A ventilation unit is for ventilating an indoor space, and incorporates an air cleaning device and mechanical restrictor for controlling a restriction to a flow resulting from a pressure differential across the unit. There is determination of the inside and outside air pressures in the vicinity of the unit and of air quality parameters inside and outside. The air cleaning device and the mechanical restrictor are controlled in dependence on the determined air pressures and air quality parameters. This provides a fan-less, and hence low-power ventilation unit, which relies on throttling the natural air flow across the unit to provide flow control, and hence enable control of air quality. The air cleaning device may be operated only when the flow is into the inside space, saving power.