Experimental charge-changing cross sections for fast hydrogen atoms and ions in a water gas as a function of impact speed. “AR” refers to the “additive rule”: when no experimental results for H₂O are available, results for H₂ and O₂ are combined to give an estimate (see text for details); experimental uncertainties for these estimates are at least 25%. Polynomial fits in thick continuous or dashed lines are also shown, whose coefficients are listed in Table 2. Smooth extrapolations at low and high energies are indicated as thin dotted lines.