The problem of the representative sampling of short-period samples of air contaminants in the breathing zone of one employee during one shift is discussed. Different types of error and different sources of variation in sampling and the analysis of such samples are dealt with. It is suggested that noncompliance with the time-weighted average limit can be established by a combined test procedure: (a) when a direct comparison between the observ,ed mean and the standard is made and the observed mean is above the standard, noncompliance is established; (b) if the observed mean (x) is below or equal to the standard, noncompliance is also established if the observed standard deviation is too large when compar,ed to µ - x, where µ is a value above the standard. The consequences of the suggested combined test in establishing noncompliance are discussed. The criteria are considered to be simple to use and comprehensible, which is a demand ,since the results of observations of air contaminant concentrations in the workroom are used by employees and laymen. The choice of the criteria imply certain tolerance limits of the population of observations during a shift. These tolerance limits are considered to be in reasonable accord with excursion factors above the time-weighted average limits.

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