The real-time dust exposures of sodium borate workers: examination of exposure variability

As part of an epidemiologic study of acute respiratory irritation, an assessment of the short term (TWA-0.25 hr) and daily (TWA-6 hr) dust and boron exposures of workers in a sodium borate production facility was undertaken. A real-time continuous aerosol monitor was used in an active mode with an in-line filter to collect a TWA-6 gravimetric sample with a datalogger to store the continuous aerosol measurements. Over 430 person-days of personal exposure measurements were made, resulting in more than 10,000 15-minute average (TWA-0.25) dust concentration measurements. The arithmetic mean total dust concentrations for the 13 job groups exposed to sodium borate dust ranged from 0.29 to 18.95 mg/m3. The geometric standard deviation of the TWA-6 total dust exposures within the sodium borate exposed job groups had a median of 2.78. The geometric standard deviation of the TWA-0.25 total dust exposures had a median of 3.97. In most jobs the "within-day" variability accounted for over 50% of the total variability in exposure levels. In jobs with constant exposure to sodium borate, the second most important source of exposure variability was attributable to "between worker" differences. Where there was only intermittent exposure, the second most important source of exposure variability was "within-worker" variability. The implications of these findings for control strategies are discussed. Based on boron measurements, a substantial portion of a total dust air sample is nonborate material such as cigarette smoke, vehicle exhaust, ambient dust, or hydration mass. Thus, even in an environment where sodium borate is being packaged, total dust measurements are an overestimate of the actual borate exposure level.