We report on a feasibility study of a free-space Quantum Key Distribution setup operating at a mid-infrared wavelength. Alice sends polarization-coded pseudo-single photons from a Quantum Cascade Laser at 4.6 μm to Bob, who uses a nonlinear crystal and a Silicon Avalanche Photodiode to perform the detection via Sum Frequency Generation. Theoretical predictions, based on a proof-of-principle experiment, show that, under certain foggy atmospheric conditions, the proposed system is barely affected, whereas other systems operating at standard near-infrared wavelengths would simply not work at all.