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In order to identify the distribution of dust grain sizes in a dental hospital and to estimate the relationship between the distribution of dust and airborne bacteria or environmental factors, measurements were carried out in the middle-size clinics (pedodontics and orthodontics) and the large clinics (restoration and prosthetics) at Asahi University Dental Hospital for two years (1994 and 1995). We measured dust grain sizes of 0.3, 0.5, 1.0, 2.0, above 5.0μm, airborne bacteria with an SY type pinhole sampler method and Koch's dropping method, and environmental factors (temperature, humidity, air flow, and number of persons, opened windows, and functioning outlets air-conditioning). Correlation between the distribution of each dust grain size and each measured factor was analysed. The estimation of airborne bacteria colonies was performed with the environmental factors and/or dust factor by multipule regression analysis, and the effect of addition of a dust factor was evaluated. The following results were obtained : 1. Total dust (above 0.3 μm) was higher inside the clinics than outdoors, and showed a tendency to be high during working times. 2. More than 98% of dust grains were 0.3-1.0 μm, but large grains of above 5.0 μm were found at a higher rate inside the clinics than outdoors. 3. A high correlation was obtained between dust of under 2.0 μm and floating airborne bacteria, and between dust of above 2.0 μm and dropping airborne bacteria. 4. The factor of operating air-conditioning had a high negative correlation to dust factor, and it was recognized that air-conditioning decreased the dust in clinics. 5. The estimated accuracy of airborne bacteria colonies by multipule regression analysis was increased by adding the dust factor to the environmental factors. As a result, we recognize that large dust grains float in dental clinics and increasing dust volume increases the airborne bacteria. The measurement of dust is useful to estimate airborne bacteria colonies in real time, and is useful for simple air pollution monitoring.