The Helsinki University Museum and the Finnish Institute of Occupational Health did some asbestos analyses for Finnish gas mask filters last year. Asbestos fibres were also present in the dust samples taken from the surface of the gas masks and in an air sample taken through the filter of a M/39.
https://blogs.helsinki.fi/hum-object-of-the-month/2020/08/18/gas-masks-and-asbestos/
