Comment by MrMcCall
Yeah, I stopped going to houses that ran those plug-in diffusers.
The rule is: don't put it into your lungs or onto your skin if you wouldn't put it into your mouth.
Those people are just paying people to poison them, however slowly.
Most (all?) "essential oil" products, used in diffusers, are edible. I like the peppermint oil as a breath freshener.