An air purification system includes a photocatalyst on a support disposed to contact airflow through an airflow channel passing across or through the support; an ultraviolet light emitting diode (UV-LED) disposed to emit ultraviolet light onto the photocatalyst, the UV-LED operated at a less than one hundred percent duty cycle, the duty cycle determined at least in part as a function of a desired minimum volatile organic compound conversion rate of air flowing through the airflow channel and a desired maximum by-product concentration of air flowing through an outlet of the airflow channel.