Now that Lacey, Olympia, Tumwater and unincorporated areas of Thurston County have implemented a ban on plastic grocery bags, Thurston County Solid Waste completed a study of what kind of grocery bag residents now use. The results show the ban has likely increased emissions of greenhouse gases and increased water pollution that contributes to "dead zones" in the ocean.

With the new mix of grocery bags being used in Thurston County, water pollution associated with grocery bags more than doubled and greenhouse gas emissions increased by 80 percent. These results are based on the best life-cycle analysis of the impact of grocery bags, completed by the United Kingdom Environment Agency in 2011.

Politicians claim the ban is working because people are, indeed, using fewer plastic bags. The Thurston county survey found most people shifted from plastic bags to reusable bags:

The numbers don't add up to 100 percent due to other types of carriers, including boxes and hand carrying.

From an environmental standpoint, however, this shift is bad news, increasing both carbon emissions and water pollution. Paper bags and reusable bags use more energy and generate more water pollution during the manufacturing process.

To calculate the total impact, we looked at how often the different types of bags are used and the total "global warming potential" and eutrophication impact of each type of bag. Research demonstrates that people reuse plastic bags about 50 percent of the time (they are used 1.5 times on average) and reusable bags are used 15 times.

For the impact, we used the best available science of life-cycle impacts from the UK Environment Agency. Cotton, reusable bags cause much more air and water pollution than woven plastic reusable bags, but it is unclear what percentage people in Thurston are using. Based on experience elsewhere and the countywide comments they received, it is likely that most reusable bags are woven plastic.

Assuming 70 percent of the reusable bags are woven plastic and 30 percent are cotton, the bag ban has significantly increased pollution:

• Greenhouse gas emissions increased by 80 percent.
• Water pollution that creates "dead zones" more than doubled, rising to 211 percent of previous levels.

You can see our entire spreadsheet here, where we also examine the impact if all reusable bags are woven plastic or if they are all cotton. You can adjust the reuse rate, the mix of bag type and see the results.

No matter how it is calculated, however, the bag ban is a bad result for the environment.

Supporters of the ban will claim fewer bags are reaching the water and harming marine life. These claims are never backed up with any data. Weighed against a doubling of "dead zone" water pollutants, basing a policy on zero data is unscientific.