Not all air is created equal. Lossnay only draws fresh air from the outside specifically – it does not draw air from the attic.
The absence of significant air movement common in attics means the air is likely to be stale. In addition, build-up of dust, dirt and other contaminants such as mould, insect and rodent droppings makes this air much harder to filter before it is distributed through your home. This is why the Balanced Pressure Lossnay system specifically utilises direct fresh air instead.