As an example, a 1 µm particle in the lung would emit about five alpha particles per year, resulting in a negligible radiation dose, assuming the particle was resident for that long a time. As a frame of reference, lung cancer has been well documented in underground miners exposed to the short-lived alpha emitting decay products of 222Rn gas (218Po and 214Po). The decay products are formed in air as atoms and rapidly attach to the ambient aerosol particles and, when inhaled, deposit on the bronchial airways. The bronchial airways are the site of the majority of lung cancers (Saccomanno et al., 1995). Lung cancer associated with the gas exchange or pulmonary region is rare.
The bronchial airways are lined with a thin layer of tissue called bronchial epithelium. The epithelium averages 30 to 40 µm in thickness and the target cells for lung cancer are in this area. The specific target cells are basal and secretory cells and lie at a depth of about 25 µm in the epithelium, within the range of all alpha particles emitted from decay products deposited on the bronchial airway surface. The function of the target or stem cells is to continually produce the cells that constitute the airway lining. These are fully differentiated cells and have a life span of a few months. Lung cancer can only arise in stem cells (i.e., cells capable of ongoing division).
The number of target cells in the bronchial epithelium required to be hit in order to develop a lung cancer can be calculated from the physical dosimetry of the inhaled decay products and the observed lung cancer risk in the miners. This calculation has been performed, and it is estimated that about 109 hits are necessary (Harley et al., 1996). Thus, although it requires perhaps only one very special or "particular hit" of a cell to develop a cancer, the "particular hit" must be exquisitely correct and the requirement is that a very large number of hits of the target cells is needed to provide that "particular hit."
An estimate of the amount and number of DU particles inhaled may be made with the stipulation that about 109 target cells must be hit to produce a cancer. If the aerosol particle size inhaled is assumed to be 1 µm, then about one gram must be inhaled per day (1011 particles/day) for several weeks, because of the requisite 109 target cells to be hit.
In the Colorado uranium mining cohort of 3,360 men, for example, there were on average 108 calculated target cells in bronchial epithelium hit per person because of their short-lived decay product exposure. Less than 10 percent of lung cancer is documented to date (NIH, 1994) with some of the lung cancer associated with smoking. In summary, lung cancer stemming from alpha particle irradiation is a very-low-probability event.