At low (400), but not at high (> 930), ppm CO2 the entropy produced per glucose mole is lower in fermentation than in respiration. Then, applied to the proliferation of the fermentative cancer cells within vertebrate body, the Prigogine theorem of the trend to minimize the rate of entropy production would predict that cancer must growth faster than normal tissues at low but not at high CO2 concentrations. Accordingly, re-examination of epidemiologic data shows link between low cancer incidence or death and jobs or habits likely exposed to high CO2 concentrations. Further experiments should confirm the suggested inhibition of cancer by CO2.