If you recycle you’ve probably turned over a plastic container to read the number on the bottom, the one surrounded by the little recycling symbol. Many recycling programs depend on these numbers to tell you which plastics you can and can’t recycle.

Do you know what these numbers mean? Did you know these numbers tell you which plastics are considered safe and not? You may recall there was a big scare recently over BPA plastic leaching chemicals into water bottles and baby feeding bottles, after studies showed that BPA mimics estrogen and interferes with hormone levels.

There are seven numbers you will find on plastic containers, reflecting seven different types of plastic available in the market. The number is a resin identification code associated with the type of plastic used in the container. Some plastics are healthier and more environmentally friendly, some less so. Some are easier to recycle, some less.

Here’s your guide to what the numbers mean, whether they’re safe, and how easily recyclable they are:

To summarize, plastics #2, #4 and #5 are generally considered safe. Plastic #1 is safe too but should not be re-used due to the risk of growing bacteria. Any other plastic should be used with extreme caution, especially around food or drink. The risk is even greater when heating food. For microwaving in particular, remember that microwave safe containers aren’t necessarily healthy. They just won’t melt. In general, it’s better to avoid microwaving plastic entirely and stick to glass.