The present invention relates to an add on filter for a packaged air handling unit, such as a heating, ventilation and/or air conditioning unit, and method incorporating an add on filter for applying ultraviolet light to an environment to create oxidizing agents for killing microbes, such as bacteria, mold, and viruses, and for destroying odors. The add on filter may be coupled to the blower housing of existing packaged heating, ventilation and/or air conditioning units. The add on filter includes an ultraviolet light source for emitting a broad spectrum of ultraviolet light and a catalytic target structure mechanically coupled to the ultraviolet light source. The system and method result in the production of Hydroxyl Radicals, Super Oxide ions and Hydro Peroxide for killing microbes.