Oscillation at $5.5\mathrm{\mu }\mathrm{m}$ wavelength has been investigated in the low-phonon, moisture-resistant ${\mathrm{RbPb}}_2{\mathrm{Cl}}_5:{\mathrm{Dy}}^{3+}$ crystal in free-running mode at room temperature. In line with our knowledge this is the longest laser wavelength for a rare-earth doped crystal that does not require any special precautions for survival. Spectra and kinetics of luminescence of the new laser crystal were investigated under pumping to $1.3\mathrm{\mu }\mathrm{m}$ absorption band, corresponding to lasing experiments. Emission cross sections were determined for mid-IR transitions. It was found that emission cross section on the ${}^{6}H_{9∕2}+{}^{6}F_{11∕2}\to {}^{6}H_{11∕2}$ laser transition at $5.5\mathrm{\mu }\mathrm{m}$ wavelength is as high as $0.6\times {10}^{-20}{\mathrm{cm}}^2$ .