Having met quite a few people with "modern living chemical allergies" unfortunately many do fit in the nocebo category. The 'allergy' often crosses many widely different chemical groups and is often precipitated by some well noticed 'initiating event' e.g. Sick Building Syndrome and, unlike most identifiable allergies e.g. peanuts, beestings where they typically only notice the first allergic response not the sensitising exposure/s.
Medical science is far from complete , especially in matters of the mind.
Please note the "nocebo effect' is NOT malingering. In malingering it is a conscious act, nocebo it is subconscious.
Many illnesses can also be 'societally defined' e.g. Neurathenia' in the 1800s and Repetitive Strain Injury 'epidemics' in the 1980s.