Using the sun has been a popular method of stain removal for nappies, and many believe that the sun sanitises. With enough of a decent detergent, adequate agitation and if needed a laundry booster (stain remover product) stains will be removed by the washing machine.
UV can only affect the surface layer of a fabric. Nappy inserts are typically multi-layered so UV won’t reach the layers underneath. As a result any stain removal 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 . The problem is 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, along with the multi layered nature of nappy inserts, means the sun is not an effective method of sanitisation for cloth nappies. If the sun was a completely effective sanitisation method, bacteria on the surface of the earth wouldn’t exist.
Stains remaining on nappies doesn’t always mean soiling (poo) hasn’t been removed, some foods simply stain eg. blueberries, carrot, tomato. Exclusively breastfeed poo can stain also. But the issue with using UV to remove the stain is it’s only temporary, after storage it will reappear. Sometimes the stains are actual poo which hasn’t washed out.
For methods to ensure your laundry comes out of the machine stain free, see the Wash Routine Basics, and Why Am I Getting Stains pages. Now you can wear those white clothes, and dress your children in those white clothes in confidence!