Obesity is largely blamed on calories (too many) and exercise (too little), but recent studies suggest that chemical exposure may also pack on pounds. And it's tough to diet from so-called "obesogens," which show up in everything from pesticides to food containers. Chemicals found to produce more and larger fat cells in mice include waterproof-paint ingredient tributyltin; diethylstilbestrol, which was widely prescribed to pregnant women from the 1940s to the '60s; and estrogen-like bisphenol A, which showed up in 95 percent of people tested by one recent study. BPA promotes fat-cell activity in utero, producing "lifetime effects" that occur at "phenomenally small levels" of exposure, says biological sciences professor Frederick vom Saal; he dismisses the chemical industry's claim that BPA poses no health risk as a "blatant lie." The production and use of BPA has quadrupled in the last couple of decades, in roughly the same timeline that obesity has noticeably risen. Coincidence? Fat chance.