Here are some good guidelines to think about before you keep using plastic to reheat any food in – from the WSJ:

When to Throw Out Microwaveable Plastic Containers

Lunch at your desk can be a downer, especially when it involves leftovers reheated in the office microwave. But are you putting more into your body than just lukewarm pad thai? Rolf Halden, the director for the Center for Environmental Security at the Biodesign Institute at Arizona State University, stirs the pot.

“We don’t know if and how many people die from plastic exposure,” says Dr. Halden, “but we do know that in the developed world we suffer from a lot of diseases—breast cancer, obesity and early onset puberty—that are less prevalent in developing countries. These are a result of our lifestyle.” He adds: “From a public health perspective, we should consider heated plastic an unnecessary source of exposure to harmful elements and eliminate it.”

Two to Watch

Since plastic was first synthesized in the early 1900s, it has evolved into everything from lifesaving medical devices to a softening agent in hair conditioner. Plastic is ubiquitous but there are two chemicals in it to watch out for when it comes to what your body ingests.

Phthalates, the chemicals that make a PVC container flexible, “can migrate out of the plastic when it’s heated,” says Dr. Halden, who has done comprehensive studies on emerging contaminants and plastics for more than a decade. Phthalates can leach into food, resulting in hormone imbalances and birth defects—although no one knows at what level those effects are triggered, he says. Phthalates are present in measurable levels in the blood of nearly every person in the developed world, he adds.

Bisphenol A, or BPA, is a potentially worse offender. Once tested for possible use as an estrogen replacement, BPA was found to be of better use in the mass production of polycarbonate plastic. It’s used in everything from the lining of metal soup cans to receipt paper. The FDA banned the use of BPA in baby bottles in July 2012, because of its link to developmental delays.

While the recycling numbers at the bottom of plastic items “are not meant to provide health information or risks,” Dr. Halden says, they can sometimes provide clues to the chemicals in them. For example, No. 7, says Dr. Halden, means “there is a high likelihood” that Bisphenol A is in it. That reusable water bottle sitting on your desk? “Think of it as one big BPA vessel,” he says.

Lunch at your desk can be a downer, especially when it involves leftovers reheated in the office microwave.

When to Toss It

The amount of chemicals leaching into food depends on the type of plastic that is put in the microwave, the time it is heated and the physical condition of the container, says Dr. Halden. Old, cracked containers and those that have been washed hundreds of times often give off more toxins when heated. Any deformities or discoloration are a sign it’s time for the recycling bin.

And reheating foods heavy in cream and butter in plastic is always a bad idea. “Fatty foods absorb more of these harmful chemicals when heated,” he says.

Another Way to Reheat

Rather than torturing yourself over what plastic is safe, use an inert container such as glass, ceramic or stainless steel, he suggests. Along with cold spots in food that could harbor bacteria, Dr. Halden points to another reason to avoid reheating in the microwave: taste. “Food tastes much better if it is prepared in a hot oven or on the stove, and not cold on the inside and too hot on the outside,” he says.