Figure 5.7

Relative risk (RR) of lung cancer in the Colorado Miner Cohort as a function of cumulative WLM exposure and fitted model. Fitted linear model shown < 3200 WLM. (NIH 1994).

One important aspect of the data is shown in figure 5.7—that the ERR at high exposures tends to flatten out. That observation is erroneously called the inverse-exposure effect. It is usually stated that the increase in lung-cancer risk per unit exposure is higher for low exposures than for high exposures. The flattening of the response curve is probably the result of cell-killing due to multiple traversals of cell nuclei. At low exposures, even a single traversal of a cell nucleus by an alpha particle is rare. Therefore, the effect is actually a reduced response at high exposure that is due to sterilization, not an increased response at low exposure.

The terminology has caused considerable confusion because it implies that domestic exposure can somehow be "more dangerous" than mine exposure. That is not the case, and it has been demonstrated that no additional risk above the linearity shown in all the cohorts is posed by domestic exposures.

The main features of the lung-cancer risk model derived from the jointly analyzed data are as follows:

  1. There is a reduction in risk after cessation of working in the mines. It is called the time-since-exposure effect (the TSE factor).
  2. There appears to be no clear age-at-start-of-exposure effect; that is, the age at the start of mining is not an obvious factor. However, the age attained after the start of mining is a factor, and risk decreases with age (the AGE factor).
  3. Longer duration (the DUR factor) or lower 222Rn concentration (the WL

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