Limites de radiação não ionizante

Limites de radiação não ionizante

(Parte 5 de 13)

Magnetic field models assume that the body has a homogeneous and isotropic conductivity and apply simple circular conductive loop models to estimate induced currents in different organs and body regions, e.g., the head, by using the following equation for a pure sinusoidal field at frequency f derived from Faraday’s law of induction:

where B is the magnetic flux density and R is the radius of the loop for induction of the current. More complex models use an ellipsoidal model to represent the trunk or the whole body for estimating induced current densities at the surface of the body (Reilly 1989, 1992).

If, for simplicity, a homogeneous conductivity of 0.2Sm 21 is assumed, a 50-Hz magnetic flux density of 100 mT generates current densities between 0.2 and 2 mA m22 in the peripheral area of the body (CRP 1997). According to another analysis (NAS 1996), 60-Hz exposure levels of 100 mT correspond to average current densities of 0.28 mA m22 and to maximum current densities of approximately 2 mA m22. More realistic calculations based on anatomically and electrically refined models (Xi and Stuchly 1994) resulted in maximum current densities exceeding 2 mA m22 for a 100-mT field at 60 Hz. However, the presence of biological cells affects the spatial pattern of induced currents and fields, resulting in significant differences in both magnitude (a factor of 2 greater) and patterns of flow of the induced current compared with those predicted by simplified analyses (Stuchly and Xi 1994).

Electric field models must take into account the fact that, depending on the exposure conditions and the size, shape, and position of the exposed body in the field, the surface charge density can vary greatly, resulting in a variable and non-uniform distribution of currents inside the body. For sinusoidal electric fields at frequencies below about 10 MHz, the magnitude of the induced current density inside the body increases with frequency.

The induced current density distribution varies inversely with the body cross-section and may be relatively high in the neck and ankles. The exposure level of 5 kV m21 for exposure of the general public corresponds, under worstcase conditions, to an induced current density of about 2 mA m22 in the neck and trunk of the body if the E-field vector is parallel to the body axis (ILO 1994; CRP 1997). However, the current density induced by 5 kV m21 will comply with the basic restrictions under realistic worstcase exposure conditions.

For purposes of demonstrating compliance with the basic restrictions, the reference levels for the electric and magnetic fields should be considered separately and not additively. This is because, for protection purposes, the currents induced by electric and magnetic fields are not additive.

For the specific case of occupational exposures at frequencies up to 100 kHz, the derived electric fields can be increased by a factor of 2 under conditions in which adverse indirect effects from contact with electrically charged conductors can be excluded.

At frequencies above 10 MHz, the derived electric and magnetic field strengths were obtained from the whole-body SAR basic restriction using computational and experimental data. In the worst case, the energy coupling reaches a maximum between 20 MHz and several hundred MHz. In this frequency range, the derived reference levels have minimum values. The derived magnetic field strengths were calculated from the electric field strengths by using the far-field relationship between E and H (E/H 5 377 ohms). In the near-field, the SAR frequency dependence curves are no longer valid; moreover, the contributions of the electric and magnetic field components have to be considered separately. For a conservative approximation, field exposure levels can be used for near-field assessment since the coupling of energy from the electric or magnetic field contribution cannot exceed the SAR restrictions. For a less conservative assessment, basic restrictions on the whole-body average and local SAR should be used.

Reference levels for exposure of the general public have been obtained from those for occupational exposure by using various factors over the entire frequency range. These factors have been chosen on the basis of effects that are recognized as specific and relevant for the various frequency ranges. Generally speaking, the factors follow the basic restrictions over the entire frequency range, and their values correspond to the mathematical relation between the quantities of the basic restrictions and the derived levels as described below:

● In the frequency range up to 1 kHz, the general public reference levels for electric fields are one-half of the values set for occupational exposure. The value of 10 kV m21 for a 50-Hz or 8.3 kV m21 for a 60-Hz occupational exposure includes a sufficient safety margin to prevent stimulation effects from contact currents under all possible conditions. Half of this value was chosen for the general public reference levels, i.e.,

Table 5. Basic restrictions for power density for frequencies between 10 and 300 GHz.a

Exposure characteristics Power density (W m22)

Occupational exposure 50 General public 10 a Note: 1. Power densities are to be averaged over any 20 cm2 of exposed area and any 68/f 1.05-min period (where f is in GHz) to compensate for progressively shorter penetration depth as the frequency increases. 2. Spatial maximum power densities, averaged over 1 cm2, should not exceed 20 times the values above.

510 Health Physics April 1998, Volume 74, Number 4

5k Vm 21 for 50 Hz or 4.2 kV m21 for 60 Hz, to prevent adverse indirect effects for more than 90% of exposed individuals; ● In the low-frequency range up to 100 kHz, the general public reference levels for magnetic fields are set at a factor of 5 below the values set for occupational exposure; c In the frequency range 100 kHz–10 MHz, the general public reference levels for magnetic fields have been increased compared with the limits given in the 1988 IRPA guideline. In that guideline, the magnetic field strength reference levels were calculated from the electric field strength reference levels by using the far-field

Table 6. Reference levels for occupational exposure to time-varying electric and magnetic fields (unperturbed rms values).a

Frequency range

E-field strength

H-field strength

Equivalent plane wave a Note: 1. f as indicated in the frequency range column. 2. Provided that basic restrictions are met and adverse indirect effects can be excluded, field strength values can be exceeded.

3. For frequencies between 100 kHz and 10 GHz, Seq,E 2,H 2, and B2 are to be averaged over any 6-min period. 4. For peak values at frequencies up to 100 kHz see Table 4, note 3.

5. For peak values at frequencies exceeding 100 kHz see Figs. 1 and 2. Between 100 kHz and 10 MHz, peak values for the field strengths are obtained by interpolation from the 1.5-fold peak at 100 kHz to the 32-fold peak at 10 MHz. For frequencies exceeding 10 MHz it is suggested that the peak equivalent plane wave power density, as averaged over the pulse width, does not exceed 1,0 times the Seq restrictions, or that the field strength does not exceed 32 times the field strength exposure levels given in the table.

6. For frequencies exceeding 10 GHz, Seq,E 2,H 2, and B2 are to be averaged over any 68/f 1.05-min period (f in GHz). 7. No E-field value is provided for frequencies ,1 Hz, which are effectively static electric fields. Electric shock from low impedance sources is prevented by established electrical safety procedures for such equipment.

Table 7. Reference levels for general public exposure to time-varying electric and magnetic fields (unperturbed rms values).a

Frequency range

E-field strength

H-field strength

Equivalent plane wave a Note: 1. f as indicated in the frequency range column. 2. Provided that basic restrictions are met and adverse indirect effects can be excluded, field strength values can be exceeded.

3. For frequencies between 100 kHz and 10 GHz, Seq,E 2,H 2, and B2 are to averaged over any 6-min period. 4. For peak values at frequencies up to 100 kHz see Table 4, note 3.

5. For peak values at frequencies exceeding 100 kHz see Figs. 1 and 2. Between 100 kHz and 10 MHz, peak values for the field strengths are obtained by interpolation from the 1.5-fold peak at 100 kHz to the 32-fold peak at 10 MHz. For frequencies exceeding 10 MHz it is suggested that the peak equivalent plane wave power density, as averaged over the pulse width does not exceed 1,0 times the Seq restrictions, or that the field strength does not exceed 32 times the field strength exposure levels given in the table.

6. For frequencies exceeding 10 GHz, Seq,E 2,H 2, and B2 are to be averaged over any 68/f 1.05-min period (f in GHz). 7. No E-field value is provided for frequencies ,1 Hz, which are effectively static electric fields. perception of surface electric charges will not occur at field strengths less than 25 kVm21. Spark discharges causing stress or annoyance should be avoided.

511Guidelines for limiting exposure to time-varying electric, magnetic, and electromagnetic fields c ICNIRP GUIDELINES

formula relating E and H. These reference levels are too conservative, since the magnetic field at frequencies below 10 MHz does not contribute significantly to the risk of shocks, burns, or surface charge effects that form a major basis for limiting occupational exposure to electric fields in that frequency range;

● In the high-frequency range 10 MHz–10 GHz, the general public reference levels for electric and magnetic fields are lower by a factor of 2.2 than those set for occupational exposure. The factor of 2.2 corresponds to the square root of 5, which is the safety factor between the basic restrictions for occupationalexposureand those for general public

Fig. 1. Reference levels for exposure to time varying electric fields (compare Tables 6 and 7). Fig. 2. Reference levels for exposure to time varying magnetic fields (compare Tables 6 and 7).

512 Health Physics April 1998, Volume 74, Number 4 exposure. The square root is used to relate the quantities “field strength” and “power density;”

● In the high-frequency range 10–300 GHz, the general public reference levels are defined by the power density, as in the basic restrictions, and are lower by a factor of 5 than the occupational exposure restrictions; c Although little information is available on the relation between biological effects and peak values of pulsed fields, it is suggested that, for frequencies exceeding 10 MHz, Seq as averaged over the pulse width should not exceed 1,0 times the reference levels or that field strengths should not exceed 32 times the field strength reference levels given in Tables 6 and 7 or shown in Figs. 1 and 2. For frequencies between about 0.3 GHz and several GHz, and for localized exposure of the head, in order to limit or avoid auditory effects caused by thermoelastic expansion the specific absorption from pulses must limited. In this frequency range, the threshold SA of 4–16 mJ kg21 for producing this effect corresponds, for 30-ms pulses, to peak SAR values of 130–520 W kg21 in the brain. Between 100 kHz and 10 MHz, peak values for the field strengths in Figs. 1 and 2 are obtained by interpolation from the 1.5-fold peak at 100 kHz to the 32-fold peak at 10 MHz. c In Tables 6 and 7, as well as in Figs. 1 and 2, different frequency break-points occur for occupational and general public derived reference levels. This is a consequence of the varying factors used to derive the general public reference levels, while generally keeping the frequency dependence the same for both occupational and general public levels.

Up to 110 MHz, which includes the FM radio transmission frequency band, reference levels for contact current are given above which caution must be exercised to avoid shock and burn hazards. The point contact reference levels are presented in Table 8. Since the threshold contact currents that elicit biological responses in children and adult women are approximately one-half and two-thirds, respectively, of those for adult men, the reference levels for contact current for the general public are set lower by a factor of 2 than the values for occupational exposure.

For the frequency range 10–110 MHz, reference levels are provided for limb currents that are below the basic restrictions on localized SAR (see Table 9).

It is important to determine whether, in situations of simultaneous exposure to fields of different frequencies, these exposures are additive in their effects. Additivity should be examined separately for the effects of thermal and electrical stimulation, and the basic restrictions below should be met. The formulae below apply to relevant frequencies under practical exposure situations.

For electrical stimulation, relevant for frequencies up to 10 MHz, induced current densities should be added according to

O i51H z

For thermal effects, relevant above 100 kHz, SAR and power density values should be added according to:

O i5100 kHz

SARL 1 O

300 GHz Si where

Ji 5the current density induced at frequency i;

JL, i 5the induced current density restriction at frequency i as given in Table 4;

SARi 5the SAR caused by exposure at frequency i; SARL 5the SAR limit given in Table 4;

SL 5the power density limit given in Table 5; and

(Parte 5 de 13)

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