Abstract: UV-C light assembly is designed to kill germs (bacteria, molds, protozoa, virus, and yeast) in the forced airstreams of HVAC systems. Thus the UV-C light kills and prevents the spreading of germs into other rooms or spaces. An air-flow activated switch is invented for turning on the UV-C lights when airstream passes through and turning off when airstream stops the flowing in the HVAC systems. The invented air-flow activated switches include electromagnetic induction and other proximity sensor switches such as infrared, ultrasonic, capacitive, laser, radar, inductive and etc. The UV-C light assembly is installed inside air duct. The UV-C germicidal assembly is an easy add-on to an existing HVAC system for indoor air purification. The UV-C light sources are either LEDs or discharge lamps.