FlavourArt's Mustard flavor says this:
"Powerful, for demanding users. This flavor is based on mustard oil, diluted and stabilised to ease its use. Handle with care. Very irritant !! NOT SUITABLE FOR VAPING."
and it links to page that says this: "We recommend using with caution in preparations for children aromas "Pepper, Mustard and all the Cheese one" because of their nature must be handled by an adult."
Gads! (yet no warning on the Bread Crust, hmmm...lol)