NEW YORK (Fortune) -- How, exactly, did Wal-Mart become the new Food and Drug Administration?

The giant retailer, along with CVS (CVS, Fortune 500) and Toys 'R Us, announced recently that it plans to stop selling baby bottles containing the chemical bisphenol-A.

The question is, why? Bisphenol-A has been widely used since the 1950s. The Food and Drug Administration, as well as Japanese and European regulators, have no problems with it. Canada is about to ban it from baby bottles, but officials term the move purely precautionary.

To be sure, other scientists worry because animal studies have linked small doses of BPA to cancer and other health problems. But scientific debate isn't driving the baby bottle war; a hard-hitting push by activist groups, politicians and trial lawyers is.

As traditional media picked up the story in the spring, spooked retailers like Wal-Mart (WMT, Fortune 500) backed away from BPA, while companies that had done so earlier scored a PR coup that boosted their fortunes.

One could argue, as BPA opponents do, that the government is too slow to take action to protect health, so private action by consumers and companies is necessary. Or one could argue, as does Steve Hentges, a chemist and industry lobbyist that "the science can't compete with the emotion."

What's inarguable, though, is how rapidly markets can by reshaped today by an activist campaign that catches fire online. The Environmental Working Group and the authors of the book Our Stolen Future have used the Internet to sound alarms about bisphenol-A. The Bisphenol-A Free portal keeps a running tally of bad news reports. Bloggers at www.safemama.com and www.healthychild.org pound away at the chemical industry.

Got BPA?

BPA is everywhere, used to make polycarbonate, a rigid, clear plastic for bottles, bike helmets, DVDs and car headlights. It's also an ingredient in epoxy resins, which coat the inside of food and drink cans. About 93% of Americans tested by the Centers for Disease Control had the chemical in their urine.

If opponents drive BPA out of the food supply, consumers will pay. Some BPA-free plastic bottles sell for $10 each, more than twice the price of bottles with BPA. Baby bottles made of glass can break, potentially causing injury. Replacing BPA in the lining of cans would mean retooling all that packaging, and it's not clear that there are safe alternatives.

A handful of companies emerged as winners this spring when the BPA story got big: Whole Foods Market (WFMI, Fortune 500), which pulled BPA baby bottles and cups off its shelves several years ago; Eastman Chemical (EMN, Fortune 500), which introduced a plastic alternative called Triton last year; and Born Free, a private company started in 2006 specifically to provide BPA-free baby bottles.

Others, including SABIC Innovative Plastics, which was formerly the plastics division of GE and is now the U.S.'s biggest manufacturer of BPA, presumably suffered. (SABIC declined to comment on the financial impact.) Baby-bottle makers including Avent America, Evenflo and Gerber Products are now being sued because they sold products made with BPA.

The BPA battles were fought like a political campaign, with catchy soundbites, press releases, personal attacks, and warring Web sites. The anti-BPA general is Dr. Frederick vom Saal. He has testified before state legislatures and appeared on TV to denounce BPA in terms that gloss over the scientific uncertainty. Referring to the fact that BPA is a mild estrogen, he says things like "the idea that you're using sex hormones to make plastic is just totally insane."

Vom Saal isn't the only scientist warning about BPA. Dozens of others are active in trying to ban it. But Vom Saal is the most visible and the most vitriolic. He accuses a Dow Chemical (DOW, Fortune 500) executive of trying to bribe him, a charge the company strongly denies.

The industry, in turn, has gone after vom Saal, noting that he has appeared in a video news release produced by Born Free, which makes BPA-free baby products. He also consulted with a New York law that's suing baby bottle manufacturers. Vom Saal says he has not taken any money from a company or law firm.

Fenton Communications, a Washington, D.C. PR firm, is another key warrior against BPA. Fenton's clients have included Born Free and its BPA-free bottles; the Environmental Working Group , which has led the fight against BPA for years; and trial lawyers. Fenton also works for liberal advocacy groups like MoveOn that support Democrats in Congress who have sponsored legislation to ban BPA from children's products.

Sometimes these groups appear to be working in concert. Last year, the Environmental Working Group tested canned foods and found that "many Americans are exposed to BPA above levels shown to be harmful in laboratory studies."

This year, a congressional investigation led by Reps. John Dingell and Bart Stupak asked manufacturers of infant formula to remove BPA from their cans. They declined. All this generated headlines - and worry.

Science for sale

The chemical industry has tried to get its message out, too. See the Web sites http://www.bisphenol-a.org/ and http://www.factsonplastic.com/ , which offer a defense of BPA. But the industry is often depicted as a "special interest group," while environmentalists and politicians are seen as serving the "public interest."

It isn't that simple, of course. Controversy helps the enviros raise money, Democratic politicians love to find fault with the Bush administration. And the trial lawyers sense a big payday.

The problem for the chemical industry is that its track record doesn't exactly inspire confidence. The Dingell-Stupak investigation of BPA looked at what the congressmen call "science for sale," finding examples of consultants promising clients how research would turn out. Needless to say, this is not how science is supposed to work.

This became a key element of the attack on BPA. Dingell has said he's concerned about whether "the science FDA relied on to approve the use of Bisphenol A was bought and paid for by industry."

But, as Dingell must know, the FDA typically uses industry research because it doesn't have the money to conduct independent studies of the thousands of chemicals on the market. It then reviews what industry produces.

By April, all the news had turned bad for BPA. "There is no safe level of BPA," declared Dr. Nancy Snyderman, an NBC medical reporter, on the Today show. (Maybe NBC is the new FDA?) The Canadian government recommended its ban on baby bottles with BPA.

A lengthy draft report from a part of the National Institutes of Health found "some concern" about the effect of BPA on fetuses, infants and children at current exposure levels. The NTP report is a model of thoroughness and nuance. Naturally, that makes it a flop in the court of public opinion.

With fear in the air, in the space of a few days Wal-Mart, Toy 'R Us, CVS and others said they will phase out baby bottles containing BPA. Nalgene, a water bottle maker, and Playtex also said they will stop using the chemical.

I asked Wal-Mart why the company is removing a legal product, which may or may not be dangerous, while continuing to sell cigarettes, which are incontrovertibly harmful. "We sell products our customers want to buy," responded spokeswoman Linda Brown Blakley. "Our customers are telling us they want this option."

That won't end the war. You can expect to see anti-BPA forces keep up the pressure. Will soup, soda and beer cans be next?

And is this any way to make judgments about public health?