There they remain indefinitely because no clearance mechanisms effectively remove them. Nature did not protect us against such particles, because none existed until very recently. Once lodged in the deep regions of the lung, fine particles, with their enormous surface area enriched with toxics, provide a particularly efficient means for delivering metals and organic pollutants directly into the blood stream. Their large surface area provides effective contact with moist tissue and the opportunity for dissolving or for other chemical reactions, putting pollutants directly into the victim's blood. Once in the circulatory system, toxics are then distributed throughout the body.
Fine particles have one other characteristic worthy of mention.
This occurs because they are so small and light that gravity does not pull them downward efficiently, so the slightest air current holds them aloft. Furthermore, as we shall see, they are not removed from the atmosphere efficiently by rain. Therefore, for a long time after they are released into the environment, they remain available for humans to breathe in.