In late March, the San Francisco Board of Supervisors voted 10-1 to become the first American city to ban the use of non-biodegradable plastic bags by larger retailers, including supermarkets and drug stores. Alternatives include bags made of recycled paper or biodegradable plastic. But neither of these options is without problems.

Critics of plastic bags argue that the petroleum products required to produce them make these types of bags an environmental problem. The feedstocks for paper bags, on the other hand, are renewable wood products. But this is only part of the story.  According to the Institute for Lifecycle Environmental Assessment, total production of a single plastic bag uses 120 kJ (kilojoules) of petroleum, but a single paper bag uses 500 kJ of petroleum. 

When all elements are considered, the total amount of energy used by a single paper bag is 1,680 kJ, compared to 735 kJ for a single plastic bag. In other words, a paper bag is, in effect, a double bag requiring more than twice the amount of energy. Plastic is also the winner from the standpoint of pollutant production. 

The lifecycle of one plastic bag produces 0.55 kg of atmospheric pollutants and 0.1 g of waterborne pollutants. A paper bag produces 2.6 kg of atmospheric and 1.5 g of waterborne pollutants. In other words, the paper bag produces more than four times the atmospheric pollutants and 15 times the waterborne pollutants.

Another argument against the plastic bags is that they can take hundreds of years to degrade in the environment.  However, in a landfill, none of the bags will truly degrade; most modern landfills are designed not to allow anything to break down and leave the landfill system because of potential dangers to nearby groundwater and soils. As a result, the most important issue for landfills is how much space the trash takes up. In that regard, easily compressible plastic bags are better than bulkier paper bags. 

The biggest environmental issue related to plastic bags, then, is littering.  An estimated one to three percent of the plastic bags consumed each year end up in the litter stream. One significant concern of plastic bags is the danger that they pose as litter in marine environments, where they can choke or strangle aquatic wildlife. However, even the biodegradable plastic bags won’t decompose in fresh or salt water environments for an estimated eight to14 months.  Thus there is still a clear danger to fish and other aquatic species from the biodegradable plastic bags. 

City officials hope that switching to biodegradable plastic bags, which can be composted, will encourage citizens to add their bags to curbside compost bins. But if proper disposal of the plastic bags (such as through the landfill or into a plastic bag recycling bin available at numerous grocery stores) were typically being practiced by every consumer, the litter problem would not exist. That the new rule relies heavily on proper disposal may not result in significant reductions in the amount of litter. Since regular plastic bags will still be available at smaller retailers and the like, it will be difficult for consumers to keep track of what sort of plastic bags they have, leading to challenges in making sure that only the biodegradable bags make it into the compost bins. 

Further, biodegradable plastic bags cost between eight and 10 cents, compared to a penny for the standard plastic bag.  Supporters of the San Francisco ban say the price of biodegradable plastic bags would drop if more municipalities required them, but this may not be the case. The biodegradable bags are made from soy and especially corn.  Given the increasing demands for corn from the ethanol industry, the cost of producing these biodegradable bags is likely to increase by a significant amount.

The desire for a cleaner environment is a valuable goal. But San Francisco's bag mandate will impose one potentially costly option on consumers with no clear environmental benefits.