Document yrYJYg3xR6y45ZOk4v1QbDkME

26 Age standarization of X-ray readings Inasmuch as the prevalence of X-ray abnormalities reflects various assaults to the lung (separate from that under study) that accrue overtime, it is necessary to consider age in comparisons of groups exposed to asbestos in different circumstances. Any age effect can, in part, be due to unknown exposures to various dusts, including asbestos, the opportunities for which increase with time. Additionally, any dust-related changes progress with time and the manifestation of rela tively minor exposures early in life can be significant in later years. As discussed previously, a selection criteria for the exposed and control groups resulted in study populations that had similar age distributions. However, to more accurately take into account any age effect, the overall percentages of X-ray abnormalities were standard ized to the age distribution of all 983 individuals for whom readable X-rays were available. A standardized percentage, ^st<j was calculated using equation (1). / - P .. = 2. P. nT/983 (1) ^ std ill Where P. is the percentage of abnormal X-rays in the ith age category 1T of the group of interest and N^, the number of all 983 individuals in the ith age group. For a group having an age cell with no data (insulation/factory, other) standardization was based on those cells with data using equation (2) P = i P. nT/983 [X pT nT/X pT nT] std ii ii ii avail i all i avail i (2) Where P. again equals the percentage of abnormal X-rays in the ith age 1T category of the group of interest and N., the number of all 983 indiX1 viduals in the ith age category; P^ is the percentage of abnormal X-rays among all 983 individuals in the ith age category. These over all age-standardized percentages are shown in Table 13. The difference between the individuals performing brake work and those with no identi fied asbestos exposure remains, while the percentage of abnormalities in those employed in a garage but not. engaged in brake repair work