ISO 7494-2:2003 specifies requirements and test methods for the materials, design and construction of the water and air supply within dental units in order to ensure that the compressed water and air supplied via the dental unit are of appropriate quality. It includes provisions for the prevention of retraction of oral fluids into the water supply of the dental unit. ISO 7494-2:2003 does not address prevention of contamination and/or proliferation of hazardous microorganisms (for example bacteria, viruses) in the dental unit.