1. The numbering system for plastic is a valuable way to differentiate between the worst offenders and those that are known to leach out a lot less. The bad guys to chuck are:
#3 - PVC, especially common in plastic wraps and, in my case, dirt cheap picnic ware from Target. Make sure you use a wrap that says "no plasticizers" on the label if you can't find an actual number.
#6 - All styro products, including takeout containers.
#7 - PC, this is a hard plastic that's very common for durable water containers. Pitch that puppy and get yourself a new aluminum one. Sigg makes some lovely versions.
2. The better plastics to look for in the store: #1, 2, 4, 5, and 7-PLA. That last one is actually made from corn and it's starting to pop up in organic store brands. It's the only one of the bunch that's totally safe. If you don't see the PLA attached, it's the bad version of #7.
3. Heat and plastic should never mix. No matter what type it is, it'll leach if you heat it up. This means no plastic in the microwave and no hot takeout containers from the Thai place down the street. Sorry. Don't leave bottled water out in the sun where it can get warm. And do I really need to tell you not to eat frozen entrees? If you really must, transfer it to a glass or ceramic plate before nuking it.
4. If the plastic is cloudy, really old, stained or scratched, pitch it. It's leaching.
5. Meet your old friends, glass, ceramics, waxed paper cartons, and aluminum. They didn't mutate your grandma's DNA and they won't mutate yours.