measurements are described in Appendix V.

Table 7 Induced and Contact Current Limits for Persons Not Classed as RF and Microwave Exposed Workers (Including the General Public)

Notes:

1. Frequency, f, is in MHz.
2. The above limits may not adequately protect against startle reactions and burns caused by transient spark discharges for intermittent contact with energized objects.

3. Where the electromagnetic radiation consists of a number of frequencies in the same or different frequency bands, shown in Column 1 of Table 7, the ratio of the square of the measured current at each frequency to the square of the limit at that given frequency shown in Column 2 or 3 (depending on whether it is the induced or contact current) shall be determined, and the sum of all ratios thus obtained for all frequencies shall not exceed unity when time averaged. See Section 2.1.3 for more details on calculating the sum.
4. For frequencies between 3 kHz and 100 kHz, the averaging time to be applied to the induced and contact currents shall be 1 second(s). For frequencies between 100 kHz and 110 MHz, time averaging shall be applied to the square of the induced and contact currents and shall be consistent with the averaging time in Table 5 (0.1 h or 6 min), provided that the time-averaged square of the current in any 0.1 h (6 min) period does not exceed the limit given in the following:

where I_av is the maximum allowable time-averaged current for exposure times less than 0.1 h (6 min), I_lm is the current limit through each foot (45 mA) as specified in Table 7, and T_exp is the exposure time in minutes during any 0.1 h (6 min) period. Shown in Table 8 are the higher values of I_av that may be allowed for exposure times less than 0.1 h (6 min).

Table 8 Time-Averaged Induced and Contact Current Limits for Different Exposure Times in the Frequency Band 0.1-110 MHz for Persons not classed as RF and Microwave Exposed Workers (Including the General Public)

*Maximum instantaneous current for exposure times less than 0.5 min. is 155 mA.

Note: The above limits may not adequately protect against startle reactions and burns caused by transient spark discharges for intermittent contact with energized objects.

For exposure times less than 0.5 minutes, a maximum instantaneous current of 155 mA shall not be exceeded.

2.2.4 Peak Field Strength Limit for Pulsed Fields

For exposures to pulsed electromagnetic fields in the frequency range of 0.1 to 300 000 MHz, the peak value of the instantaneous field (temporal peak), in terms of electric field strength, shall not exceed 100 kV/m. For pulse duration less than 100 ms, and in the preceding frequency range, peak limits shall be derived using the formula given in Appendix IV. If the derived peak limits are greater than 100 kV/m, the exposure limit shall be taken as 100 kV/m.

2.3 Time Averaging

2.3.1 Frequency 0.003 MHz - 15 000 MHz

A single measurement is sufficient, unless the field is changing significantly (more than 20%) within a period of 0.1 h, in which case the time-averaged values must be calculated from multiple measurements. Some modern instruments have time-averaging capabilities. If this feature is not available on the instrument being used, the time-averaged values over 0.1 h can be obtained by using the following formulae:

1. To obtain the time-averaged rms electric (E) or magnetic (H) field strength, use the applicable formula:

where E_i and H_i are the sampled rms electric and magnetic field
strengths, respectively, which are considered to be constant in the i-th
time period, Dti is the time duration, in minutes, of the i-th time period
and n is the number of time periods within 6 min (0.1 h).

2. To obtain the time-averaged power density W, use the
   formula

where W_i is the sampled power density in the i-th time period.

3. To obtain the time-averaged SAR, use the formula:

where (SAR)_i is the sampled SAR in the i-th time period.

Notes:

1. In all of the previous formulae, the following relationship
   shall be satisfied:

2. For pulsed fields, E_i and H_i are rms values, and W_i is the value averaged over the time interval . If peak values are measured, the rms or average values shall be calculated.

2.3.2 Frequency 15 000 MHz - 300 000 MHz

In the frequency range 15 000 MHz-300 000 MHz, the averaging time, in minutes, shall be evaluated for field strengths using the formula:

• Averaging Time = 616 000 /f ^1.2 ,

where f is the frequency in MHz.

2.4 Measurements and Evaluation

2.4.1 Field Measurements - Spatial Averaging

Measurements to determine conformity with the limits specified in Sections 2.1.1 and 2.2.1 shall be performed with field sensors (probes) placed at least 0.2 m away from any object or person. To determine the spatially averaged value, local values including the maximum value shall be measured over a surface area of 0.35 m (width) x 1.25 m (height) perpendicular to the ground and at a reasonable distance (e.g., 0.5 m) above it. It is advisable that the measurement points are uniformly spaced within the sampling area. Local values should be measured in nine or more points. Where the field is reasonably uniform (within ± 20%), e.g., in the far-field, measurements suffice in one location, representative of the space that is occupied by a person. Appendix V may be consulted for more information on field measurements and spatial averaging. The spatially averaged values shall be calculated from the following formulae:

where n is the number of locations, E_i , H_i andW_i are the electric field strength, the magnetic field strength and the power density, respectively, measured in the i-th location.

2.4.2 Determination

To satisfy the requirements of Sections 2.1.2 and 2.2.2, the SAR shall be determined with an uncertainty not greater than ± 25 percent. Any suitable and reliable computational or measurement method may be used. Appendix V may be consulted for more information.

2.4.3 Induced and Contact Current Measurements

Under certain conditions, the induced current can exceed the limits specified in Tables 3 and 7, even though the electric field strengths, which are the major contributor to the induced current, are belowthe limits specified in Tables 1 and 5^(8,9). These conditions may occur when the electric field strength is as low as 25% of the exposure limits.

For any conducting metallic object that a person may come in contact with and that is located in a high-intensity RF field, contact currents shall be measured.An electrical circuit having the impedance of the human body shall be used for measurements. Under certain conditions, the contact current can exceed the current limits specified in Tables 3 and 7, even though the electric field strengths, which are the major contributor to the contact current, are below the limits specified in Tables 1 and 5. These conditions may occur when the electric field strength is as low as 20% of the exposure limits shown in Tables 1 and 5. Appendix V maybe consulted for more information.