Cang Man^{1, *}, Takemasa Watanabe¹, Kana Oshimura¹, Nandingbaolige², Shuangjin Li³

¹Graduate School of Health and Sport Science, Chukyo University, Toyota, Aichi 470-0393, Japan
²Hohhot MinZu College, No.56, Tongdao North Road, Xincheng District, Hohhot 010051, China
³Zhalantun Vocational College, No.4, Xiushuilu Linzhong Lane, Zhalantun 162650, China

*Corresponding Author: Cang Man, Graduate School of Health and Sport Science, Chukyo University, Toyota, Aichi 470-0393, Japan

Abstract
In this study, we investigated gender differences in the effects of air pollution levels on physical fitness levels of 6^th-grade students at Mongolian elementary schools in five cities in Inner Mongolia, an autonomous region of China. Significantly, a weak negative correlation between the 50m x 8 shuttle run execution time and good day rate for both boys and girls was found. There was a significant weak positive correlation between the vital capacity and the percentage of good days for boys, while almost no correlation was found for girls. Therefore, it was suggested that the degree of air pollution may cause a decrease in cardiovascular endurance due to a decrease in daily physical activity in both boys and girls, and a decrease in lung function in boys due to an increase in exposure to air pollutants by playing outside.

Keywords: Mongolian children, air pollution, vital capacity, cardiovascular endurance, gender differences.

Abbreviations
Analysis of variance (ANOVA), Carbon monoxide (CO), Chinese National Student Physical Fitness Standard (CNSPFS), fine particulate matter (PM2.5), inhalable particles (PM10), Inner Mongolia Autonomous Region (IMAR), nitrogen dioxide (NO2), ozone (O3), sulfur dioxide (SO2), World Health Organization (WHO)

In recent years, with the development of the market economy, the expansion of international trade, and the diversification of culture, many ethnic festivals, such as Madam, have been held on a large and small scale in the Inner Mongolia Autonomous Region (IMAR) of China. In Naadam, the traditional games that captivate boys such as "horse racing", "archery", and "Mongolian wrestling" remain deep- rooted. As boys in Inner Mongolia prefer an active, outdoor lifestyle in a traditional cultural context, there is a concern that the level of air pollution may affect their physical fitness levels. At school, boys spend most of their time outside when not in class, while girls spend most of their time in the classroom. Boys prefer to play soccer and basketball, while Mongolian wrestling is a very popular sport.

In a comparison of five cities, the authors found that the degree ofair pollution was associated with a lower vital capacity and systemic endurance in sixth-grade boys in Inner Mongolia, China [6]. However, the effects of sex and age on children's physical fitness levels cannot be ignored. Internationally, girls around age 12 was found to be less physically active and more inactive than boys of the same age [7]. In China, although few areas was surveyed, it was reported that elementary and middle school male students in Shanghai performed exercise and sports for longer than girls [8], indicating that the reduction in the amount of physical activity in daily life may be because of air pollution on the health status as well as the physique and physical fitness levels of Mongolian children living in the IMAR and that this effect may be different between boys and girls.

Therefore, the purpose of this study was to compare the physique and physical fitness levels of Mongolian children living in five cities with different levels of air pollution in the IMAR and to clarify how the effects of air pollution on the physique and physical fitness levels differ between boys and girls by comparing the physique and physical fitness levels of Mongolian children attending the 6th-grade in elementary schools in those cities.

Methods
Geographical and demographic characteristics of the Inner Mongolia Autonomous Region
Founded in 1947, the IMAR is in the east and west of the northern frontier region of China. The total area of the autonomous region is 1.183 million km², about three times the size of Japan. The IMAR is a vast expanse of land far from the sea, with a temperate monsoon climate and an average elevation of 1,500 to 2,200 m above sea level. In winter, January is the coldest month, and the average monthly temperature ranges from -10 °C to -32 °C. Summer, which is short at approximately 1–2 months, is not very hot, and the average monthly temperature ranges from 16 °C to 27 °C. However, in the hottest month of July, the temperature can rise to approximately 36 °C–43 °C. At the end of 2014, according to the 2015 IMAR Statistical Yearbook, the total population of IMAR was 25.048 million of which 77.54 % was Han Chinese and 18.65 % was Mongolian [9].

Selection of participants
Based on the IMAR Environmental Mass Report [10], in each of the western, central, and eastern regions with different levels of economic development and air pollution, five cities, Hohhot, Baotou, Chifeng, Bayannur, and Xilinhot, where Mongolian elementary schools had been established, was selected (Figure 1). At the end of 2014, the population of each city was 3,089,000 in Hohhot, 2,857,000 in Baotou, 1,062,000 in Chifeng, 550,000 in Bayannur, and 265,000 in Xilinhot [9]. All the sixth graders (average age, 12.0 ± 0.5 years old) attending all the Mongolian elementary schools in each city was selected as the participants in this study. The total number of participants for the 4 years from 2013 to 2016 was: Hohhot City, 1,031 (483 boys and 548 girls); Baotou City, 437 (214 boys and 223 girls); Chifeng City, 486 (243 boys and 243 girls); Bayannur City, 188 (98 boys and 90 girls); and Xilinhot City, 791 (405 boys and 386 girls); with a total of 2,933 (1,443 boys and 1,490 girls) participants.

Figure 1: Geographical location of five cities in the IMAR selected for the study.

Physique and physical fitness measurements
(1) Physique and physical fitness measurements was carried out every September for four years from 2013 to 2016 at each school according to the Chinese Education Department's "National Student Constitution Health Standard" [11].
(2) There was nine measurement items for both boys and girls, namely, height (cm), weight (kg), BMI (kg/m2), vital capacity (ml), 50 m sprint (s), sit and reach (cm), 50m × 8 shuttle run (s), timed sit- ups (number of times per minute), and timed rope skipping (number of times per minute). In China, the 50m × 8 shuttle run time is used as an evaluation of the overall endurance of 6th-grade elementary school students.

Analytical methods
Comparison of the air pollution levels between cities.
The determination of the air pollution level was based on the "Environmental Air Mass Index (Air Quality Index, hereinafterAQI) Technical Regulation (HJ633-2012) [12]", and "Excellent Day", "SO2", "NO2", and "PM10" (fine particles below 10 μm particle size) was used as evaluation indices. The annual average air pollution data between cities was compared using annual government-published reports from 2013 to 2016. The proportion of "good days" (hereinafter referred to as "good days") was used as the main air pollution index of this study. A "good day" was defined as an AQI of "excellent" (0– 50) or "good" (51 – 100) based on the "Environmental Air Mass Standard (GB3095 - 2012) "[13]. The AQI is classified into six stages, with 101–150 rated as "mild pollution", 151 – 200 as "moderate pollution", 201 – 300 as "severe pollution", and 301 – 500 as "very severe pollution " [13]. The AQI is an index that was created independently by the Chinese Ministry of Environmental Protection and its value is calculated using the concentration values of six types of target substances as PM10, PM2.5, SO2, NO2, CO, and O3, and set so that the higher the AQI, the higher the air pollution level, and the greater the health hazard to the human body. The average concentration values of SO2, NO2, and PM10 were treated as supplementary indicators in this study. The average of the percentage of good days per month for four years was used as the percentage of good days; for the average of SO2, NO2, and PM10, the average of the annual averages for four years was used as representative values of each city. For the comparison of mean values between cities, a one- way analysis of variance (ANOVA) was performed. For items where the F values was recognized as significant, multiple comparison tests using the Bonferroni method were performed. The statistical significance level was set at P < 0.05.

Results
Air pollution levels in target cities
From 2013 to 2016, the average percentage of good days from the lowest was in the order of Hohhot, Baotou, Chifeng, Bayannur, and Xilinhot. The average percentage of good days in Hohhot and Baotou cities was significantly lower than that in Bayannur and Xilinhot. The average concentration of SO2 in Chifeng City and Baotou the city was significantly higher than that in Xilinhot. The average concentration of NO2 in Hohhot and Baotou cities was significantly higher than that in Chifeng, Bayannur, and Xilinhot. In addition, the NO2 average pollution concentration in Chifeng and Bayannur was significantly higher than that in Xilinhot. The mean concentration of PM10 in Hohhot and Baotou cities was significantly higher than those in Xilinhot. (Table 1) summarizes the levels of air pollution in each of the five target cities and shows the results of multiple comparisonsof ANOVA F values and post-analysis.

Table 1: Air pollution levels in cities in the IMAR from 2013 to 2016

1. p < 0.05 vs Hohhot, (2) p < 0.05 vs Baotou, (3) p < 0.05 vs Chifeng, (4) p < 0.05 vs Bayannur, (5) p < 0.05 vs Xilinhot

Physique and physical fitness level
To compare the average physique and physical fitness among the cities, a one-way ANOVA analysis using cities as a fixed factor was carried out, and the results according to boys and girls are shown in (Table 2).

Table 2: Comparison of physique and physical fitness averages between cities by boys and girls

In girls, there was a significant positive correlation of r = 0.08 between the percentage of good days and vital capacity, and a significant negative correlation of r= -0.32 with the 50 m × 8 shuttle run times. Vital capacity showed significant negative correlations with SO2, NO2, and PM10 concentrations (r = -0.02; r = -0.09; r = - 0.07, respectively) The 50m × 8 shuttle run time showed significant positive correlations with SO2, NO2, and PM10 concentrations at r= 0.16; r = 0.34; r = 0.31, respectively)

Table 3: Results of the correlation analysis between air pollution level, physique, and physical fitness level by boys and girls

**p < 0.01, *p < 0.05

(Figure 2) shows the relationship between the average vital capacity and the percentage of good days by boys and girls. In boys, the mean vital capacity was higher in the order of Baotou, Hohhot, Chifeng, Bayannur, and Xilinhot. Meanwhile, in girls, the relationships observed in boys was not observed. In the regression analysis of the vital capacity and the percentage of good days, the regression equations for boys y = 11.47 x + 1097.63 and girls y = 3.12 x + 1575.77 was obtained. The regression coefficient obtained for boys was significantly higher than that obtained for girls. (Figure 3) shows the relationship between the 50 m×8 shuttle run time and the percentage of good days for both boys and girls. The average run time of the 50m×8 shuttle run was higher in Hohhot and Baotou than the times in Bayannur, Xilinhot, and Chifeng. No significant differences was observed between boys and girls. In the regression analysis, the regression equations for boys y = -0.38 x + 140.72 and girls y = -0.40 x + 144.75 was obtained. There was no significant difference in the regression coefficients between the boys and girls.

Figure 2: Relationship between vital capacity and percentage of good days by boys and girls

Figure 3: Relationship between the 50m × 8 shuttle run times and percentage of good days by boys and girls.

Discussion
Among the five cities in the IMAR, China, that was the targets in this study, there was a clear difference in the percentage of good days, the main indicator of air pollution level. The air pollution in Hohhot and Baotou was more pronounced than the air pollution in Chifeng, Bayannur, and Xilinhot. The average annual concentration of SO2 was 42 µg/m3 in Hohhot and 46 µg/m3 in Baotou, exceeding the WHO reference value (24 µg/m3), but below the Chinese reference value (60 µg/m3). The average annual concentration of NO2 was 41.3 µg/m3 in Hohhot and 41.8 µg/m3 in Baotou, exceeding the WHO and Chinese reference values (40 µg/m3). In addition, the annual average concentration of PM10 in the five cities was in the range of 58.3 µg/m3–119.5 µg/m3, exceeding both the WHO reference value of 20 µg/m3 and the Chinese reference value of 70 µg/m3. Children in Shenyang, which is adjacent to the IMAR, experienced pulmonary dysfunction caused by air pollution [14], and according to 2016 China Air Pollution ranking, Hohhot experienced more severe air pollution than Shenyang [15]. This suggested that the same pulmonary dysfunction may have occurred in Hohhot, thus, it was a suitable city selection to elucidate the present research topic.

Funding: This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

References