This part of ISO 18560 specifies a test method for the determination of the air-purification performance of materials that contain a indoor-light-active photocatalyst or have indoor-light-active photocatalytic films on the surface, usually made from semiconducting metal oxides, such as titanium dioxide or other ceramic materials, by continuous exposure of a test piece to the model air pollutant under illumination with indoor light. Formaldehyde (HCHO) is chosen because it is a typical indoor air pollutant that causes the so-called sick building syndrome. This part of ISO 18560 is intended to evaluate the photocatalytic performance for building materials, such as boards, wallpapers. This part of ISO 18560 does not apply to powder or granular photocatalytic materials. This test method is usually applicable to indoor-light-active photocatalytic materials produced for air purification. This method is not suitable for the determination of other performance attribute of photocatalytic materials, i.e. decomposition of water contaminants, self-cleaning, antifogging and antibacterial actions. This test method is based on ISO 16000-23 and is adjusted for the measurement of indoor-light-active photocatalytic materials. NOTE Another test method for the determination of air-purification performance of photocatalytic materials by using formaldehyde is described in ISO 22197-4. The test methods comprising of ISO 22197 are prepared for evaluation of material-based air-purification performance under irradiation of ultraviolet light, while this part of ISO 18560 is intended for providing a direct index to the improvement of indoor air quality by the indoor-light-active photocatalytic materials under the simulated conditions. Approximate correlation between the results by ISO 22197-4 and this part of ISO 18560 has been confirmed.