One set of convincing evidence that different fiber compositions can have significantly different biological effects even when the doses and dimensions of the fibers are the same comes from chronic inhalation studies in rats performed at the Research and Consulting Company (RCC) in Switzerland. References to the original publications of these studies are given in this paper. The following table compares the response of three distinct compositions of synthetic vitreous fibers. All three fibers had virtually the same airborne concentrations of the same size distribution, both in length and diameter, as shown in the bottom section of the table. But the disease incidences, shown in the top section, could scarcely have been more different.
|Lung Tumors (%)||13||3||3|
|WHO Fibers (f/cc)||190||250||210|
|Average Diameter (µm)||1.0||0.9||1.0|
|Average Length µm)||22||18||21|
|Long Fibers (f/cc)||60||80||90|
For perspective, the lung tumor incidence in unexposed animals is in the range of 2% to 4% in this strain of rat and the standard error of these incidence percentages is about the same.
The data in the above table are quite compelling evidence of a significant difference among fibers, one that cries out for a scientific explanation. However, many studies carried out in the past did not allow such an unambiguous comparison of different fibers because of the difficulty in exposing animals to the same dose of the same size of fibers of different types. The table below illustrates this difficulty by comparing studies with two types of asbestos fibers, crocidolite and chrysotile, with two vitreous fibers from the previous table.
|Lung Tumors (%)||13||19||3||3|
|WHO Fibers (f/cc)||1600||11 000||250||210|
|Average Diameter (µm)||0.3||0.1||0.9||1.0|
|Average Length (µm)||6||2||18||21|
|Long Fibers (f/cc)||160||330||80||90|
Although there is once again a great difference in the disease incidences (top section of the table), there is also a great difference in the exposures, no matter how expressed, caused by the great difference in fiber sizes. One cannot be sure from data like these alone whether different fibers have different biological effects or whether dose and fiber size could explain it.
So there is an important scientific question here, one that can be answered by recent research. Return to the previous page for the answer.