Sunning has been a popular method of stain removal for nappies, and many believe that sunning is also antibacterial. Poo stains mean that poo has not been removed effectively. With a good detergent and adequate agitation, stains are removed by the washing machine.
UV from the sun has a short wavelength, it is only capable of penetrating the very top layer of fabrics. It also works via ‘line of sight’, meaning that it can’t reach areas that are shadowed. Nappy inserts and fabrics are typically multi-layered and have many shadows due to their textured surface, so it is impossible for UV light to reach most of the surface area of the fabric. Any bleaching effect from the sun is effective at the surface layer only.
UV from the sun is UVA, UVB and UVC wavelengths. When UV light is used to sanitise (in a method called Ultraviolet Germicidal Irradiation), UVC wavelengths are used . However, the majority of UVC is blocked by ozone in the Earth’s atmosphere (as a side note, this is also why sunscreens contain only UVA and UVB filters). The lack of UVC at ground level, coupled with the multi-layered, shadowed nature of nappy inserts mean that the sun is not an effective method of sanitisation for cloth nappies. If the sun sanitised, bacteria on the surface of the earth wouldn’t exist.