I find myself completely nauseated when exposed to lavender, so I doubt it's anything to do with the chemicals being used.

For my family (since my aversion is familial), I attribute it to the relatively strong sinus reactions my dad's side has. Whenever exposed to certain strong smells, we start making mucus, and then it collects in the sinuses and causes a major pressure headache followed by mucus dripping out.

I'm guessing it's related to allergies? For us, it's nothing to do with modern times, since I have it, my dad has it, his family did, etc.