blackmold.awardspace.com/kill-remove-mold.html#teatree this says 1 teaspoon tea tree oil per cup of water (NB the particular type of tea tree recommended). I used less than this for a playmat that had green mould & some black spots, as I ran out of oil. You do have to follow up with scrubbing off mould, the oil only kills it. I then put playmat in for a 60C wash. Tea tree oil not suitable for under 1 year olds, so do it ASAP and I'd wash it a few times after to get rid of smell and air plenty.