Cats are prolific producers of allergen that is associated with skin scales. The allergen or allergens are digestive proteins that are transferred to the skin and hair during preening activities.

Skin particles are relatively small and as such become airborne easily and can remain airborne for hours in a still house.

I have measured elevated concentrations of cat allergen in a house that previously had cats despite the fact that the house was unoccupied for two years. As such, cat allergen is quite persistent.

As protein cat allergen is subject to decomposition by bacteria and fungi as well as sunlight. If environmental conditions are optimal, 90% or more of the allergen should be denatured or rendered harmless in about 6 months. If environmental conditions are less optimal, active allergen can persist for years.

Given the fact that a cat has lived in your house for such a long time, one would expect cat allergen to be present almost everywhere. However, highest concentrations would be expected in those rooms where the cat spent most of its time. Highest concentrations would also be expected in dust on horizontal surfaces.

In preparing the house for your fiancé, I would first initiate a program of surface cleaning in those rooms with the highest cat allergen concentrations (based on where the cat spent most of its time).

The surface cleaning should include the use of a portable HEPA vacuum cleaner (these are available from a number of main line vacuum cleaner manufacturers as well as providers of allergy products on the web) and wet wiping.

HEPA vacuuming works best on soft surfaces such as overstuffed furniture, carpeting, clothing and other fabrics. Multiple vacuumings of frequently cat- visited areas is desirable.

HEPA vacuuming can also be applied to hard surfaces such as floors, furniture, etc. However, since particles are often bonded to hard surfaces by electrostatic forces, such vacuuming has only limited effectiveness. Hard surfaces should be wet-wiped and then wash rags rinsed often.

Allergen will be found in higher concentrations on horizontal surfaces where there is a higher probability that it will be re-suspended on disturbance. It will also be found on walls, curtains and drapes in much lower concentrations; it is also at lower risk of re-suspension. It is questionable whether it is worthwhile cleaning walls, ceilings, windows, etc.

Over that 12-year period your heating/cooling ducts are likely to have become heavily contaminated with cat allergen. As such, duct cleaning is recommended.

Lastly, most cat owners come in contact with their pets. As such, allergen is likely to be present on the clothes you have worn. In cleaning the house the last step is to make sure that clothes which may have come into contact with your cat or cat allergen-containing dust in your house are washed or when appropriate dry cleaned.

It goes without saying that all cat "toys", scratching posts, litter boxes, etc. should be removed (permanently) from your home.