Introduction

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 [12], as do emergency ambulance dispatches [13] after Asian dust events in Japan. Non-accidental mortality and cardiovascular mortality also increase significantly after Asian dust events in Taiwan [14]. 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 [15].

Given that people spend approximately 90 % of their time indoors [16], 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 Our preliminary investigation (unpublished data) revealed that approximately half of the pregnant women in Kyoto, Japan, opened windows every day during April 2012 (1794 respondents/2107 queried).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.

Methods

Monitoring situation

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 apartment is a 4LDK with 86 m2 floor area, occupied by two adults and an 11-year-old girl. None of the occupants are at home during the daytime and all three are non-smokers. The floor is wooden and shoes are strictly prohibited in the rooms in accordance with Japanese culture. The monitored room of the apartment has a 22 m2 floor area and is shown in Fig. 1. The door in the monitored room was kept closed throughout the study period except when occupants went in or out, but there is a 0.8 cm-wide space at the bottom of the door even when it is closed. An air purifier was constantly used in the living/dining room during the observational period (2 m3/min airflow) and was moved to the monitored room for the experiment.

Fig. 1
figure 1

Floor plan of the monitored apartment. Two particle counters (solid circles) were used. One was placed 20 cm above the floor in the monitored room and the other was placed 120 cm above the veranda floor. An air purifier (solid rectangle) was placed in the living-dining room during the observational period and moved to the monitored room for the experiment

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The study period was composed of an observational period and an experimental period. An observational period was composed of a control period and an Asian dust period (Fig. 2). In the experimental periods, we performed experiments of opening windows (20 cm, 10 min) or air-purifier use to see how these factors affect the PM counts indoors. We also performed an experiment of crawling (a researcher, 160 cm height, crawled on the floor for 10 min) to see if dust fallen on the floor affects the indoor air again because of activities.

Fig. 2
figure 2

Local SPM measurements during the control and Asian dust periods

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Monitoring equipment

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.

Results

Forecast and station data during the study period (22–25 April 2012)

The Chemical Weather Forecast System (CFORS) predicted the arrival of an Asian dust cloud in Kyoto at noon on 23 April (Fig. 3) [21, 22], and LIDAR in Osaka measured high concentrations of soil dust during the same period (Fig. 4) [17, 18].

Fig. 3
figure 3

Asian dust distribution prediction by the Chemical Weather Forecast System [17]. The Asian dust clouds were predicted to arrive in Kyoto at noon on 23 April 2012

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Fig. 4
figure 4

Estimation of non-spherical and spherical particle concentrations by light detection and ranging (LIDAR) [21]

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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).

Notably, CFORS predicted increased sulfate in the air during the same period (Fig. 5) [21], and an increase in the spherical particulate matter was observed by LIDAR during this Asian dust event (Fig. 4) [17], which was considered to be trans-boundary air pollution flying simultaneously with Asian dust.

Fig. 5
figure 5

Sulfate distribution prediction by the Chemical Weather Forecast System [17]

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Indoor and outdoor PM counts (/L) before and during dust storms

Figure 6 shows indoor and outdoor air PM counts (/L) before and during a dust storm with closed windows, without air-conditioner use in the monitored room, and without cooking throughout the study period. A room door was opened twice during each period and the apartment door was opened twice for short periods of time (several seconds) during each period. Before the dust storm, the PM (particles > 0.3 μm) counts were very low both indoors and outdoors (indoors: mean 5186/L, range 921–12 670/L; outdoors: mean 4779/L, range 1154–23 637/L). During the dust storm, the indoor PM (particles > 0.3 μm) counts increased approximately 20-fold, while the outdoor PM counts increased approximately 50-fold (indoors: mean 115 340/L, range 44 737–243 399/L; outdoors: mean 250 867/L, range 65 152–375 367/L; Fig. 6). Indoors, smaller PM levels seemed to be more influenced by outdoor PM levels than larger PM levels (Fig. 6).

Fig. 6
figure 6

Comparison of particle counts/L inside and outside during control (left) and Asian dust (right) periods

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Factors that affect indoor PM counts

Figure 7 shows the time course of PM counts through the experiments. Opening a window (20 cm) for 10 min resulted in a rapid increase in PM counts up to 70 % of outdoor levels. Smaller PM sizes remained longer in room air, as high as 50 % of outdoor levels for PM greater than 0.5 and 0.3 μm even after 3 h. A researcher’s crawling caused an increase in counts of PM larger than 1 μm. An air purifier (non-HEPA filter, 3 m3/min airflow) reduced PM counts for all sizes in 30 min. With the air purifier on, window opening (20 cm) for 10 min still caused elevation of PM counts up to 50 % of outdoor levels, but counts for all sizes rapidly returned to baseline levels after closing the window.

Fig. 7
figure 7

PM counts/L in outdoor (upper) and indoor (lower) air under various conditions

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Discussion

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 [23]. 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 [24]. United States researchers quantitatively confirmed that having a single window open can increase air change rates [25].

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.