For questions like this, I get my information from the Environmental Working Group. They're an American environmental organization that specializes in research and advocacy in the areas of toxic chemicals, agricultural subsidies, public lands, and corporate accountability.
I'd like to err on the side of safety, so while corporations may have their own statement on microwaving their containers, they have a vested interest to sell their product. I'd rather get my information from somebody not interested in helping the corporation in question make a profit.
While the smaller question is "Is It Safe To Put Tupperware In The Microwave?" the bigger question is: "Is It Safe To Put Plastic In The Microwave?"
There is some evidence that some molecules - phthalates in some flexible plastic, and another chemical plasticizer DEHA - can migrate into high-fat foods such as meats or cheeses. And that's not just if you're microwaving plastic. Many environmentally conscious Web sites tell consumers to avoid wrapping high-fat foods in plastic altogether for fear that you'll end up eating minuscule plastic bits.
Step 1 - Is Your Plastic A Safe number?First, determine if your plastic is safe for food. Here are the numbers deemed as safe for food:
#2, HDPE, a usually opaque plastic used for milk jugs, detergent bottles, juice bottles, toiletries and the like.
#4, LDPE, used for things like plastic bags, food storage, bread bags, some food wraps, squeezable bottles.
#5, polypropylene, used for a wide variety of applications such as yogurt cups, medicine bottles, ketchup and syrup bottles, and straws.
If your item doesn't have a number on it, assume the worst, and throw it out. Better safe than sorry.
Step 2 - Is Your plastic In Perfect Condition?
The container must be completely free of scratches, dents, and bubbles. All of these things cause chemicals to leach out, and likely contaminating your food.
If your items pass both of these steps, then it is technically safe to microwave. However, if you're microwaving things often, I recommend using glass. That way, you know it's safe. You don't have to constantly inspect it for scratches or bubbles.
There Are Many Alternatives To Plastic Food Storage
Looking for food storage alternatives? I like to go with good old glass. I store anything and everything in mason jars & a few Glasslock Snapware Containers. I love they they don't stain, don't hold odors, are easily washable, and can even be thrown in the dishwasher. :)