Photos: Plastic by the numbers

High-density polyethylene, or No. 2 HDPE, used in shampoo and detergent bottles, milk jugs, cosmetics, motor oil, toys, and sturdy shopping bags, is considered one of the safer plastics. HDPE is often opaque or cloudy. Some recycling centers can only handle clear No. 2 plastics, such as milk jugs, but not colored bottles.

As rule of thumb, bottles, jars, and jugs are most likely to be collected for recycling, particularly those labeled 1 or 2.

Tubs, lids, spray pumps, buckets, films, bags, and items containing toxic chemicals such as drain cleaner, are less likely to be accepted by recyclers.

Environmental groups generally warn against using plastics No. 3, 6, and 7, whose toxic side effects are best understood. However, the making of No. 2 HDPE has involved toxic chemicals such as hexane and benzene. Plastics makers stress the safety of their products and aim to debunk myths.

But most plastic is "downcycled," meaning that unlike materials such as pure aluminum, it loses valuable properties and strength the more it is recycled. Unfortunately, there are no hard and fast plastics recycling rules, which vary by region. Many communities lack the resources to handle all types, so it's necessary to check local guidelines before tossing something in a recycling bin. Plus, the majority of plastics collected, even in the San Francisco Bay Area, are sent to Asia, particularly China, which further increases the carbon emissions of the plastics life cycle.

In terms of shipments, making plastics is the fifth largest U.S. manufacturing industry in 2005, according to the American Chemistry Council. The sector employs more than 1.1 million people, according to the Society of the Plastics Industry.