Thanks Sue, I'm afraid I need a translation for that study. I think it's saying a lot of microbacteria are resistant to chlorine but is hypochlorous solution the same as the commonly used chlorine? Or do different formulations not matter? I've bought a Purefy brand humidifier additive which is a hypochlorous solution & was looking to see exactly whether it's available to just dilute myself since the product is expensive. But as you say, I don't find anything about proportions. The Purefy company doesn't say anything about very short shelf life (what I bought would last at least a month). But then, the info they make available is very limited which isn't so reassuring either. So I'm hitting a wall at this point. Maybe Dr Falkinham as Scoop suggests!