Air in industrial plants is subject to contamination by many different chemicals, and companies must monitor ambient levels of hazardous species to be sure they are below limits specified by the National Institute for Occupational Safety and Health (NIOSH). In personal breathing-zone sampling (as opposed to area sampling), workers wear devices that periodically collect air samples less than 10 inches away from their noses. Breathing-zone sampling and analysis methods for hundreds of species are set forth in the NIOSH Manual of Analytical Methods. For benzene, NIOSH specifies a recommended exposure limit (REL) of 0.1 ppm time-weighted average exposure (TWA), and the Occupational Safety and Health Administration (OSHA) permissible exposure limit (PEL) is 1.0 ppm TWA. 

A worker in a petroleum refinery has a personal breathing-zone sampler for benzene clipped to her shirt collar. Following the NIOSH prescription, air is pumped through the sampler at a rate of 0.200 L/min by a small battery-operated pump attached to the worker’s belt. The sampler contains an adsorbent that removes essentially all of the benzene from the air passing through it. After several hours, the sampler is removed and sent to a lab for analysis, and the worker puts on a fresh sampler. On a particular day when the temperature is 21°C and barometric pressure is 730 mm Hg, samples are collected during a 4-h period before lunch and a 3.5-h period after lunch. The analytical laboratory reports 0.17 mg of benzene in the first sample and 0.23 mg in the second. 

(a) Calculate the average benzene concentration, CB(ppm), in the worker’s breathing zone during each sampling period, where 1 ppm = 1 mol C₆H₆/10⁶ mol air. 

(b) The worker’s TWA is the average concentration of benzene in her breathing zone during the eight hours of her shift. It is calculated by multiplying CB in each sampling period by the time of that period, summing the products over all periods during the shift, and dividing by the total time of the shift. Assume that the worker’s exposure during the unsampled 30 minutes was zero, and calculate her TWA. 

(c) If the worker’s exposure is above the recommended limits, what actions might the company take?