Safety and Health Program- RF Energy Upgrade Chapter

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RF Energy and Microwave Radiation Safety

Purposes:

The purposes of this chapter are:

(a) to specify maximum levels and durations of exposure to RF fields of frequencies between 3 kHz and 300 GHz in order to prevent human health effects;

(b) to specify maximum allowable RF contact and induced body currents to prevent the physical perception of RF fields to the general public and RF shock or burns to RF and microwave exposed workers;

(c) to recommend general procedures for ensuring that exposure of the general public and of personnel working in the vicinity of RF and microwave devices is not greater than the levels specified in this chapter; and

(d) to recommend working conditions that will lead to high standards of safety for all personnel engaged in the manufacture, installation, operation and maintenance of RF devices.

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 that 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 that 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:

(a) occupancy of areas;

(b) actual duration of exposure and time averaging (including ON/ OFF times of the RF generators, direction of the beam, duty factors, sweep times, etc.);

(c) spatial characteristics of exposure, i. e., whole body or parts thereof;

(d) 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 shall be averaged over one-tenth hour period (0.1 h or 6 min).

Siting and Installation:

Basic Principles-In order to institute protective measures in areas where RF devices are used, it is necessary to take into account the time workers may spend in these areas. When surveying around RF devices, field levels shall be known in controlled areas where restricted occupancy is allowed. These areas shall be designated accordingly, and the maximum worker occupancy time shall be posted where applicable

General Recommendations-

(a) Warning signs, specified in the Warning Signs section, or suitable substitutes, indicating the presence of RF fields, shall be posted according to recommendations of the Warning Signs section.

(b) The areas surrounding unmanned, high- power sources of RF radiation shall be fenced off to prevent unauthorized access to places where an overexposure could occur. If a metallic fence is used, the contact current limits specified per state regulations shall be met.

(c) The siting of a RF device shall take into account the practical possibility of multiple source exposures from fields and leakage from other devices in the vicinity.

(d) Metallic objects that are not necessary shall not be present near any radiating RF device, as they may cause high intensity fields in some locations.