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Asthma is the most common occupational respiratory disorder in the industrialized world. In this research we investigated the effectiveness of quality control chart as a method to explore the work-relatedness of asthma using peak expiratory flow (PEF) as the variable of interest.PEF measurements were recorded from several workers suspected of having occupational asthma. The measurements were recorded for aperiod of 2 weeks at work and two away from work. For each subject, average (X) quality control chart was developed and analyzed using Shewhart methods.Using the ratio of average daily PEF diurnal variation at work to the baseline average diurnal variation, quality control chart detected and characterized increased variation in PEF between the periods at workplace and away from workplace. This ratio was evaluated at 15% and 20% and compared to the results of the gold standard. A ratio of greater than 15% produced a sensitivity of 94% and specificity of 61%. These results are as good as and in some cases better than the previously published work.Quality control chart is an effective and inexpensive method for early intervention to detect workers suspected for occupational asthma. Also, the findings in this article are an invitation for further research in occupational asthma using quality control chart methods for other indices of airways obstruction (FEV-1, RAW) or inflammation (NO).