A method for reducing biological contamination of indoor air, which method includes providing a concentrated salt solution, which is either (i) a brine capable of responding to aeration thereof by a rapid increase of its Redox potential, wherein the rate of said increase is greater than the rate of increase observed for a 45% (w/w) calcium chloride solution subjected to identical aeration conditions; or (ii) a brine which is passed through an electrolytic cell in order to raise its Redox potential; circulating said concentrated salt solution through a treatment zone; causing a stream of biologically contaminated air to flow through said treatment zone, such that said contaminated air is contacted with said salt solution in said treatment zone; and withdrawing purified air from said treatment zone.