what do those numbers on plastic containers mean?

hi all,
did you ever wonder what those numbers.. 1, 2, 7.. on your plastic containers mean?
there is so much to learn about plastic!
here is some info from the national geographic green guide:

Plastic Containers

What To Look For

Plastic is the most widely used material in the United States, and it crops up in everything from toys to clothes to food containers. But not all plastics are created equal, particularly in regards to food storage: Some plastics can transmit chemicals into your food, while others are perfectly safe [Kerry's note: safe for sea turtles?].

Before you know which type of plastic container to buy the next time you hit the store, you first need to know how to tell them apart. Plastics are typically classified by a number from #1 to #7, each number representing a different type of resin. That number is usually imprinted on the bottom of your container; flip it upside down, and you'll see a recycling triangle with the number in the middle.

Here's a quick breakdown of plastic resin types:

#1 polyethylene terephthalate (PET or PETE)
Product examples: Disposable soft drink and water bottles, cough-syrup bottles

#2 high density polyethylene (HDPE)/
Product examples: Milk jugs, toys, liquid detergent bottles, shampoo bottles

#3 polyvinyl chloride (V or PVC)
Product examples: Meat wrap, cooking oil bottles, plumbing pipes

#4 low density polyethylene (LDPE)
Product examples: Cling wrap, grocery bags, sandwich bags

#5 polypropylene (PP)
Product examples: Syrup bottles, yogurt cups/tubs, diapers

#6 polystyrene (PS)
Product examples: Disposable coffee cups, clam-shell take-out containers

#7 other (misc.; usually polycarbonate, or PC, but also polylactide, or PLA, plastics made from renewable resources)
Product examples: Baby bottles, some reusable water bottles, stain-resistant food-storage containers, medical storage containers

Now that you know what each of the numbers represents, here are the kinds you should look for at the store:

Safer Plastics

#2HDPE, #4LDPE and #5PP

These three types of plastic are the healthiest. They transmit no known chemicals into your food and they're generally recyclable; #2 is very commonly accepted by municipal recycling programs, but you may have a more difficult time finding someone to recycle your #4 and #5 containers.

#1 PET

#1 bottles and containers are fine for single use [Kerry's note: what is fine about single use???] and are widely accepted by municipal recyclers. You won't find many reusable containers made from #1, but they do exist. It's also best to avoid reusing #1 plastic bottles; water and soda bottles in particular are hard to clean, and because plastic is porous, these bottles absorb flavors and bacteria that you can't get rid of.

PLA

PLA (polylactide) plastics are made from renewable resources such as corn, potatoes and sugar cane and anything else with a high starch content. The starch is converted into polylactide acid (PLA). Although you can't recycle these plant-based plastics, you can compost them in a municipal composter or in your backyard compost heap. Most decompose in about twelve days unlike conventional plastic, which can take up to 100 years.

Plastics to Avoid

#3 PVC

#3 polyvinyl chloride (PVC) is often used frequently in cling wraps for meat. However, PVC contains softeners called phthalates that interfere with hormonal development, and its manufacture and incineration release dioxin, a potent carcinogen and hormone disruptor. Vinyl chloride, the primary building block of PVC, is a known human carcinogen that also poses a threat to workers during manufacture.

#6 PS

Extruded polystyrene (#6 PS; commonly known as Styrofoam) is used in take-out containers and cups, and non-extruded PS is used in clear disposable takeout containers, disposable plastic cutlery and cups. Both forms of PS can leach styrene into food; styrene is considered a possible human carcinogen by the International Agency for Research on Cancer. It may also disrupt hormones or affect reproduction.

#7 PC

#7 Polycarbonate (PC) is found in baby bottles, 5-gallon water bottles, water-cooler bottles and the epoxy linings of tin food cans. PC is composed of a hormone-disrupting chemical called bisphenol A, which has been linked to a wide variety of problems such as cancer and obesity.

Source: http://www.thegreenguide.com/products/Kitchen/Plastic_Containers