I've heard this-and-that but haven't heard about cancer, but there sure is a lot of elecromagnetic radiatian effects if you place a sheet of tin foil in a microwave, or a food content placed inside is surrounded by the same! The most important thing is to keep the seals around the microwave door clean and secure to seal it shut.
The Food and Drug Administration (FDA) has regulated the manufacture of microwave ovens since 1971. On the basis of current knowledge, the FDA believes that ovens that meet the FDA standard and used according to the manufacturer's instructions are safe for use.
http://www.fda.gov/cdrh/consumer/microwave.html
"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....
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."
http://hps.org/hpspublications/articles/microwaveoven.html
"Radiation leaks
Exposure to high levels of microwave radiation is known to cause health problems including cataracts and burns. Radiation leaks from a microwave oven are possible if the oven door is damaged or doesn't seal properly.
In most cases, the leaks are too small to cause a significant health risk, according to the National Health and Medical Research Council. If your microwave is in good condition and is used according to the manufacturer's instructions, it is safe. However, safety suggestions include: Regularly check the door of your microwave oven for corrosion or damage, and that it fits securely. Never tamper with or inactivate the interlocking devices. These are designed to stop the production of microwaves the moment the latch is released or the door opened. Clean the oven regularly to make sure that burnt food isn't stuck to the interior and door. Don't run the microwave oven when it's empty."
http://www.betterhealth.vic.gov.au/...es/Microwave_ovens_safety_issues?OpenDocument
"Microwave radiation is odorless and invisible and therefore hard to detect. Microwave oven radiation is present whenever a microwave oven is turned on. The microwave energy causes the water molecules in the food to vibrate rapidly. This rapid vibration produces heat which, in turn, cooks the food. It can also penetrate through living tissue which is why exposure is harmful to our health. Once you turn off the oven, the microwaves disappear but until then, you may be exposing yourself to dangerous levels of radiation leakage. A survey conducted among the Professional Service Associates, a group of microwave repair servicemen, indicated that over 56% of microwave ovens two years or older leaked levels of radiation 10% higher than the safety standards set by the FDA. More often than not, a simple adjustment was all that was needed to stop the leakage. Slamming the oven door, dirt or food particles caught in the door seals and hinges, or basic wear and tear all cause the oven door to slightly seperate from the oven enclosure. Once this happens, invisible, but highly dangerous microwaves leak through the small space and cause irreversible damage and injury to the user."
http://home.cvc.org/microwaves/detecto.htm
