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Introduction Historic dust concentrations are available in a large-scale cohort study of workers in a chrysotile mine and processing factories in Asbest, Russian Federation. Parallel dust (gravimetric) and fibre (phase-contrast optical microscopy) concentrations collected in 1995, 2007 and 2013/14 were used to determine if dust to fibre conversion factors can be estimated. Materials/subjects and methods Daily medians of multiple parallel dust and fibre concentrations by sampling points were used to derive fibre to dust ratios. Applying linear mixed models, we estimated best linear unbiased predictions for the fibre to dust ratios. Results and discussion A total of 620 daily median fibre to dust ratios were derived. In the factories, modelled ratios varied by unit, increasing along the stages of asbestos enrichment as expected. In the mine, ratios were higher in winter compared to summer. Overall, the ratios showed a strong negative dependency on dust concentration. Conclusions Our study shows that dust to fibre conversion is possible by unit but extrapolations are needed. The patterns for exposure by dust and fibre will be similar but estimated fibre levels will show less contrast due to the conversion factor being smaller at higher dust concentrations.