China Strontium Carbonate Company
Sr was detected in water samples from 314 cities (see Table S1). Table 2 presented the Sr levels in drinking water in Chinese cities. In general, the range of Sr concentration was 0.005–3.11 mg/L with a mean value of 0.360 mg/L. Compared with Sr concentration in the southwest part of Cairo (mean value 0.867 mg/L) and the USA (mean value 1.10 mg/L), it was relatively low in Chinese cities. As shown in Fig. S2, there were 295 cities whose Sr concentration mainly fell in the range of 0.005–1 mg/L. Thus the number of these cities accounted for 93.95% of the total number of cities (314 cities). Among them, there were 76 cities whose Sr concentration was below 0.1 mg/L, which occupied 24.2%; the number of cities with Sr concentration of 0.1–0.2 mg/L and 0.2–0.3 mg/L was 61 and 60, respectively.
Table 2 Strontium concentrations in public drinking water in Chinese cities (mg/L)Full size table
As presented in Table 2, Sr concentrations were the highest in NWC with a range of 0.041 to 1.43 mg/L, mean of 0.667 mg/L; the lowest Sr concentration was detected in SC with a range of 0.005–1.59 mg/L, mean of 0.179 mg/L. The Sr concentration in NC and QT was 0.051–3.11 mg/L with an average of 0.537 mg/L and 0.094–1.08 mg/L with an average of 0.439 mg/L, respectively. It could be found in Fig. S3 that Sr concentration in public drinking water in different Chinese cities followed the sequence NWC > NC > QT > SC. According to the Kruskal-Wallis non-parametric test, the concomitant probability value was 0, which meant the relationship of Sr concentrations between different cities was statistically significant.
In Fig. 1, it was clear to see that the Sr concentration in public drinking water in Chinese cities presented geographically aggregated distribution. Sr level was relatively low in the southern region of the Yangtze River. Sr concentration was below 0.1 mg/L in most cities except the middle part of Guizhou province (Sr concentration was above 0.5 mg/L). Sr concentration in the cities along the Yangtze River was mostly between 0.2 and 0.3 mg/L. Cities in which Sr concentration was above 0.5 mg/L were found in the Yellow River Basin and Xinjiang. As shown in Fig. 1, Sr concentration was higher in the northwest than in the southeast.
The average Sr concentrations intaking through drinking water in different age groups in different regions were shown in Fig. 2a. In general, the average Sr intakes of infants, children, teens, and adults through public drinking water were 0.273, 0.503, 0.633, and 0.784 mg/day, respectively, which was below the daily intake of Sr from drinking water in the USA (2 mg/L) (WHO 2010). In terms of geographical distribution, the intake of Sr via public drinking water was the lowest in SC, which was half of the national average intake of Sr. However, in the QN (the combination of QT and NWC), the Sr intake by drinking water was the highest, and the Sr intake in adults could reach 1.19 mg/day. At present, the total daily intake of Sr in China is not clear. Nevertheless, we know the total daily intakes of Sr in Japan, Finland, and the USA are 2.3 mg/day, 1.91 mg/day, and 3.3 mg/day, respectively (Shiraishi et al. 1994; Varo et al. 1982; WHO 2010). Assuming that the total daily intake of Sr in China is between Finland and the USA, namely, the daily intake of Sr in China is estimated in the range of 1.91–3.3 mg/day. Therefore, based on the intakes from various countries, the Sr intake from public drinking water accounts for 24–41% of the total daily intake of Sr for an adult in China. As you see, public drinking water is one of the most important ways to get Sr. In Fig. 2b, the 95th percentile value of the national average daily intake of Sr through drinking water was 2.72 mg for the adults, especially in NC and QN which values were 2.92 mg and 2.86 mg, respectively. As a result, for the cities which have a relatively high Sr concentration in NC and QN, drinking water is the main source to intake Sr. Although there are some studies reported the Sr content in human urine, the Sr content in feces and sweat and the amount of Sr retained in the bones are unclear (Yang et al. 2019). So it is still difficult to determine the estimated average (daily) requirement (EAR) and recommended daily intake (RDA) of Sr.
Fig. 2The Sr intake through drinking water in different age groups in different regions: (a) mean values; and (b) 95th percentile values (NC, Northern China; SC, Southern China; QN, Northwest China and Qinghai-Tibet Plateau)
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Although Sr is a trace element having a high concentration in drinking water and the mineral water containing high Sr concentration is quite popular among customers (Misund et al. 1999), the studies about the relationship between Sr content and body health are limited. A previous study proposed that Sr content in drinking water was significantly negatively correlated with the incidence and mortality of cardiovascular diseases. However, as shown in Fig. S4, the cities having a relatively high Sr concentration in drinking water also have a relatively high total hardness. It is widely accepted that the hardness of drinking water could prevent cardiovascular diseases to some extent (Monarca et al. 2003). As a result, the relationship between Sr concentration and cardiovascular diseases is still being concerned.
Moreover, Sr has been utilized in the prevention of osteoporosis (Alexandersen et al. 2011). Figure 3 presented a significant correlation between the bone mineral density (BMD) of the 60–70 years older people and Sr concentration in drinking water. For example, the elderly people from the cities with a lower Sr concentration had a lower BMD in the lumbar spine and femoral neck bone. As presented in Table 3, the correlation between Sr concentration in drinking water and the BMD of the lumbar spine was stronger than that between Sr concentration and the BMD of the femoral neck bone. Besides, the correlation coefficient between Sr concentration and male elderly people was 0.692 (p < 0.01); meanwhile, the correlation coefficient of female elderly people was 0.483 (p < 0.01), which presented a significantly positive correlation.
Fig. 3Scatter plot of bone mineral density (BMD) of 60–70 years old residents vs. Sr concentration in public drinking water in 31 cities in China
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Table 3 The correlation between bone mineral density (BMD) and Sr as well as that between BMD and Ca in public drinking waterFull size table
As shown in Fig. 4, it is clear to see that the high incidence of rickets in children was found in the cities with high Sr concentration and low Ca/Sr ratio in drinking water. Besides, among the age group of 1–3 years, the correlation coefficient of Sr concentration and the incidence of rickets was 0.411 (p < 0.05), while there was a negative correlation between Ca/Sr ratio and the incidence of rickets with a correlation coefficient of − 0.410 (p < 0.05).
Fig. 4Scatter plot of correlation between prevalence rate of rickets of 1–3 years old children, Sr concentration and Ca/Sr ratio in public drinking water in 48 cities in China
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The hazard quotient and hazard index of Sr through the oral intake and skin absorption in different age groups in different regions are shown in Table S4. The mean value of HQd and 95th percentile values were in the order of 10−4–10−3; the average value of HQi and 95th percentile values were mostly in the order of 10−1–10−2. HQi was two orders of magnitude higher than HQd. Therefore, oral intake was the main exposure route of Sr in public drinking water. In general, the mean value of HI and 95th percentile values of exposed Sr in public drinking water in Chinese cities were all less than 1, so the non-carcinogenic risk of exposed Sr was not obvious. Among people of different ages, the infants’ HI was the highest (average value 0.066; 95th percentile value 0.247), followed by the children’s HI (average value 0.041; 95th percentile value 0.149) and adults’ HI (average value 0.021; 95th percentile value 0.075). The teens’ HI was the lowest (average value 0.019; 95th percentile value 0.066). The infants’ HI was nearly two times higher than that of the children, and the HI of the children was almost two times than that of the teens; the HI of the teens was close to that of the adults. In Fig. 5a and Fig. 5b, HI of four age groups in SC was the lowest; the mean values of HI and 95th percentile values were one time lower than the national average level. HI of four age groups in NC and QN were all above the national average level. The mean values of HI and the 95th percentile values of four age groups in QN were all higher than those in NC. In Table S4, however, the maximum value of infants’ HI in NC was 0.795, which was close to the theoretical threshold of risk. Therefore, it was necessary to conduct detection on Sr content in the NC where there was a high Sr concentration in public drinking water.
Fig. 5Hazard index distribution in different age groups in different regions: (a) mean values; and (b) 95th percentile values (NC, Northern China; SC, Southern China; QN, Northwest China and Qinghai-Tibet Plateau)
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The results of sensitivity analysis were presented in Fig. S5. Except for the cities in QN, Sr concentration (Cw) and exposure duration (ED) had the most significant influence on different age groups, and their correlation coefficients were 0.466–0.667 and 0.276–0.389. The drinking water ingestion rate (DR) has a significant impact on the children, teens, and adults (their correlation coefficients fell in the range of 0.077–0.177), but its influence on infant could be ignored. In QN, Sr concentration (Cw) and drinking water ingestion rate (DR) were the most influential factors on the output risk of adults, and the exposure duration (ED) was a minor factor. The results of the sensitivity analysis indicated that the better definition of the probability distribution of Cw, ED, and DR could obtain a more accurate risk evaluation.