Harold McGee writes on page 389 of the revised edition (2004) of his magisterial McGee on Food and Cooking: An encyclopedia of kitchen science, history and culture (1984):
When eaten as is, most spices and herbs are acrid, irritating, numbing. And the chemicals responsible for these sensations are actually toxic. The purified essence of oregano and of thyme can be bought from chemical supply companies, and come with bright warning labels : these chemicals damage skin and lungs, so don’t touch or inhale. This is precisely the primary function of these chemicals: to make the plants that produce them obnoxious and therefore resistant to attack by animals or microbes.