A method of treating air passing through an air handling unit having a housing extending around an air handling space and a primary air treatment system that treats air as the air passes through the air handling unit between air inlet and outlet locations on the air handling unit. First and second fixtures are obtained, each having a housing and a UV light source. The first and second fixtures are strategically mounted to the air handling unit so that UV light from each of the UV light sources on the first and second fixtures treats air moving through the air handling unit with the UV light sources on the first and second fixtures energized. The first and second fixtures are serially electrically connected to a power source to thereby cause the UV light sources on the first and second fixtures to be energized through the power source.
