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This study evaluates how equilibrium vapor concentrations above petroleum solvent mixtures are affected by total aromatic content and the implications for estimating benzene vapor exposures. Headspace vapor concentrations over mixtures with liquid benzene content ranging from 0.001 to 1.0% and varying percentages of 1,2,4-trimethylbenzene and n-nonane were studied using a direct-injection gas chromatography/flame ionization detection method that showed good precision. The measured values were compared to predictions based on Raoult's Law, with and without non-ideality corrections using activity coefficients. Ratios of vapor to liquid benzene concentrations decreased with increasing total aromatic content; that is, mixtures with 10% to 20% trimethylbenzene simulating non-hydrotreated mineral spirits had much lower ratios compared to the ≥99% aliphatic mixtures that simulate hydrotreated mineral spirits. Positive deviations from Raoult's Law were greatest at liquid benzene concentrations less than ...

Historically, the greatest contributions to airborne asbestos concentrations during brake repair work were likely due to specific, short-duration, dust-generating activities. In this paper, the available short-term asbestos air sampling data for mechanics collected during the cleaning and machining of vehicle brakes are evaluated to determine their impact on both short-term and daily exposures. The high degree of variability and lack of transparency for most of the short-term samples limit their use in reconstructing past asbestos exposures for brake mechanics. However, the data are useful in evaluating how reducing short-term, dust-generating activities reduced long-term exposures, especially for auto brake mechanics. Using the short-term dose data for grinding brake linings from these same studies, in combination with existing time-weighted average (TWA) data collected in decades after grinding was commonplace in rebuilding brake shoes, an average 8-h TWA of approximately 0.10 f/cc was estimated for auto brake mechanics that performed arc grinding of linings during automobile brake repair (in the 1960s or earlier). In the 1970s and early 1980s, a decline in machining activities led to a decrease in the 8-h TWA to approximately 0.063 f/cc. Improved cleaning methods in the late 1980s further reduced the 8-h TWA for most brake mechanics to about 0.0021 f/cc. It is noteworthy that when compared with the original OSHA excursion level, only 15 of the more than 300 short-term concentrations for brake mechanics measured during the 1970s and 1980s possibly exceeded the standard. Considering exposure duration, none of the short-term exposures were above the current OSHA excursion level.

This article presents a historical analysis of published data regarding the exposure of brake mechanics to asbestos as a result of doing brake work. Concerns about this possible hazard were first raised in the late 1960s. This analysis focuses on 30 years of data collected during the brake repair event (e.g., a brake job) and 8-hour time-weighted average (TWA) personal samples. A brake job TWA represents the average concentration a mechanic experienced during brake servicing, rather than throughout the workday, and an 8-hour TWA represents the average airborne concentration of asbestos for the entire workday (which would involve brake work and other activities). Nearly 200 brake job and 8-hour TWA airborne asbestos samples were analyzed to assess how asbestos concentrations varied by type of vehicle serviced, country in which mechanics worked, time period, and brake-cleaning method. To facilitate comparisons, brake job TWAs were converted to estimated 8-hour TWAs using the durations and number of brake jobs performed per mechanic each day. Estimated and measured 8-hour TWAs for mechanics servicing automobiles and light trucks ranged from0.002 to 0.68 f/cc, with a mean of 0.04 f/cc. In contrast, the 8-hour TWAs for mechanics servicing heavy trucks and buses ranged from 0.002 to 1.75 f/cc, with a mean of 0.2 f/cc, suggesting that these mechanics experienced higher daily asbestos exposures than automobile and light truck mechanics. Brake job and 8-hour TWAs for brake mechanics worldwide were found to be similar during the same time periods, and they were consistently below contemporaneous occupational health standards in the United States. The increased use of brake-dust control measures in some garages resulted in at least a 10-fold decrease in the TWA airborne concentrations of asbestos from the 1970s to the late 1980s.

The performance of many processes and operations can be improved appreciably by the controlled unsteady operations that are provided by sequencing batch (fill and draw) biological reactors. The lack of design and operation experience in batch treatment has resulted in an experience void that has fostered the selection of continuous flow rather than batch treatment schemes. System selection should depend instead upon suitability of the system, reliability, efficiency, consistency, and economics. Bench, pilot, and full-scale investigations, and desk top and computer analyses must supplement existing bench scale studies if the experience void is to be filled. Several hypothetical examples are used to partially fill the void by comparing volumes for both the batch and continuous flow system. In the examples, sequencing batch treatment provides the potential for achieving effluent limitations in a total volume notably less than that for a conventional continuous flow system. This adds to previously recognized advantages that include holding a waste until proper treatment is achieved.