Photos: Plastic by the numbers

The No. 7 SPI code is generally a wild card marking plastics that don't fall within the other six categories. These include polycarbonate bottles, which are understood by scientists to wreak havoc on human hormones by leaching bisphenol-A into hot beverages. As a result, polycarbonate baby bottles are losing favor with the public, and retailers including Toys R Us are phasing out BPA-free bottles.

Some popular Nalgene-brand, reusable, colored water bottles are labeled as No. 7, although the company also makes bottles from No. 2 HDPE plastic.

However, No. 7 plastics include those that are gentler to the environment.

For example, the microwaveable beige bowl pictured here is made of plant-based, biodegradable plastic. Plastic dishware and cutlery from companies such as Cereplast are increasingly available in natural food stores and in San Francisco restaurants. It and other companies use plants including corn, tapioca, rice, and potatoes. However, opponents of genetic modification warn against the use of modified corn by producers such as Cargill's NatureWorks.

Among the more "natural" plastics are new bags that are made from petroleum, but that break down more quickly than others and can be added to a compost heap. Companies including Metabolix are experimenting with growing plastics within plants. However, some green watchdogs argue that these materials are too new to be fairly labeled as eco-friendly.

The market for bioplastics will grow to 15 to 20 percent of the market by 2025, according to a report by Helmut Kaiser Consultancy.