@cece55 I can smell a small amount when I open the SoClean machine after it's done but it's not bad or much - for me anyway. I generally start it right when I take a shower and it runs for 2 minutes and makes the buzzing noise that some people don't like. Then it has to stay closed for 2 hours which is the time it takes for the sanitation process. So I usually open the cover in the afternoon and change the water in the humidifier tank so that when I go to bed I don't notice any ozone smell.