While there are many oils that may be irritating to more highly sensitive individuals, some are almost universally and to a greater degree likely to cause discomfort. These are marked with a (!) in the text to accentuate that the greatest amount of caution should be exercised.
Anise
Lemon
Bergamot
Lemongrass
Camphor
Lemon Verbena
Cassia
Melissa
Cayenne
Myrrh
Chili
Pepper
Oakmoss
Cinnamon
Orange
Citronella
Peppermint
Clove
Rosemary
Frankincense
Wintergreen