An igniter for a flare stack for waste gases is actuated periodically by an intermittently firing spark plug. A fuel gas for ignition aspirates combustion air through openings in a housing enclosing a supply line for the fuel gas. Flashback from the intermittent ignition of the fuel gas is prevented and noise is reduced by closure means responsive to the pressure differential between the interior of the housing and the ambient air. The closure means close the openings when the pressure within the housing is higher than the pressure of the ambient air.