BLACK DUST ON CEILINGS
There are a few factors that cause this. The main reason for it happening with lighting is to do with heat and the resultant charged particles in the atmosphere. There is no real solution, except to look at air filtration systems to reduce dust, or to suggest to customer to reduce dust levels. Most likely both these suggestions will not be too well received by the customer.
Firstly, the luminaire itself is surrounded by an electro magnetic field. This tends to positively charge particles that pass close to it.
Also, in the case when it is halogen lighting, the fittings run very hot (relative to incandescent), and as such create a convection affect – the heat source sucks the air up and sideways when it reaches the fitting. The dust particles become positively charged as they heat up, and deposit themselves on the neutrally charged ceiling. Basically speaking, this is static.
The problem is worse in dusty conditions and in winter when the convection effect is most pronounced (difference between hot and cold air).
You will notice this problem is most prevalent in retail stores where dust is more difficult to control, and lighting and air-conditioners are common.
Incidentally, this problem is also common in the air-conditioning industry.
If someone is going to attempt removing the statically charged dust, it is best to try use a duster rather than wash it off.