smelly open fires in the winter

what do my neighbours be burning in their open fires in the winter. I have vents in the walls of our house and their open fire smell is coming inside. but it does not smell like coal (sulphur) , it does not smell of burning wood, it doesnt even smell of turf, it smells more like stale smoke like literally they are tipping the contents of their ashtray on their open fire and its going up their chimney into the outside air and then coming into the vents of the house - do you reckon thats what it is, they are tipping the contents of their ashtray into their fire? - it stinks real bad!