  Where dust and gas are uniformly mixed, atomic hydrogen can be traced through
the detection of far-infrared (FIR) or UV emission of dust. We considered, for
the origin of discrepancies observed between various direct and indirect
tracers of gas outside the Galactic plane, possible corrections to the zero
levels of the Planck-HFI detectors. We set the zero levels of the Planck High
Frequency Instrument (HFI) skymaps as well as the 100 $\mu$m map from
COBE/DIRBE and IRAS from the correlation between FIR emission and atomic
hydrogen column density excluding regions of lowest gas column density. A
modified blackbody model fit to those new zero-subtracted maps led to
significantly different maps of the opacity spectral index $\beta$ and
temperature $T$ and an overall increase in the optical depth at 353 GHz
$\tau_{353}$ of 7.1$\times$10$^{-7}$ compared to the data release 2 Planck map.
When comparing $\tau_{353}$ and the HI column density, we observed a uniform
spatial distribution of the opacity outside regions with dark neutral gas and
CO except in various large-scale regions of low NHI that represent 25% of the
sky. In those regions, we observed an average dust column density 45% higher
than predictions based on NHI with a maximum of 250% toward the Lockman Hole
region. From the average opacity $\sigma_{e
353}$=(8.9$\pm$0.1)$\times$10$^{-27}$ cm$^2$ we deduced a dust-to-gas mass
ratio of 0.53$\times$10$^{-2}$. We did not see evidence of dust associated to a
Reynolds layer of ionized hydrogen. We measured a far-ultraviolet isotropic
intensity of 137$\pm$15 photons s$^{-1}$cm$^{-2}$sr$^{-1}$$A$$^{-1}$ in
agreement with extragalactic flux predictions and a near-ultraviolet isotropic
intensity of 378$\pm$45 photons s$^{-1}$cm$^{-2}$sr$^{-1}$$A$$^{-1}$
corresponding to twice the predicted flux.
