Radiation Leakage of Before-Sale and Used Microwave Ovens

Introduction

Because of potential hazards, Health Canada introduced regulations for microwave ovens on October 23, 1974, and later amended them on December 12, 1979 [Health Canada, 1994].

The regulations were promulgated under the Radiation Emitting Devices (RED) Act and are referred to as "Microwave Ovens, part III, Schedule II in Chapter 1370 of the Consolidated Regulations of Canada, 1978." They governed the design, construction and functioning of microwave ovens sold, leased or imported into Canada. Some of regulatory requirements are indicated as follows:

• Oven doors must have at least two independent interlocks to ensure that the microwave power is turned off when the door is open and that the microwave power generator cannot be turned on while the door is open. If a door interlock fails, the oven must automatically be rendered inoperable.
• Microwave radiation leakage at 5 cm from any outer surface of the oven must not exceed 1 mW/cm² with a test load in the cavity, and 5 mW/cm2 without a test load. For an oven that is designed for cooking and has a total microwave power-generating capacity not greater than 1.5 kW, the test load must be 275 ± 15 ml of water at an initial temperature of 20 ± 5oC. The 1 and 5 mW/cm² allowable limits have been established to protect oven users and are at least 10 times lower than the threshold for adverse health effects that is generally accepted by scientific consensus [Health Canada, 1999].
• Ovens must have permanently affixed labeling on the external surface showing the name of the manufacturer, model number, serial number, date and place of manufacture, type of microwave generator, normal operating voltage and frequency and the normal maximum output power.
• A warning sign, clearly visible from a distance of at least 1 m, must be on the external surface of the oven. This sign must be a two-contrasting-color, inverted triangle containing a symbolic representation of a microwave generator and must contain the words "CAUTION - MICROWAVES" and "ATTENTION - MICRO-ONDES."

In Canada, the Radiation Protection Bureau (RPB) of Health Canada is responsible for the regulatory compliance of before-sale microwave ovens. Manufacturers intending to import, sell or lease microwave ovens voluntarily submit to RPB technical information showing that their oven models are in compliance with the regulations. However, after-sale (used) microwave ovens are not covered by these regulations. As ovens age, there is a possibility that the effectiveness of the door interlocks, hinges and seals could deteriorate with use and may consequently result in excessive leakage.

Over the past two decades, several surveys were carried out in order to evaluate radiation leakage levels from used microwave ovens. Based on the survey conducted at the US Fermi National Accelerator Laboratory between 1974 and 1985, the mean maximum leakage within 5 cm of the oven surface was 0.2 ± 3.1 mW/cm² [Miller, 1987]. Annual surveys investigated in the UK from 1980 to 1987 showed that only a small number of the inspected ovens leaked in excess of 5 mW/cm² at 5 cm from the surface [Moseley and Davison, 1989], while none of the inspected appliances in Germany emitted microwave radiation exceeding 1 mW/cm² [Matthes, 1992].

After the Canadian microwave oven regulations were introduced, there was a marked improvement in the leakage levels [Stuchly, et al., 1978]. Despite the available data on microwave oven leakage, Canadian consumers remain concerned.

Advancement in microwave oven technology has resulted in lower oven prices, thus increasing use and ownership of ovens in Canada. Microwave ovens are now common in most Canadian homes as well as in restaurants and in commercial and industrial establishments. According to data published by Statistics Canada, Canadian households that have a microwave oven increased from 10.2% in 1982 to 86.3% in 1997. Because of low prices in the last few years, many Canadians have expressed concerns whether current generation microwave ovens are made as safe as older models. Some have even questioned whether their present ovens, which have been in use for several years, leak excessive microwave radiation. In order to address the issue, two separate surveys were conducted in the greater Ottawa area for both before-sale and used ovens. A total of 60 before-sale microwave ovens, representing 14 manufacturers, were inspected in eight retail outlets in the greater Ottawa area in the spring of 1997.

Subsequently, with the cooperation of oven owners, a survey of 103 used ovens representing 17 manufacturers was carried out. The age of the used ovens varied from less than a year to 23 years, with an average of eight-and-a-half years. This paper gives a summary of measurement data from these surveys and compares the measured leakage levels with the emission standards specified in the regulations.