Indoor particle counts during Asian dust events under everyday conditions at an apartment in Japan
© The Japanese Society for Hygiene 2013
Received: 8 June 2013
Accepted: 21 July 2013
Published: 10 August 2013
Asian dust storms originating from arid regions of Mongolia and China are a well-known springtime phenomenon throughout East Asia. Evidence is increasing for the adverse health effects caused by airborne desert dust inhalation. Given that people spend approximately 90 % of their time indoors, indoor air quality is a significant concern. The present study aimed to examine the influence of outdoor particulate matter (PM) levels on indoor PM levels during Asian dust events under everyday conditions.
We simultaneously monitored counts of particles larger than 0.3, 0.5, 1, 2, and 5 μm using two direct-reading instruments (KC-01D1 airborne particle counter; Rion), one placed in an apartment room and another on the veranda, under everyday conditions before and during an Asian dust event. We also examined how indoor particle counts were affected by opening a window, crawling, and air purifier use.
An Asian dust event on 24 April 2012 caused 50- and 20-fold increases in PM counts in outdoor and indoor air, respectively. A window open for 10 min resulted in a rapid increase of indoor PM counts up to 70 % of outside levels that did not return to baseline levels after 3 h. An air purifier rapidly reduced PM counts for all particle sizes measured.
It is important to account for occupant behavior, such as window-opening and air purifier use, when estimating residential exposure to particulate matter.
Asian dust events are well-known spring phenomena in East Asia that originate from the deserts of Mongolia and China. Asian cities experience yellow air on several days in the spring when the dust is blowing. The dust includes quartz, an amorphous and crystalline silica known to cause respiratory disease in people with occupational exposure or high levels of exposure from living close to deserts [1–4] and inflammation in the lungs of rats in experimental studies [5–8]. Further, dust particles contain chemicals derived from air pollutants, such as sulfate (SO4 2−) and nitrate (NO3 −), as well as microbial agents, including bacteria, fungi, fungal spores, and viruses, that sometimes survive long-distance transportation [9–11]. The impact of airborne dust may be exacerbated by these potential allergens and pathogens.
Epidemiological studies also provide increasing evidence of adverse health effects from airborne desert dust inhalation. Hospitalization risk increases significantly for asthmatic children , as do emergency ambulance dispatches  after Asian dust events in Japan. Non-accidental mortality and cardiovascular mortality also increase significantly after Asian dust events in Taiwan . A Korean study, where the influence of desert dust was not necessarily specifically investigated, also shows that particulate matter (PM) number as well as mass concentration are significantly associated with respiratory and cardiovascular disease-related mortality among the elderly .
Given that people spend approximately 90 % of their time indoors , indoor air quality is a significant concern. In Japan, the season for Asian dust events coincides with one of the most comfortable periods of the year. Our preliminary investigation (unpublished data) revealed that approximately half of the pregnant women in Kyoto, Japan, opened windows every day during April 2012 (1794 respondents/2107 queried).
We investigated how indoor PM counts (larger than 0.3, 0.5, 1, 2, and 5 μm, respectively) are influenced by various factors, including window/door openness, activity (crawling), and air purifier use, under everyday conditions in an apartment in Japan on days with Asian dust events.
We monitored PM counts in a room on the 10th floor of an apartment building in a residential area of Kyoto. There is a 100 m distance from the apartment building to the nearest two-way road. The building is a reinforced concrete structure built in 2001, 11 years before this study.
The direct-reading instrument used to measure particle size and count was a Rion KC-01-D1 airborne particle counter. The machine simultaneously counts particles larger than 0.3, 0.5, 1, 2, and 5 μm. We concurrently monitored the indoor PM counts 20 cm above the floor, and outdoor PM counts 120 cm above the veranda floor (Fig. 1). The flow rate was 0.5 L/min, and particle counts were measured every 2 min.
Room variables recorded during the study period included the following: window (open/closed), room door (open/closed), air conditioner (on/off), ventilation system (on/off), cooking in the apartment (yes/no), and the number of persons present during the testing.
Information regarding desert dust concentration was provided by light detection and ranging (LIDAR) with a polarization analyzer in Osaka [17, 18], which distinguishes soil dust (non-spherical particles) from atmospheric pollutants (spherical particles) by measuring the extent of scattered reflected light [19, 20]. We used the data for an altitude of 135 m. Suspended particulate matter (SPM; PM7) was measured at an air quality monitoring station in Kyoto located approximately 5 km from the apartment building.
Forecast and station data during the study period (22–25 April 2012)
SPM measured by the Atmospheric Environmental Regional Observation System at a local site in Kyoto, 5 km from the apartment, increased from the afternoon of 23 April until the morning of 26 April (Fig. 2).
Indoor and outdoor PM counts (/L) before and during dust storms
Factors that affect indoor PM counts
This study measured indoor and outdoor particle counts during an Asian dust event under everyday conditions. An Asian dust event on 24 April 2012 caused a 50-fold increase in PM counts outdoors and a 20-fold increase indoors in an apartment room in Japan. As far as we know, there is one report regarding PM changes in indoor air during Asian dust events at an office building in Taipei . This report showed that indoor PM2.5 and PM10 increased threefold during the dust storm while outdoor PM2.5 and PM10 increased 1.7-fold. The ventilation systems in this high-rise building utilize air from outside, and the authors concluded that this was likely the primary reason that air particle concentrations inside the building were significantly affected by outside air pollutants during dust storms. The ventilation system in our apartment was used once for 20 min, 2 h before observation began. We speculate that, in addition to this ventilation, the occupants opening doors and their movements into and out of the rooms were the main routes for PMs entering the monitored room.
The PM increase observed in this study was larger than in the Taipei study. One explanation for this is that the baseline count was very low in our study (mean SPM during the control period was 1.7 μg/m3, range 0–5 μg/m3) compared to the control period in Taipei (PM2.5 and PM10 were 45 and 70 μg/m3, respectively). Second, in this study, air pollution other than desert dust was also observed during the Asian dust period; CFORS predicted sulfate aerosol arrival in our Asian dust period and LIDAR observed spherical as well as non-spherical particulate matter during this time period. Accordingly, the observed PM count increase is considered to be a mixture of desert dust and other air pollution. Finally, this study and the Taipei study may have also differed in the original scale of the Asian dust storms observed.
An open window (20 cm) for 10 min resulted in a rapid increase of indoor PM counts up to 70 % of outside levels, which was maintained for 3 h after closing the window. An air purifier rapidly reduced the PM counts for all particle sizes larger than 0.3 μm.
Previous reports have often noted that air change rates in occupied houses are highest when weather conditions are mild, and several investigators have speculated that this is due to increased window-opening behavior under mild conditions. Iwashita and Akasaka measured ventilation rates using gas tracers and questionnaire surveys assessing indoor environment and residents’ behavior, and concluded that 87 % of the total air change rate was due to occupant behavior . United States researchers quantitatively confirmed that having a single window open can increase air change rates .
This study’s PM observations indoors and outdoors during an Asian dust event are consistent with the previous reports above and suggest the importance of accounting for occupant behavior, such as window-opening and air purifier use, when estimating residential exposure to particulate matter.
In conclusion, Asian Dust arrival caused a 50-fold increase in PM counts outdoors and a 20-fold increase indoors under everyday conditions on 24 April 2012, in Kyoto, Japan.
A window open for 10 min resulted in a rapid increase of indoor PM counts up to 70 % of outside levels that was maintained for 3 h. An air purifier rapidly reduced PM counts for all particle sizes larger than 0.3 μm.
The results suggest it is important to account for occupant behavior, such as window-opening and air purifier use, when estimating residential exposure to particulate matter.
We thank Dr. Nobuo Sugimoto for providing LIDAR data, the Kyoto City Environmental Protection Guidance Division for SPM data, Dr. Itsushi Uno for CFORS images, and Mr. Tadashi Hioki for fruitful discussion. This research was supported by the Environment Research and Technology Development Fund (C-1152) of the Ministry of the Environment, Japan.
Conflict of interest
The authors declare no conflict of interest.
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