The behavior of the elemental depletions in the ISM along lines of sight with a very low value of the average hydrogen density is examined. A simple model in the frame of the McKee-Ostriker (1977) theory has been used to estimate the line-of-sight filling factors as a function of n-bar for those interstellar phases in which a significant degree of ionization is expected. The results show that a substantial fraction of ionized gas outside classical H II regions may well be present along the line of sight when n-bar is less than about 0.1/cu cm. If this effect is neglected, the chemical abundances relative to hydrogen can be overestimated up to 0.5 dex in logarithm, and the depletions underestimated by the same amount. A recent IUE survey of interstellar abundances has been reanalyzed, and it is found that for all the elements investigated the slope of the correlation D(app)-log n-bar is steeper in the low-density range. This disagrees with recent models for elemental depletions.