We investigate the possibility of selective absorption of ionizing photons of energy >= 13.6 eV by the dust in HII regions. Selective absorption could possibly produce the observed low helium ionization in the warm ionized medium (WIM) and in the diffuse ionized regions surrounding HII regions. New measurements of the singly ionized helium to hydrogen ratio (n_{He^+}/n_{H^+}) toward diffuse gas surrounding three Ultra-Compact HII regions (G10.15-0.34, G23.46-0.20 & G29.96-0.02) were made near 5 GHz with the Green Bank Telescope. These observations show that helium is not uniformly ionized in the three observed sources and the upper limits of the n_{He^+}/n_{H^+} ratio obtained is 0.05. Our analysis indicates that the dust absorption cross section needs to be larger by a factor >= 1.8 for photons of energy 24.6 eV compared to that at 13.6 eV to account for the low helium ionization. We compare the estimated energy dependence of the absorption cross section with that given by the Weingartner & Draine (2001) dust model and discuss possible causes for the discrepancies.