Particulate matter (PM) adversely affects human health. Even though PM tends to consist of higher concentrations outdoors, indoor PM concentrations should not be overlooked, given that people spend most of their time indoors. When there is no strong indoor PM source, indoor concentrations change within a certain ratio following the diffusion of outdoor concentrations indoors. The Indoor/Outdoor (I/O) concentration ratio is affected by static building properties such as airtightness, ventilation system type, and the system's air filter performance. However, natural ventilation can have a dynamic influence on the I/O ratio because of significant changes in ventilation rates and flow patterns within the building.
This study measured PM concentrations for particles ranging from 0.3 to 10 μm in size. The study was conducted in a room of an office building in Tokyo, over a three-week period. The size ranges were divided into 16 channels and 10-min averages of the number of particles at each channel were measured using an optical particle counter. Occupants in the room utilized natural ventilation by manually opening a window. The window was opened and closed twelve times during the entirety of the measurements.
Because outdoor PM consisted of higher concentrations than that found indoors, the natural ventilation lead to larger Indoor/Outdoor (I/O) concentration ratios. The PM concentrations were observed to increase and decrease just after the window was opened and closed, respectively. These concentration changes were more remarkable for PM consisting of larger particle sizes. Where PM measured 10 μm, concentrations increased 8 times after opening the window and decreased 0.5 times after closing the window on an average hourly basis.
Particle-size is a factor for concentration changes caused by natural ventilation as discussed using concentration prediction based on the mass balance equation of indoor PM. As a result, the dependency was explained quantitatively in terms of the difference in the gravity-settlement velocity of the particles. Using ventilation parameters estimated in this analysis, the I/O ratio of PM concentration as a function of the particle size was predicted. During natural ventilation, indoor PM consisting of particles smaller than 1 μm can have the equivalent concentration of outdoor PM. However, particle size increase leads to a larger gravity-settlement velocity and lower I/O ratio. Therefore, particles measuring 10 μm are predicted to have an I/O ratio of 40% in accordance with the room analyzed in this study.