As air is inhaled, the shape of the throat causes the air to swirl, so heavy dust particles are thrown outward by centrifugal force, where they strike the mucous-lined walls. As the tubes and passageways leading to the lungs twist and branch, they provide many opportunities for particles to collide with sticky walls and become trapped before they enter the lungs. Once trapped by mucous, coarse particles are coughed up and excreted. Thus, the deep regions of the lung provide direct access to the blood stream and, by this means, to every part of the body. Unfortunately, humans now produce huge numbers of fine particles that elude the body's protective mechanisms entirely.
Fine particles pass easily into the deepest regions of the lungs, the alveoli, or alveolar sacs.
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.