Application of Experimental Design in the Steady State Particulate Exposure Levels in a 1992 International School Bus

A steady state mobile test was developed to measure the concentration of breathable particles that can enter the cabin of a school bus. The principles of experimental design were used to identify the experimental conditions for the test and to analyze the data. The design consisted of a series of steady-state tests using a 1992 International school bus. The testing was performed on a closed three mile track at the Army Test Center in Aberdeen, MD. The mass concentrations of particles smaller than 2.5 microns were measured at three locations inside the bus and at the air intake into the engine. The number concentration of particles was measured at the tailpipe. Three factors were varied at three levels in a Box-Benhken design. The steady state speed was set at 5, 30, and 55 mph. A load was applied to the engine with a mobile dynamometer to simulate a 0, 0.67% and 1.33% road grade. Tests were performed with three window configurations: all windows open, all windows closed, and all windows half open. Steady state averages for PM2.5 mass concentrations inside the bus ranged from 2.2 μg/m³ to 111 mg/m³. An analysis of variance indicates that there was a significant difference in the concentration of particles inside the bus as a result of the varying speed and windows conditions. At the 95% confidence interval the factors of speed and windows had a significant effect on the particle mass concentration. Furthermore, the effect of speed on the particle concentration was dependent on the window configuration used.