Are clothes considered impure if they fall in a dry place whose purity is doubtful, or whose impurity is certain, such as the street or the building's staircase?

The default state of places is purity, not impurity, for the Prophet, peace and blessings be upon him, said: "The earth has been made for me a place of prayer and a means of purification." Therefore, one must abandon obsessive doubts and suspicions unless certain of the presence of actual impurity. If the impurity is dry and does not cling to the garment, then there is no issue. But if it is wet, then whatever part of the garment it touched must be washed for purification.