  Recent radio surveys have revealed pulsars with dispersion and scattering
delays induced by ionized gas that are larger than the rest of the observed
pulsar population, in some cases with electron column densities (or dispersion
measures; DMs) larger than the maximum predictions of Galactic electron density
models. By cross-matching the observed pulsar population against HII region
catalogs, we show that the majority of pulsars with $\rm DM > 600$ pc cm$^{-3}$
and scattering delays $\tau(1\ {\rm GHz}) > 10$ ms lie behind HII regions, and
that HII region intersections may be relevant to as much as a third of the
observed pulsar population. The fraction of the full pulsar population with
sightlines intersecting HII regions is likely larger. Accounting for HII
regions resolves apparent discrepancies where Galactic electron density models
place high-DM pulsars beyond the Galactic disk. By comparing emission measures
(EMs) inferred from recombination line observations to pulsar DMs, we show that
HII regions can contribute tens to hundreds of pc cm$^{-3}$ in electron column
density along a pulsar LOS. We find that nearly all pulsars with significant
excess (and deficit) scattering from the mean $\tau$-DM relation are spatially
coincident with known discrete ionized gas structures, including HII regions.
Accounting for HII regions is critical to the interpretation of radio
dispersion and scattering measurements as electron density tracers, both in the
Milky Way and in other galaxies.
