Introduction

• Pages: 1-1

1

Introduction

This document discusses the technical aspects of developing control technology

that may be used to limit exposure of workers to emissions of radiofrequency (RF)

electric and magnetic fields from dielectric heaters and sealers (hereafter referred to as

RF heaters). RF heaters present the most common source of exposure to excessive RF

fields. Protective measures can be and have been implemented to minimize such

exposure. This guide provides practical methods to install, design or retrofit this type of

equipment which will minimize the user's exposure to RF fields.

Workers in many industrial settings have a false sense of security because their

exposure to RF energy has not been properly assessed. The relatively high output power

of dielectric heaters and use of unshielded electrodes in many of them can produce

relatively high-stray RF fields. It was not until the mid-1970s that monitoring

instruments were developed to measure RF exposure accurately. With these instruments,

worksite studies were done to evaluate the hazards where RF heaters are used. The

results of several surveys (Stuchly et al., 1980; Grandolfo et al., 1983; Bini et al., 1986;

Joyner and Bangay, 1983, 1986a, 1986b; Stuchly and Mild, 1987; Conover et al., 1980)

have shown that RF heaters produce exposure fields exceeding the limits recommended

in various countries and by the International Commission on Non-Ionizing Radiation

Protection (ICNIRP). Some operators were exposed to over 100 times the guideline

levels. Surveys in the United States indicated that 60 per cent of 82 devices measured

exposed the operators to levels exceeding those recommended. However, shielding of

dielectric heaters and other protective measures can reduce worker exposure to

acceptable levels.

Absorption of high levels of RF energy from stray fields can cause localized and

whole-body heating, which may result in adverse health effects. Certain tissues, such as

the lens of the eye and the male reproductive organs, are heat sensitive. RF-induced heat

has also been shown to adversely affect the developing foetus. Hazards to operators can

also arise indirectly when contact is made with the metallic parts of the heater. This

contact can result in RF burns, which are frequently reported as being very painful, deep

seated and slow to heal.

Although work practices and administrative controls can and should be used to

reduce worker exposure, the use of shielding is emphasized in this book, as it is more

effective, dependable and efficient. Because many applications of dielectric heaters are

unique, they will require variations in the shielding design. The information required to

design, construct and install the shield is provided in Appendix A. This document should

provide a basis for understanding the problems that may be encountered and for

determining their solutions. Additional background information is given in the

appendices.

1