Your lunch bag is a bacteria magnet

Conservatives often point out that laws, no matter how benign they may appear, have unintended consequences. They can reverberate in ways that not many people foresaw and nobody wanted: Raising the minimum wage can increase unemployment; prohibition can create black markets.

The efforts in many cities to discourage the use of plastic bags demonstrate that such unintended consequences can be, among other things, kind of gross.

San Francisco has been discouraging plastic bags since 2007, saying that it takes too much oil to make them and that used bags pollute waterways and kill marine animals. In 2012, it strengthened its law. Several West Coast cities, including Seattle and Los Angeles, have also adopted bans for environmental reasons.

The government of Washington, D.C., imposes a 5-cent plastic-bag tax. (Advocates prefer to call it a "fee" because taxes are unpopular.) Environmental groups and celebrity activists, including Eva Longoria and Julia Louis-Dreyfus, support these laws.

The plastic-bag industry, predictably, wants to throw them away. It says that the making of plastic bags supplies a livelihood to 30,000 hardworking, law-abiding, patriotic Americans, many of whom have adorable children to support. It cites a 2007 report by San Francisco's Environment Department that said plastic bags from retail establishments, the target of the ban, accounted for only 0.6 percent of litter.

Most alarmingly, the industry has highlighted news reports linking reusable shopping bags to the spread of disease. Like this one, from the Los Angeles Times last May: "A reusable grocery bag left in a hotel bathroom caused an outbreak of norovirus-induced diarrhea and nausea that struck nine of 13 members of a girls' soccer team in October, Oregon researchers reported Wednesday." The norovirus may not have political clout, but evidently it, too, is rooting for plastic bags.

Warning of disease may seem like an over-the-top scare tactic, but research suggests there's more than anecdote behind this industry talking point. In a 2011 study, four researchers examined reusable bags in California and Arizona and found that 51 percent of them contained coliform bacteria.

That study also found, happily, that washing the bags eliminated 99.9 percent of the bacteria. It undercut even that good news, though, by finding that 97 percent of people reported that they never wash their bags.

Jonathan Klick and Joshua Wright, who are law professors at the University of Pennsylvania and George Mason University, respectively, have done a more recent study on the public-health impact of plastic-bag bans. They find that emergency-room admissions related to E. coli infections increased in San Francisco after the ban. (Nearby counties did not show this increase.)

And this effect showed up as soon as the ban was implemented. ("There is a clear discontinuity at the time of adoption.") The San Francisco ban was also associated with increases in salmonella and other bacterial infections. Similar effects were found in other California towns that adopted such laws.

Klick and Wright estimate that the San Francisco ban results in a 46 percent increase in deaths from foodborne illnesses, or 5.5 more of them each year. They then run through a cost-benefit analysis employing the same estimate of the value of a human life that the Environmental Protection Agency uses when evaluating regulations that are supposed to save lives. They conclude that the anti-plastic-bag policies can't pass the test -- and that's before counting the higher health-care costs they generate.

The authors argue, not completely convincingly, against the idea that regular washing and drying of reusable bags would solve the problem. They point out that the use of hot water and detergent imposes environmental costs, too. And reusable bags require more energy to make than plastic ones. The stronger argument, it seems to me, is that 97 percent figure: Whatever the merits of regularly cleaning the bags, it doesn't appear likely to happen.