# Cotton Terry Textiles with Photo- and Bio-Activity in a Model Study and Real Conditions
## 4 Discussion
In the present study, under real conditions, a statistically significant decrease in the number of fungi from \(10^{3}\) CFU/cm\({}^{2}\) to \(10^{1}\) CFU/cm\({}^{2}\) was observed on the towel surfaces. The identification of the dominant fungi revealed the presence of _Candida parapsilosis_ and _Cryptococcus humicola_ yeast originating from human skin. The literature indicates that yeast microorganisms are moderately sensitive to photocatalysis [32]. No mold was detected on the towel surfaces. The present study conducted under real conditions of towel use reveals the self-cleaning effectiveness of photocatalytically modified towels and the sensitivity of the selected groups of microorganisms (Gram-negative bacteria in particular), which reside on these surfaces. One should bear in mind that the photocatalytic self-cleaning of towels can be affected by the number, types and condition of physiological microorganisms, environmental pH and the presence of organic contaminants. In real-condition studies, we can assume that there are two impact mechanisms on microorganisms: (_i_) direct reaction of biocides on the towel surface, and (_ii_) light-induced catalysis in the air in the bathrooms where the modified towels were used. Lack of statistically significant differences in the biostatic activity of the towels used in both bathrooms can suggest that the main mechanism, which occurred on the material surfaces, involved the direct impact of TiO\({}_{2}\) and ZnO nanoparticles on microorganism cells. In the present study, it was discovered that microclimate parameters and lighting in the rooms have a significant impact on photocatalytic processes in the air. Similar relations were described in the model studies using different photocatalytic arrays, which were significantly affected by air humidity, and lighting type and time [30; 31]. In the present real-condition studies, the duration of exposure to visible light was discovered to contribute most to the photocatalytic effectiveness of textile fabrics modified with TiO\({}_{2}\) and ZnO. It was demonstrated that natural lighting with intensity of 92.3-95.0 lux and lighting duration of 420 min/day contributes more to the reduction in the number of microorganisms in the air than artificial light with light intensity of 152.8-166.7 lux and lighting duration of 80.7-87.8 min/day. Similarly, [PERSON] et al. [59] found that low light intensity with long exposure time was more effective in inactivating microbes than high light intensity with short exposure time with the same UV dose, which was defined as the product of intensity and irradiation time. Model studies carried out by other researchers revealed that the light exposure duration necessary for the inactivation of microorganisms in the air in different photocatalyst systems and at different light exposure times ranges from 15 min to 72 h [7; 32]. The duration of exposure to visible light seems to be the key parameter determining the effectiveness of air cleaning of microorganisms. For the bathroom with natural lighting, this was 5.5 h longer than in the bathroom with artificial lighting. Moreover, the studies were carried out in winter, when natural lighting in the bathroom lasted for only 7 h/day. Most probably, photocatalytic activity in other seasons can become more intensive as a result of the extended duration of exposure to visible light. The present studies revealed that the photocatalytic activity of modified towels supports the cleaning of air from microorganisms with a light intensity range between 92 and 167 lux. At these parameters, no relationship had higher effectiveness at higher intensity of visible light. One should bear in mind that the process of photocatalytic air cleaning under real conditions simultaneously affects a number of parameters, such as the kind of photocatalytic material, its surface, lighting intensity, type and time of light exposure, and air humidity [31].