Exposure variability and determining factors of urinary metals for schoolchildren in Taiwan

Urinary heavy metal levels in schoolchildren vary greatly over time, and research on the determinant variables explaining variance components in urinary metal exposure is limited. This study assessed metal concentrations and variability in the urine of schoolchildren and explored their important determining factors. We collected spot urine samples from schoolchildren (n = 321) living in urban, suburban, and rural areas during the warm and cold seasons. The toxic (As, Cd, Ni, Cd, and Pb) and essential (Co, Cu, Mo, and V) elements of urine samples were analyzed by inductively coupled plasma mass spectrometry. We assessed the within- and between-subject variability in urinary metal concentrations, calculated the "probability of overexposure (θ)," compared with the reference values. We evaluated the spatiotemporal and biological factors that determine average exposure by using a mixed-effects model. The within-subject variance accounting for 63.8%-95.4% of the total variance in exposures was predominant for selected urinary metals. Urinary As levels (subject-specific mean = 69.0 μg/g creatinine) in schoolchildren were remarkably high and presented ∼99.9% of θ. After adjusting for the selected variables, we found that urban schoolchildren had significantly higher urinary metal levels (As, Cr, Co, and Ni) than suburban and rural schoolchildren. The urinary levels of most metals (except for Cu and Pb) increased during the cold season. Girls had higher urinary Cr, Co, Cu, Ni, and Pb levels than boys. Body mass index and urinary creatinine could also affect urinary metal levels. Those variables explained 15.8% (Pb), 11.6% (Cr), and 6.5% (Cd) of total variance in urinary concentrations. The repeated measurements of spot urine samples across seasons in individuals for long-term exposure estimates of metals were suggested. This biomonitoring survey of a large number of urine samples provided useful information on toxic metals and on the important determinants of exposure in schoolchildren.