toxicology of fibres has shown that persistence of some asbestos fibres in the lung as a result of their insolubility was an important determinant of carcinogenicity and fibrogenicity, and that fibre length was also critically important in determining toxicity. This research expanded to consider fibres and other materials introduced as substitutes for asbestos, such as calcium silicate and aramid fibres. From 1990, the Colt Foundation, relevant industries and the Health and Safety Executive supported a programme of laboratory research into the health effects of man-made mineral fibres that helped to clarify the quantitative relationships between health risks and fibre dimensions and biopersistence. This work was awarded the prestigious Bedford Prize by the British Occupational Hygiene Society. In 1977, Henry Walton and Dr Steve Beckett invented a microscope eyepiece graticule used for counting asbestos fibres. This device was designed to improve the reliability of the measurements and it has since become part of the international standard methodology for fibre measurement. A programme ensued on the measurement of fibrous aerosols such as asbestos, raising important issues in comparability of counts by different laboratories, and leading to the establishment of quality control schemes for asbestos. In 1979 the IOM was appointed by the Health and Safety Executive as the British Central Reference Laboratory for asbestos fibre counting, and the following year the World Health Organization similarly appointed IOM as its central reference laboratory for man-made mineral fibre counting. From 1985 IOM collaborated with the International Agency for Research on Cancer in a major European epidemiological study of the carcinogenicity of mineral wool fibres. As part of this work IOM helped develop novel methods to estimate retrospectively exposure of workers in the study, ultimately demonstrating that there was no evidence of a carcinogenic effect of these fibres on the lung. IOM scientists also conducted a study of the respiratory health of workers manufacturing Refractory Ceramic Fibre in Europe, showing small though inconsistent effects on respiratory health associated with inhalation of these fibres. Workplace exposure, measurement and modelling From its earliest years, IOM has had a tradition of using quantitative exposure measurements to explore links between the working environment and health and has pioneered the development of new methods to measure the concentration of aerosols in ways that are relevant to human biology. The MRE 113A respirable dust sampler developed for use in our pneumoconiosis research was the first landmark and led to greater insight