I worked in lawn/pest control and my boss said that after a time, some guys take a hit from the chemicals, and become 'exquisitely sensitized' as is referenced

I worked in lawn/pest control and my boss said that after a time, some guys take a hit from the chemicals, and become 'exquisitely sensitized' as is referenced here. Chemicals that had no noticeable effect previously, suddenly have immediate consequences, like rashes or illness.