LEUVEN, BELGIUM—Although the presence of localized thickening of the parietal pleura (pleural plaques) is an established sign of past asbestos exposure, it has been unclear if plaque size offers any clues about exposure intensity. To answer this question, Belgian investigators studied 73 long-term employees of an asbestos-cement factory and found no significant relationship between pleural plaque size and cumulative asbestos exposure.[1]

“People with large plaques do not necessarily have a high cumulative asbestos exposure and vice versa,” explained Benoit Nemery, MD, PhD, a principal investigator and Professor of Toxicology at the University of Leuven in Belgium.

The study subjects, men who averaged 43.5 years of age, had been hired by the factory from 1965 to 1969 and had worked there for at least 23 years. Determining each worker’s total exposure was difficult because asbestos levels at the factory were measured less frequently and less systematically before 1978 than they were in later years. The investigators therefore estimated asbestos exposure as accurately as possible using the available measurements.

Each factory worker underwent plain radiography and high-resolution computed tomography (CT) of the chest for pleural plaque. The total surface area of the plaques was determined through the CT scan. Lung function tests for vital capacity, forced expiratory volume in one second, and maximal expiratory flows were also ordered.

The imaging and lung function test results were compared with those from a control group of 21 men (mean age, 44.5 years) employed by the University of Leuven’s cleaning or catering departments. Eighty-five percent of the factory workers and 76% of the controls were smokers or ex-smokers.

NOT A QUANTITATIVE INDICATOR

The estimated cumulative asbestos exposure among the factory workers ranged from 16.4 to 98.7 fiber-years/mL (mean, 26.3). Pleural plaques were detected initially by chest film in 19 (26%); subsequent CT scan raised the number to 51 (70%). None of the controls had plaques on chest films or CT.

On average, the factory workers’ total pleural plaque surface area was 47.9 cm2 (the total pleural surface area being about 2,000 cm2 in adults), and none of the studied subjects had very large plaques. There was a small tendency for the surface area to increase with cumulative asbestos exposure, but this finding was not significant.

No significant correlations were found between the plaques’ surface area and cumulative asbestos exposure, pack-years of smoking, or time since first asbestos exposure. Also, there was no difference in lung function between the factory workers and controls or between the factory workers with and without pleural plaques. Further, no correlation was found between the plaques’ surface area and lung function.

The bottom line: Although the presence of pleural plaques is a good qualitative index of past asbestos exposure, pleural plaque size should not be used as a quantitative indicator of past cumulative exposure.

—Timothy Begany

Reference
1. Van Cleemput J, De Raeve H, Verschakelen JA, et al. Surface of localized pleural plaques quantitated by computed tomography scanning: no relation with cumulative asbestos exposure and no effect on lung function. Am J Respir Crit Care Med. 2001;163:705-710.

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