A method and system for eliminating, in whole or in part, contaminants in a gas stream or gaseous body such as air or an exhaust gas or gas emission stream is provided. The method includes the steps of contacting the gas stream or gaseous body with a biocide containing liquid so as to eliminate, in whole or in part, the contaminant(s). The biocide containing liquid is typically an aqueous solution, suspension, or emulsion comprising hydrogen peroxide (H₂O₂) in combination with a catalyst for enhancing the activity thereof, the catalyst preferably comprising a superoxidedismutase formed from the combination of elemental copper (Cu), silver (Ag), manganese (Mn) and zinc (Zn). The method and system find particular application in the combating of sick building syndrome (SBI) and building related illness (BRI).