To test proposed methods for reducing SBS symptoms and to learn about the causes of these symptoms, a double-blind controlled intervention study was designed and implemented. This study utilized two different interventions designed to reduce occupants' exposures to airborne particles: (1) high efficiency filters in the building's HVAC systems; and (2) thorough cleaning of carpeted floors and fabric-covered chairs with an unusually powerful vacuum cleaner. The study population was the workers on the second and fourth floors of a large office building with mechanical ventilation, air conditioning, and sealed windows. Interventions were implemented on one floor while the occupants on the other floor served as a control group. For the enhanced filtration intervention, a multiple crossover design was used (a crossover is a repeat of the experiment with the former experimental group as the control group and vice versa). Demographic and health symptom data were collected via an initial questionnaire on the first study week and health symptom data were obtained each week, for eight additional weeks, via weekly questionnaires. A large number of indoor environmental parameters were measured during the study including air temperatures and humidities, carbon dioxide concentrations, particle concentrations, concentrations of several airborne bioaerosols, and concentrations of several microbiologic compounds within the dust sampled from floors and chairs. This report describes the study methods and summarizes the results of selected environmental measurements. The environmental conditions measured during this study were typical of the conditions commonly encountered inside office buildings. Air temperatures were sometimes outside of the boundaries of the ASHRAE thermal comfort zone. The spatial and temporal variations in air temperature ranged up to a few degrees Celsius — large enough to suggest that it may be important to account for variations in temperature in the evaluations of the interventions. Relative humidities were maintained within a narrow and widely accepted range (42% to 58%). Carbon dioxide concentrations were always below the 1000 parts per million guideline concentration recommended by. ASHRAE but occasionally above the lowest suggested guideline value of 800 parts per million. Carbon dioxide concentrations tended to be higher on the second floor. The effective time-average steady ventilation rates (ESVRs) were approximately equal to or above the recommended minimum rates in the ASHRAE ventilation standard. For the nineweek study period, the average ESVRs were relatively constant (e.g., f 15%), thus, temporal variation in ventilation rates were not likely to be a major confounding factor distorting the apparent effects of the interventions on SBS symptoms. The use of high efficiency air filters greatly decreased the indoor concentrations of submicron particles. Indoor concentrations of 0.3 pm to 0.5 pm size particles were, on average, a factor of 20 lower when the high efficiency filters were used compared to the concentrations with the standard air filters. Because the processing and analyses of the questionnaire data are still underway, the influence of the interventions on symptoms is not yet known.