The invention pertains generally to dental units (e.g., a high-speed dental handpiece (11), an ultrasonic scaler (13) and an air/water syringe (12)) connected to a water supply that provides coolant and rinse water to the dental units. More particularly the invention pertains to a dental unit using water that is continuously treated with a water treatment agent, such as microbiocidal silver ions. The treated dental unit water is supplied to the dental units via dental unit waterlines (6, 7, 8, 9, and 10).