An air purification system configured to remove contaminants from air in an air handling system, the air purification system comprising: a power supply providing variable power to an AC transformer; an ozone generating device coupled to the power supply, wherein a generation rate of ozone is a function of input power from the power supply, the input power varying in response to the variable power provided to the AC transformer, an ozone sensor configured to sense a concentration of ozone in an airstream and located downstream of the ozone generating device; a contaminant sensor, a controller configured to vary input power from the power supply to the ozone generating device to control the rate of ozone generation, wherein the input power is a function of the at least one of the concentration of the ozone and the concentration of volatile organic compounds; the controller also configured to vary the flow rate of air; characterised in that the air purification system further comprises: a particle filter, the contaminant sensor configured to sense a concentration of volatile organic compounds in the air stream and located upstream or downstream of the ozone generating device; and wherein the controller is configured to vary the flow rate of air through the ozone generating device as a function of the concentration of ozone and the concentration of volatile organic compounds, and wherein the concentration of ozone in the airflow through the ozone generating device decreases as the airflow increases. a power supply providing variable power to an AC transformer; an ozone generating device coupled to the power supply, wherein a generation rate of ozone is a function of input power from the power supply, the input power varying in response to the variable power provided to the AC transformer, an ozone sensor configured to sense a concentration of ozone in an airstream and located downstream of the ozone generating device; a contaminant sensor, a controller configured to vary input power from the power supply to the ozone generating device to control the rate of ozone generation, wherein the input power is a function of the at least one of the concentration of the ozone and the concentration of volatile organic compounds; the controller also configured to vary the flow rate of air; characterised in that the air purification system further comprises: a particle filter, the contaminant sensor configured to sense a concentration of volatile organic compounds in the air stream and located upstream or downstream of the ozone generating device; and wherein the controller is configured to vary the flow rate of air through the ozone generating device as a function of the concentration of ozone and the concentration of volatile organic compounds, and wherein the concentration of ozone in the airflow through the ozone generating device decreases as the airflow increases. a particle filter, the contaminant sensor configured to sense a concentration of volatile organic compounds in the air stream and located upstream or downstream of the ozone generating device; and wherein the controller is configured to vary the flow rate of air through the ozone generating device as a function of the concentration of ozone and the concentration of volatile organic compounds, and wherein the concentration of ozone in the airflow through the ozone generating device decreases as the airflow increases.