First of all, WD-40 is not a slate cleaner. It is used to oil your flooring to make it shiny if you don’t want its original dull look. Although it’s highly flammable, it’s safe if you just rub it into your slate floor. But if you are bothered by this, better use other products. You can refer to this video on how to polish natural slate hearth with WD-40. Another thing to consider is that although many recommend WD-40, it doesn’t really keep your slate floor shiny. There are people saying that olive oil does a better job than WD-40.