Daytime profile of residential PM_2.5 concentrations in a ger, a traditional residence in Mongolia

Introduction

Nearly 50% of the population in developing countries use solid fuels [1]. Solid fuels are often burned in traditional open stoves without a proper ventilation system. Thus, a significant level of indoor air pollution occurs due to incomplete combustion [2, 3]. Among the various solid fuels, the World Health Organization (WHO) recommends against the use of unprocessed coal in the indoor environment [4, 5].

Indoor air pollution related to the burning of solid fuel for heating and cooking in a house affects human health. Coal combustion in a house has been linked to respiratory illnesses, lung cancer, chronic obstructive pulmonary disease (COPD), immune system impairment, and reduction in lung function in China [6]. The PM_2.5 related to biomass burning has been associated with hospital admissions for respiratory diseases in children under the age of 5 in Cuiabá city [7]. In Shanghai in China, past use of coal among women was associated with all-causes mortality [8].

About 1.2 million people reside in Ulaanbaatar (UB), the capital city of Mongolia, accounting for more than 40% of the population of Mongolia [9]. The city has a peri-urban ger area where a significant part of the UB population is living in a ger. According to the UB Statistics Department, there are about 164,127 households in the ger areas of six districts surveyed by World Bank (WB) at the end of 2011, an increase of approximately 42,000 since the end of 2007 [10].

A ger is a traditional Mongolian house, which has a one-room tent structure with five or, in some cases, six walls, and typically with an average floor area of 28 m². The stove for heating and cooking is located in the center of the ger. Households living in a ger in UB are costly to keep warm and comfortable because UB is the coldest capital in the world with an average daily temperature of about - 13ºC and an average night temperature of -40 ºC. The stove is used throughout the winter months from October to March. With the heavy use of coal, indoor and outdoor air pollution in UB can be very high during the winter.

The purpose of this study was to measure indoor PM_2.5 levels in gers, and determine the effects of cooking and heating on indoor PM_2.5 levels. The indoor PM_2.5 levels were measured in 28 gers during the daytime, and cooking and heating activities were observed continuously.

Methods

Results

During the winter of 2011-2012, traditional stoves have been replaced by low-emission stoves (called "improved stoves") by WB’s project, as a part of a major effort to reduce outdoor air pollution in Ulaanbaatar [10]. However, there was no significant difference in fuel usage, times of adding fuel, PM_2.5 concentration, and indoor temperature.

Eighteen of the 28 gers used a traditional stove, while the others used an improved stove. Up to 16 of the 28 gers used coal, 4 used wood, and 8 used coal with additional materials (wood, plastic bags, nut cover, and paper). The average fuel usage in 26 gers was 8758 ± 4074 g, excluding the two gers where no fuel was added during the observation period. The average number of times fuel was added into the stove was 0.4 ± 0.3 times/h. The average PM_2.5 concentration, temperature, and relative humidity in the 28 gers were 208±173 μg/m³ (n=28), 23±4ºC (n=27, one home is missing due to malfunction of the monitor), and 21±7% (n=28), respectively. The descriptive statistics of the PM_2.5, temperature, and relative humidity are listed in Table 1. The average PM_2.5 concentration in 61% of the gers exceeded 100 μg/m³ (17 of 28 gers) and was under 50 μg/m³ in only 2 of 28 gers. The average temperature was over 21℃ in 70% of the gers (19 of 27 gers) and less than 18℃ in 3 of 27 gers.

Table 1.

Descriptive statistics of indoor PM2.5, temperature, and relative humidity

Overall, the PM_2.5 concentration was higher in the morning and decreased gradually until late afternoon. Five temporal profiles of the indoor PM_2.5 concentration in the 28 gers were determined (Figure 1) using the concentration and temporal pattern of the real-time PM_2.5 concentration. The average PM_2.5 concentrations in the five groups were 384±171 μg/m³ (n=10), 171±71 μg/m³ (n=6), 123±56 μg/m³ (n=4), 74±27 μg/m³ (n=5), and 27±20 μg/m³ (n=3). In the first group, the indoor PM_2.5 concentrations were very high, and an extreme peak of about 8,500 μg/m³ was observed. In the second and third groups, the indoor PM_2.5 concentrations in the morning were 600 and 400 μg/m³, respectively, and decreased gradually. In the second group, peaks of about 1,100 μg/m³ occurred in the afternoon. In the third group, less peaks were observed. In the fourth group, the indoor PM_2.5 concentrations in the morning were 200 μg/m³. There was a peak of about 180 μg/m³ in the afternoon. In the fifth group, the PM_2.5 concentration remained below 50 μg/m³ in the afternoon.

The correlation between the PM_2.5 concentration and the pattern of stove use is shown in Table 2. The number of times the stove was opened was significantly associated with the PM_2.5 concentration. The PM_2.5 concentration increased when the amount of fuel usage was higher, but they were not significantly correlated (p=0.078). Fuel usage and the frequency of opening the stove were significantly correlated (R=0.77, p-value<0.01). The peak frequency of PM_2.5 was not statistically correlated with the number of times the stove was opened and the fuel usage.

Table 2.

Correlation between the PM2.5 concentration and pattern of stove use (opening and fuel usage)

The differences in the PM_2.5 concentration based on stove type are listed in Table 3. The PM_2.5 concentration did not differ based on stove and fuel type. The number of times the stove was opened was significantly higher with the traditional stove. However, 2 of the 5 gers using an improved stove did not add fuel during the observation period. The PM_2.5 concentration in these two gers were lower than the other three gers. The differences in the PM_2.5 concentration based on fuel type are listed in Table 4. The peak frequency of PM_2.5 was higher with the use of mixed fuel. The stove was opened more often when mixed fuel was used.

Table 3.

Difference in PM2.5 concentration based on stove type(N=20)

Table 4.

Difference in PM2.5 concentration based on fuel type(N=18)

Figure 1.

Five types of temporal profiles of the PM2.5 concentration in the 28 gers.The boundary of the box closest to zero indicates the 25th percentile, the line within the box marks the median, and the boundary of the box farthest from zero indicates the 75th percentile. Whiskers (error bars) above and below the box indicate the 90th and 10th percentiles, respectively. The dots indicate all data points that are outside the 10th and 90th percentiles.

Discussion

The indoor temperature in the gers was on average 23.0 ± 4ºC, which was on the high end of the acceptable indoor comfort temperature range. The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) recommends an indoor temperature of 19.2 ºC to 27.8 ºC in winter [11]. In 4 of the 27 gers, the indoor temperature was greater than 28ºC. Such high temperatures suggest the overuse of coal. Education to ensure the reasonable use of coal is required.

The relative humidity was very low (21% on average). Low relative humidity adversely affects human health. Eye and skin irritation may be caused under relative humidity of less than 20% [12-14]. Several reports have recommended maintaining indoor relative humidity above 30% to 40% to prevent drying of the mucous membranes and to maintain adequate ciliary activity [15, 16]. The survival of the rhinovirus and the influenza virus also improved in low relative humidity environments [17, 18]. Particles in dry air can transfer to deeper parts of the respiratory system, because particles can become smaller due to the loss of surface water [19].

Coal combustion for heating and cooking in gers caused high concentrations of PM. The indoor PM_2.5 concentration was comparable with other studies using solid fuels. In Inner Mongolia, China, the highest respirable particulate matter (RPM) concentration of 719 μg/m³ was observed in a province with biomass burning in the single cooking/living/bedroom [20]. In the primarily coal-burning provinces, RPM concentrations were 202−352 μg/m³ in Guizhou and 187−361 μg/m³ in Shaanxi. In Xuanwei and Fuyuan, China, the geometric means (GM) of indoor PM_2.5 air concentrations were GM(GSD):162 (2.1) and 136 (2.0) μg/m³, respectively [21]. In an area of extreme poverty in Metropolitan Santiago, higher pollutant concentrations were observed during hours when heating was used in houses that used coal (mean PM10 250 μg/m³, CO 42 ppm, and SO2 192 ppb). However, the 24-hour average PM₁₀ concentration was 173 ± 14 μg /m3. In four states in India, the 24hr concentration of PM_2.5 in households burning solid cook fuel ranged from 163 μg/m³ in the living area to 609 μg/m³ in the kitchen [22].

The temporal profiles of indoor PM_2.5 concentration tend to include high levels in the morning. It has been suggested that indoor PM_2.5 concentrations may be higher at night-time. We classified the temporal profiles into five groups. Group 1 with the highest concentration may have been affected by smoking and incense burning. Five of the ten gers in this group had either indoor smoking or incense burning. They used more mixed fuel types and opened the stove more frequently. The proportion of traditional improved stoves did not differ between groups. Groups 2 and 4 had slightly more fluctuating PM_2.5 concentrations than groups 3 and 5. Only coal was used as fuel in groups 2 and 4. No indoor smoking and incense burning was observed in groups 4 and 5. The stove was opened much less frequently in groups 4 and 5, recorded as 0.2 and 0.3 times/h per house, respectively. Based on the observations, the peaks in the daily profile of the PM_2.5 concentration coincided with smoking, incense burning, opening the stove for refueling, cooking fried food, and using the vacuum cleaner.

The residents in the ger used various materials for heating and cooking, although coal was the main fuel type. WHO recommends against the use of unprocessed coal in the indoor environment, because it can release various toxic pollutants. However, other materials, such as plastic and rubber, also release toxic chemicals. Plastic combustion can release hazardous chemicals, including, hydrochloric acid, sulfur dioxide, dioxins, furans, and heavy metals, as well as particulates. These emissions are known to cause respiratory diseases, to stress the human immune system, and are potentially carcinogenic. Therefore, the Mongolian government must educate residents to refrain from burning these materials while limiting the burning of unprocessed coal.

The study could not determine the significant factors causing the indoor PM_2.5 concentration. It was not associated with stove or fuel type. In a previous study, the indoor PM_2.5 and CO were not significantly decreased by the use of an improved stove [23]. The amount of fuel used was also not associated with the indoor PM_2.5 concentration. The peak frequency of the PM_2.5 concentration was higher with mixed coal. In previous research, the indoor PM concentration using coal mixed with additional materials was higher than when using coal only [21]. Further study with a larger sample size should be conducted to determine the significant factors. The temporal profile suggests that the number of times the stove door was opened may be associated with the peak frequency. This may be due to the release of PM during the door opening.

This study was limited by sample size. Because the observation of resident behavior was required, the number of gers studied was limited. However, the observations were important to determine the use of other fuels. The effects of stove type and other factors should be further determined with 24hour sampling. Another limitation was that ambient air concentrations were not measured. Ambient monitoring was not possible because of extremely cold temperature. In addition, there might be no significant difference in outdoor concentration for four sampling days. Therefore, the effect of the stove usage pattern on indoor PM_2.5 concentration were valuable information.

Conclusion

The indoor PM_2.5 concentrations in the gers reached high peak concentrations of up to 8500 μg/m³. However, our measurements may have underestimated the 24-hour exposure due to lack of monitoring during the night and early morning. The indoor PM_2.5 concentration was associated with opening the stove. A stove should be developed where the PM emissions are reduced during the opening of the stove. Further study is also necessary to measure the indoor PM concentration for 24 hours and identify the factors associated with daily indoor exposure in a large sample population.

Acknowledgments

This study was partially supported by the China Medical Board (CMB) and the Institute for Global Social Responsibility, Seoul National University (SNU). We thank C. Han of the Department of Preventive Medicine, College of Medicine, SNU, and D.G. Park, S. Lee, S. Xu, and K. Ha of the Graduate School of Public Health, SNU, who have helped and co-operated with us during the field measurements. We also thank T. Monkhnaran, B. Buyuntushig, B. Jeljejtji, D. Otgongerel, Sh. Baynjargal, D. Bayrmaa, Ts. Odontuya, G. Odmaa, Sh. Purevdorj, and B. Yanjinlkham from the School of Public Health, Mongolian National University of Medical Sciences, Ulaanbaatar, Mongolia for their kind support in translation and helping us observe residents in the gers.

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