Evaluation of a passive sampler for volatile organic compounds at ppb concentrations, varying temperatures, and humidities with 24-h exposures. 1. Description and characterization of exposure chamber system

A dynamic exposure chamber was constructed to evaluate the performance of the 3M 3520 organic vapor monitor (3520 OVM, 3M Co., St Paul, MN) when exposed during 24 h to combined test atmospheres of benzene, 1,3-butadiene, carbon tetrachloride, chloroform, 1,4-dichlorobenzene, methylene chloride, styrene, tetrachloroethylene, and toluene at target concentrations of 10, 20, and 200 mu g/ m(3) in combination with temperatures of 10, 25, and 40 degrees C and relative humidities of 12 50, and 90%. These conditions are generally representative of the range of community air environments, both indoor and outdoor. The system consists of five distinct units: (i) dilution air delivery, (ii) humidification, (iii) VOC generation and delivery, (iv) mixing chamber, and (v) exposure chamber. High-emission permeation tubes were utilized to generate the target VOCs. Both the target temperatures and humidities were achieved and maintained for multiple consecutive days. The variation of the temperature in the exposure chamber was controlled within +/-1 degrees C, while relative humidity was controlled within +/-1.5% at 12% RH, 12% at 50% RH, and +/-3% at 90% RH. Under constant preset temperatures and stable nitrogen flow through the VOC generation unit, various temporal patterns of permeation rates were observed over time. The lifetimes and permeation rates of the tubes differed by compound, length of the tube, and manufacturer. For tubes with a long shelf life, an initial conditioning period of up to 50 days in the VOC generation unit was necessary before permeation rates became stable. A minimum of 3 days of reconditioning was required when the tubes were stored in the refrigerator before they were used again. 1,3-Butadiene tubes had a short shelf life, and the permeation rates changed significantly and relatively quickly over time; however, the rates could be estimated by using a best-fit equation for the tube weight loss data for each exposure period. By closely monitoring weight loss over time, the permeation tubes could be used for delivering low and stable concentrations of VOCs over multiple months.