If, under the conditions of the ISO test used, no activity is discerned, do not completely despair. With some materials it is essential some degree of weathering/use occurs for their true potential to be revealed, as in the case of some photocatalytic paints. For example, it has been shown for some systems from NO removing ability vs. accelerated weathering time that such films need to be used for a little while before realising their optimum performance, presumably because the pigment particles first destroy some of the organics that coat their surface and which form part of the paint formulation that bind them. In this case, it can be argued that the presence of NOx actually helps preserve the coating, as the air-borne pollutant is oxidised in preference to the binder. What is particularly interesting is that some paints exhibit no initial photocatalytic activity, implying that an initial assessment would indicate no activity and so possibly prompt its rejection, even though accelerated weathering or repeated use would reveal its true photocatalytic potential.
In addition, sometimes the test chosen is simply too insensitive with respect to the photocatalyst’s activity, and so inappropriate. For example, many commercial photocatalyst self-cleaning films show little or no activity when tested using the NOx system, but are very active when assessed using the methylene blue test. Thus, alternative test(s) may be needed. The photocatalyst indicator inks, not an ISO standard yet, are particularly good for identifying samples with a low activity.