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Are bans on plastic bags making people sick? Not so fast.

Everyone loves counterintuitive studies. They make for catchy headlines. And so, last fall, when two academics posted a working paper suggesting that San Francisco's eco-friendly ban on plastic bags might actually be killing people, it was too good to ignore.

The paper, by Wharton's Jonathan Klick and Joshua Wright, found that food-borne illnesses in San Francisco increased 46 percent after the bag ban went into effect in 2007--with no such uptick in neighboring counties. Most likely, the authors concluded, this was due to the fact that people were putting their food into dirty reusable bags and not washing them afterward. The kicker: "Our results suggest that the San Francisco ban led to, conservatively, 5.4 annual additional deaths."

That sounds terrible. And it gave rise to plenty of alarming headlines. But was the conclusion actually warranted? Are plastic-bag bans really killing people? At least one public-health expert is very skeptical.

Let's start with the original study itself. One thing Klick and Wright did was to examine the number of emergency room visits for E. coli infections in San Francisco County in the 10 quarters before and after the plastic bag ban took effect. They found a big jump right after the ban:

For comparison's sake, there was no such jump in E. coli-related emergency room visits in nearby Bay Area counties that didn't have a ban:

This is certainly suggestive. But according to Tomás Aragón, an epidemiologist at UC Berkeley and health officer for the city of San Francisco, these graphs don't prove nearly as much as you might think.

In a memo (pdf) released earlier this week, Aragón explained that this is an example of the "ecological fallacy." In order to establish a link between the bag ban and illnesses, the authors would have to show that the same people who are using reusable bags are also the ones getting sick. This study doesn't do that. Aragón also points out that emergency-room data can be very incomplete—under an alternate measure, there's been no rise in E. coli at all.

Aragón also offers an alternative hypothesis for the recent rise in deaths related to intestinal infections. A large portion of the cases in San Francisco involve C. difficile enterocolitis, a disease that's often coded as food-borne illness in hospitals. And this disease has become more common in lots of places since 2005, all around the United States, Canada, and Europe (for yet-unexplained reasons). "The increase in San Francisco," he notes, "probably reflects this international increase."

Klick, for his part, told the San Francisco Chronicle that it was hard to "rule out the possibility that there was something peculiar that happened in San Francisco." That's fair enough. But it suggests his study could only raise the question, not prove a link.

In any case, it's a question worth asking. Researchers have found that reusable grocery bags tend to contain plenty of bacteria—not surprisingly, it's a bad idea to carry meat or fish in a cloth sack and then reuse it without washing. And, true, there's a case to be made that reusable bags aren't any more eco-friendly than plastic bags if they need endless scrubbing.

But be wary of claims that bag bans are killing people—at least for now. "The idea that widespread use of reusable bags may cause gastrointestinal infections if they are not regularly cleaned is plausible," Aragón concludes. "However, the hypothesis that there is a significant increase in gastrointestinal foodborne illnesses and deaths due to reusable bags has not been tested, much less demonstrated in this study."

(Link via Mark Thoma of Economist's View)