A Practical Approach to Microwave Oven Safety

Dewey D. Sprague, LSO, UC Berkeley Office of Radiation Safety

Perhaps no "high technology" device has been so readily accepted by modern society as the microwave oven. Its convenience and usefulness have resulted in widespread use both at home and in the workplace. As these devices became widespread in work environments, it was inevitable that occupational safety issues would develop.

All new microwave ovens produced for sale in the United States must meet the Food and Drug Administration/Center for Devices and Radiological Health (FDA/CDRH) performance requirements in Title 21, CFR, Part 1030.10. This requirement states that new ovens may not leak microwave radiation in excess of 1 mW cm^–2 at 5 cm from the oven surface. It also states that ovens, once placed into service, may not leak microwave radiation in excess of 5 mW cm^–2 at 5 cm from the oven surface. The "Procedure for Field Testing Microwave Ovens" (HEW Publication (FDA) 77-8037) is the standard method for verifying that these oven performance criteria are met.

Most concerns about microwave oven safety are generated by employees who have accidentally damaged an oven, read a mass media article about electric and magnetic fields (EMF) dangers, or are suffering from some illness which they associate with exposure to an oven. Regardless of the reason, it is important to address these employee concerns. Ignoring these concerns may result in exacerbating employee fears and escalating the complaint. NOTE: Some years ago there were concerns about leaking microwave ovens interfering with cardiac pacemakers. These concerns often resulted in hazard postings near the ovens to warn pacemaker wearers. Today, due to the shielded designs of modern pacemakers and ovens, leakage is not considered to be a realistic concern and the postings are no longer used.

Ovens can be damaged in several ways. The use of metal objects inside the oven may result in high-voltage arcing, damaging the enclosure or causing a fire. Interlock connectors or switches may fail in an unsafe condition through abuse of the door, allowing microwave leakage. Finally, the oven may be dropped, damaging the door seal, enclosure, switches, or power supply.

In reality, ovens are notoriously resistant to leaking microwaves. In the past 10 years, I have only heard of two instances of oven leakage exceeding the FDA/CDRH standard of 5 mW cm^–2 at 5 cm. The first oven had a physical puncture of the protective metal grid at the viewing window (caused by an exploding metal food container). The second oven had a defective door seal as a result of dropping the oven off the top of a refrigerator. In addition, I once found an oven that had been intentionally disassembled to remove the magnetron tube and power supply for use in a laboratory experiment (apparently research funds were short). In all of these cases, the oven had been severely damaged either through misuse or abuse.

Modern microwave ovens use a magnetron tube producing microwaves at 2,450 MHz. These microwaves are normally contained inside a continuous metal housing covering the interior of the cooking chamber. The door/viewing window contains a metal grid with small holes through which optical radiation can pass but which contains the microwave frequency. Besides the metal housing, the oven has at least one interlock switch used to deenergize the magnetron tube when the door is opened. The high-voltage electrical components (up to 4,000 V DC at 300 mA) are contained in a grounded metal enclosure. Most ovens are Underwriters Laboratory (UL) approved for electrical and fire safety. These design elements tend to make ovens extremely safe during normal use.

Due to the inherent safety of microwave oven design, it may not be justifiable to expend the resources needed to measure the microwave leakage for all the ovens on your site. Instead, I recommend a more practical approach to microwave oven safety. You may wish to consider using one or more of the program elements listed below:

• Maintain a basic inventory of your microwave ovens (model, year of manufacture, serial number, wattage, contact person, and location).
• Post a one-page microwave oven safety document next to each oven. (For a sample, contact the author at sprague6@llnl.gov.) This action alone will prevent many of the unsafe acts that cause ovens to leak.
• Visually inspect those ovens that are suspected of leaking. Most of these are found through employee complaints. If the oven is clearly damaged (loose doors, broken switches, penetrations of the metal enclosure) either have it repaired or discard the oven (I recommend cutting off the power cord).
• If (and only if) calibrated survey instrumentation is available, survey those ovens suspected of leaking and document the results. Ovens found to not meet the performance standard should be repaired or discarded.

NOTE: A number of inexpensive "home" microwave meters may (or may not) be accurate enough for survey work. Unfortunately, these inexpensive meters usually cannot be calibrated to meet the Institute of Electrical and Electronic Engineers, Inc.-American National Standards Institute, Inc., standards. Meters equivalent to the LORAL/NARDA Model 8217 are recommended to assure accurate microwave oven surveys.

As with any safety program, your microwave oven safety program should be properly documented. Although microwave oven safety is usually a small part of any nonionizing radiation safety program, it is a part that is highly visible because it potentially impacts so many people. A small amount of effort placed into such a program yields large rewards in employee relations while mitigating occupational safety concerns.