A method for reducing the biological contamination of indoor air and sanitizing the walls and surfaces in a closed space, which method comprises causing a stream of air to flow upwardly through a treatment zone, and to contact, on a substantially horizontally aligned perforated surface, with a stream of brine solution having Redox potential of not less than 500 mV, whereby an active layer is formed on said horizontally aligned perforated surface, in which layer the brine and air mix, and delivering the treated air which exits said treatment zone into the space to be sanitized.
