Your Home May Be 'Sucking' It All In.
If there are holes, the next question is, "What is the force that would cause the house to suck in the dust?" In your house, the most likely force would be negative pressures caused by leaks or imbalances in the forced air system.
If, for instance, the supply ducts in the basement are very leaky, then the system is blowing less air into the living space than is coming back through the return ducts. This will cause the house to have a negative pressure relative to the outside (and the attic). In other words, it will cause the house to suck air out of the attic, if there are holes.
In some houses, this same effect is created by closing bedroom doors. The heating/air conditioning system is blowing air into the bedrooms, but the air can't get back out to the return duct located in a central hallway. The bedrooms are being blown up like a balloon,(except the sheetrock doesn't stretch) and rest of the house is under a negative pressure, with the return trying to suck in air from wherever it can get it. If the easiest place for that air to come from is the attic, then that's where it will come from, complete with dust, 'critter' droppings, insulation fibers, and whatever else might be up there between the house and the attic.
Of course, it's also quite possible that these same forces are sucking air into the house from the basement or from outside, rather than from the attic. The only way to know for sure is to have the house tested with a blower door, a device designed to locate the holes in the house and measure their size.
Aside from the dust, the air is probably also hot and wet in the summer, cold and dry in the winter, and contaminated with a variety of pollutants all year round. Identifying the problems and fixing them will not only improve your dust situation, but will also reduce your energy bills, increase your comfort, and enhance the health of you and your family.