San Francisco wants to trash the plastic bag.

San Francisco’s Board of Supervisors passed an initiative in late March to phase out plastic bags in the city’s grocery stores over the next six months and in pharmacies over the next year. Although local media sources and environmental groups around the country praised the move, some critics have voiced concerns that one of the proposed alternatives — compostable plastic bags — are hardly an improvement.

According to the city’s Department of the Environment, grocery stores and pharmacies hand out 90 percent of plastic bags used in San Francisco. Under the new law, the stores will be allowed to use only recycled paper bags or compostable plastic bags that are certified by the American Society for Testing and Materials and verified by the Biodegradable Plastic Institute.

Reducing landfill waste and litter is a primary stated goal of the law. While some critics have voiced concerns that the compostable bags don’t break down, Jerry Bartlett of Seattle-based Cedar Grove Composting says certified compostable bags that have been through stringent simulation tests actually do compost in a commercial setting.

Still, Bartlett points out, even though San Francisco has curbside compost bins, not all grocery store and pharmacy customers compost the bags. If they don’t, the bags will go into landfills, and Bartlett says not all bags that are suitable for composting are designed to breakdown in landfill conditions. It remains to be seen what sorts of bags San Francisco grocery stores and pharmacies will purchase, but BioBags, manufactured by Norwegian company Polar Gruppen, are recommended by both Cedar Grove and the Biodegradable Plastic Institute.

The ban also aims to cut the use of petroleum. “As a polymer chemist, I love plastic,” says Mark Michalovic, consultant for educational services in the Roy Eddleman Institute for Interpretation and Education at the Chemical Heritage Foundation. “But while I appreciate that plastics are amazing materials, I also know they are made from a finite resource: petroleum.”

Michalovic says petroleum-based plastic should be saved for products that (unlike grocery bags) can’t be made with another material. The California Grocer’s Alliance and the Progressive Bag Alliance (PBA), a group of plastic bag manufacturers that advocates for plastic bag recycling, worked with the city to build up plastic bag recycling programs, says Andy Devilling, vice president of sales and marketing for PBA member Starpak.