Microwave popcorn contains a Teflon-like chemical in the lining of the bag to keep it dry and non-stick. This chemical, PFOA (which the EPA classified as a likely carcinogen), has been shown to cause cancer in lab mice. And it has been found in small amounts in the blood of nearly 95 percent of humans. I don't know how much evidence links it to cancer in humans, but I don't want to find out in thirty years that there is a connection.
Many of the companies who make microwave popcorn have plans to remove PFOAs by 2015, but I'm not sure that the chemical with which they will replace it will be any better. It's best to avoid as many chemicals as you can.
Luckily, you can still use the microwave to cook up a quick, healthy, and non-toxic snack. Just buy a container or bag of plain popcorn at the grocery store.
Place 1/4 cup of the popcorn into a plain paper bag, and fold it over twice. Make a nice, strong fold, so your snack doesn't start popping out all over the microwave. Press the popcorn button on your microwave (or pop for three to four minutes).
Top as you like: butter, salt, nutritional yeast, Old Bay, or cinnamon and sugar. Easy, and no unwanted chemicals.
I wanted to take a picture of the popped popcorn so you could see how pretty and fluffy it was, but I ate it all before I thought of it. Oops.