Abstract: Air is mechanically entrained in an aqueous solution of surfactant and poly(oxyalkylene) polyisocyanate polymer, the poly(oxyalkylene) portion of which contains sufficient oxyethylene units to render the polymer water-soluble and hydrophilic, to form a fluid, water-based, air foam which is sprayed or otherwise applied to exposed surface of a substrate, such as a body of hazardous material, the applied foam gelling or increasing in viscosity, due to reaction of the polyisocyanate polymer with water to form a poly(oxyalkylene) polyurea polymer, and forming a persistent gelled air foam or viscous air foam in the form of a coating on the exposed surface, thus sealing or otherwise protecting or controlling the substrate.