4. Discussion
Environmental pollution by heavy metals in industrialized countries is the consequence of present or past emissions by petrochemicals industries. Unfortunately, few stringent measures and controls have been put into place and, as a consequence, high levels of these pollutants still persist in the soils and sediments causing concerns for the food chain integrity and unwanted consequences for the populations living in those areas.
In the present study, we evaluated heavy metals exposure in the adolescent population, resident in the Milazzo-Valle del Mela area, identified as a high-risk area by local authorities. A total of 215 adolescents aged 12–14 years were recruited in the seven schools located in the municipalities of Condrò, Gualtieri Sicaminò, Milazzo, Pace del Mela, San Filippo del Mela, Santa Lucia del Mela, and San Pier Niceto. An age-matched control group resident in a rural area of Sicily was also studied. Adolescents represent an essentially important population sample: in fact, exposure to heavy metals from the early childhood could have a negative impact on the developmental status. In addition, this population has not yet too many complicating influences, such as occupational exposure, moving house, which may cause variability in the data evaluation and interpretation.
The first most relevant result emerging from our survey is the presence of high levels of urinary cadmium in exposed adolescents compared with control ones, having levels within the range of the reference values [7, 9][7][9]. As a matter of fact, there are several industrial plants and metal-work-related factories, currently operating in the area, that use cadmium compounds as stabilizers for PVC products, color pigments, alloys, and rechargeable nickel cadmium batteries. In addition, the thermal power plant and metallurgical industries, still active and located in the area, use cadmium as an anticorrosion agent (cadmiation). In the present study, we identified gender difference in the urinary levels of cadmium in the exposed group: this may reflect either a different tissue accumulation or absorption in the two sexes, as already reported [16][21]. Airborne cadmium could be readily absorbed by plants and taken up through the roots to edible leaves, fruits, and seeds [17[22][23], 18], and this might also explain the elevated values observed in the adolescents: in fact, results from the questionnaire show that 67% of adolescents eat fruits and vegetables grown in their own garden. All these observations, taken together, led us to hypothesize that the elevated urinary levels of this heavy metal are due to a high burden of pollutants in both the ecosystem and the food chain.
The urinary cadmium concentration is mainly influenced by the body burden and is proportional to the concentration in the kidneys. In general, nonsmokers have urinary cadmium concentrations of 0.02–0.7 μg/L, and their cadmium levels slowly increase with age together with the accumulation of cadmium in the kidney [19][24]. Therefore, elevated urinary cadmium excretion is a read-out of high intake of cadmium and also of elevated gastrointestinal absorption of the heavy metal in children, as recently pointed out by Kippler et al. [20][25]. However, the increased levels of cadmium of exposed adolescents compared with controls do not allow drawing any final conclusion about the health risk of the population living in the Milazzo-Valle del Mela area.
As a matter of fact, the German Human Biomonitoring Commission recommends the use of two different kinds of criteria to assess exposure: reference values and HBM values. The reference values indicate the upper margin of background exposure to a given pollutant in a given population at a given time [7]. By contrast, HBM values derive two different kinds of values: HBM I represents the concentration of a substance in human biological material below which there is no risk for adverse health effects and, consequently, no need for action; HBM II represents the concentration above which there is an increased risk for adverse health effects. Adverse health effects should be considered for concentrations in the range between HBM I and HBM II. The geometric mean of the urinary cadmium levels in our exposed adolescents was higher than the specific HBM I value (0.50 μg/L) settled by German Human Biomonitoring Commission [8] and fell in the range between HBM I and HBM II (2 μg/L). In another study, called DEMOCOPHES, a pan-European HBM project involving 120 mother and child (6–11 years) couples, cadmium levels were found higher in younger children, suggesting that the prepubertal age is even more sensitive to metals' accumulation [21][26]; given the different age-range, it is difficult to compare those results with our study.
Intense industrialization and other activities have led to the global occurrence of soluble chromium, which is readily leached from soil to ground water in concentrations above permissible levels. This naturally occurring element was found elevated in either exposed or control adolescents when compared with reference values [22][16]. This could be due to nonanthropogenic contamination of drinking water: in fact, most of the local mineral water has shown an elevated content of this metal (up to 50 μg/L) during routine checks by the Sicilian Environmental Protection Agency (ARPA, Sicilia) [23][27]. Besides, it should be pointed out that we measured total chromium but not its hexavalent form that more correlates with an increased healthy risk. Finally, there is no survey showing reference value for this specific heavy metal in adolescents; of the previous surveys, only one carried out in a cohort of Spanish adolescents evaluated urinary chromium, demonstrating lower levels compared to the present study [24][28]. Therefore, our present data represent a warning for the local, regional, and national authorities that should undertake immediate action to reduce exposure to cadmium and chromium.
Surprisingly, we found higher blood lead levels in the control population than in exposed adolescents: this unexpected finding is likely linked to high levels of lead water pipes that have not yet been replaced in the rural houses. Another explanation could be the higher frequency of smoking parents as suggested by the results of our life style and habits questionnaire.
Additionally, adolescents living in Milazzo-Valle del Mela area had an increased risk of being more exposed to arsenic than those living 45 km away from the industrial site; this mainly reflects the frequency of fish consumption, in agreement with results of questionnaires, where it appears that the 40% of exposed adolescents (versus the 14% of controls) eat fish twice a week. Furthermore, the Sicilian Environmental Agency Report (ARPA, Sicilia) has detected overruns for arsenic in groundwater in the Milazzo-Valle del Mela area [23][27]. However, arsenic urine levels of the adolescents living in the industrial area were below the reference values [22, 25][16][29].
Mercury urinary levels were above reference value, in exposed children; however, the high percentage of samples below the limit of quantification makes the comparison with other studies particularly difficult. Besides the industrial emission, mercury levels are most likely due to the large use of dental amalgam fillings, and methylmercury exposure derives also from fish consumption [26][30].