According to the ILEA’s excellent summation of the study, which I’m paraphrasing here, paper bag takes wood, petroleum and coal to make, and uses the equivalent of 550 kilojoules of wood as feedstock, plus 500 kJ of petroleum and 350 kJ of coal for process energy, making its total energy amount to 1,680 kJ.

Plastic bags are made of natural gas, petroleum, and coal.  Because it takes about two standard grocery store plastic bags to equal the carrying capacity (and consequently, usefulness) of one paper bag, the researchers measured plastic bags in pairs.  Two take about 990 kJ to produce, and combined with their feedstock energy, the total energy use of two plastic bags amounts to 1,470 kJ, or about 87% of the energy claimed by the paper bag. 

HOWEVER.  According to the EPA, in 2002 Americans recycled 0.6% of our plastic bags and 19.4% of our paper bags.  Recycling makes a big difference in energy consumption, since the feedstock energy gets used over and over again and thus decreases.  So, as recycling rates change for the better (let’s hope), it might eventually more sense to choose paper—for now, plastic is the better bet. 

In terms of pollutant emissions, plastic fared better than paper, too.  Physically, plastics produce less solid waste since they take up less space, and as recycling rates improve the difference will only aggravate, since every recycled bag avoids becoming postconsumer waste.  Atmospherically, plastics also produce less pollutants, and if we’re talking waterborne pollutants, plastics lead again, with two plastic bags producing only 7% of the waterborne waste of one paper sack (1.5g vs. 0.1g). 

I wonder how many times you’d have to use (“recycle”) a canvas bag to make up for its feedstock energy, which must be much higher than paper and plastic’s?