[Originally uploaded 2015-12-09. Updated 2016-03-24 to add the chrysotile data.] Size distribution measurements of South African amosite, South African crocidolite, chrysotile, and New York tremolite by scanning electron and transmission electron microscopy. These materials were characterized by the University of Maryland as part of a project undertaken by the US Bureau of Mines on behalf of the National Institute of Environmental Health Sciences (NIEHS) which used the materials in an animal oral ingestion study. Data were collected in ca. 1976. See documentation file for full study abstract, an explanation of the content of the data files, and references to associated publications. The csv versions contain the same data as the corresponding xlsx files. The csv files are provided as a software-independent alternative to the xlsx format.