As chef says, fumes will find their way upward and, if (as in the majority of UK buildings) the floor structure is wooden, then there is plenty of scope through all manner of gaps, even if (but especially if not) your living space is cooler than the ceiling temperature levels below. If on the other hand the floor structure is a solid slab (concrete, say) then fumes will for all intents and purposes be barred from passing straight up from one storey to the next. Note any patterns (weather, doors/windows open or shut, time of day, etc.) when discussing with authorities or others doing a survey. But before anything else, raise the matter (politely) face to face with the hairdresser who, if he/she is any good at all (and deserves to know before the heavies move in), will want to sort the matter out amicably. He/she will have the option of installing a good ventilation system (fumes will not enhance the shop environment either). If the reaction is unhelpful even after following up in writing then take chef's advice and prepare to apply serious pressure (if you choose). Be warned, suing is likely to be very costly and is not at all guranteed to be successful.