Accurate values for the density shift and broadening of five isolated ${\mathrm{H}}_{\ensuremath{\alpha}}$ fine-structure transitions have been measured. Doppler-free polarization and double-quantum saturation spectroscopy with a cw dye laser are used in a He dc discharge with 1%
H2. The negative shifts and the huge broadening values are correctly predicted by an impact-approximation theory taking into account the large cross sections for fine-structure transfer (quenching) collisions and only Van der Waals interaction between
H∗ and He. Consequences of a comparison with the positive shift of the unresolved ${\mathrm{H}}_{\ensuremath{\alpha}}$ line and of the alkali-metal
D lines in He buffer gas are discussed. For pressures below 0.1 Torr hfs decoupling collisions give rise to a positive shift of single ${\mathrm{H}}_{\ensuremath{\alpha}}$ fine-structure lines and to a collisional narrowing effect analogous to that observed in nuclear magnetic resonance.