Exposure assessment

• Pages: 17-19

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Exposure assessment

Stray fields from RF heaters are complex in nature and dependent upon the

machine, the applicator type and the material being treated. Mathematical modelling of

a simplified exposure to fields from an RF heater has been carried out (Chen and

Gandhi, 1989a) but such modelling of realistic exposures from a wide variety of

equipment is not practical. It is necessary therefore to use instrumentation to measure

the external fields and to assess induced currents.

The guidelines for restricting exposure of people at work to RF fields are given in

Chapter 6. The objectives of any measurement programme are to ensure: (a) that the

exposure of machine operators and other people in the vicinity of RF heaters is below

the appropriate restrictions; and (b) to assist in evaluating the effectiveness of

protection measures such as shielding.

In order to demonstrate full compliance with the guidelines, two types of

measurement must be made. The first is to measure both the external electric and

magnetic fields, and the second is to obtain a measure of the induced current. The latter

is the more reliable method of assessing the potential hazard.

Assessment of the work situation should be made in order to protect against RF

burns that may arise due to accidentally touching metal objects in the RF fields.

7.1. Measurement of RF fields

Instruments capable of measuring both E- and H-field strengths should be used to

measure the RF fields in the vicinity of RF heaters. Additional details including an

example of an exposure evaluation for an RF heater are given in NCRP (1993).

Field-strength measurements are made in the absence of the operator but at the

operator's normal location to demonstrate that the measured value is below the

appropriate guideline level. Instrument limitations may prevent measurement closer

than 20 cm from the machine. In this case compliance with the guidelines should be

established by measurements of induced currents only.

Fields close to RF heaters are highly non-uniform. To evaluate compliance with

the exposure guidelines using field-strength instruments, measurements should be made

at least 20 cm from the machine in the vertical plane at the position of nearest whole

body approach to the applicator.

For accurate measurements the following important factors should be considered.

(1) Measurements should be made at the normal positions of the operator but, if

possible, in his or her absence to minimize field perturbations and to standardize

procedures.

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