In recent decades, the study on biological monitoring in epidemiological and toxicological research to assess the risk of human exposure to pollutants from different routes has been of great importance. In this regard, due to the widespread occurrence of arsenic in the environment and drinking water sources, as well as its potential for adverse effects on human health, biological monitoring is used as a valuable tool to assess exposure to arsenic through natural and anthropogenic pathways15,16.
The main source of groundwater contamination with arsenic could be attributed to the geological structure of the study area, which comprised limestone, Jurassic sandstone, marl, metamorphic rocks, and shale. Also, most portions of the area belong to the northwestern part of the Sanandaj-Sirjan zone and the southwestern part of the Zagros thrust belt27,28. Another reason for arsenic contamination in drinking water sources is the proximity of the area to Kurdistan Province, where the presence of arsenic in drinking water sources has been reported in some areas29,30. According to the results of the present study, the average, minimum, and maximum concentrations of arsenic in the fingernail samples were higher in the participants living in arsenic-contaminated areas than in those living in areas without arsenic exposure.
Spatial variations and regional distribution of arsenic contents in toenail samples and drinking water supplies of Nova Scotia in Canada were examined by Dummer et al. They also assessed the geological and environmental characteristics related to high concentrations of arsenic in drinking water sources. Based on the findings of this study, the levels of arsenic in drinking water samples where less than the detection limit of the method (478 µg/L). In their research, they stated that the contamination of drinking water sources with arsenic originated from the geological structure of the study area. Also, in this research close associations were observed between the high levels of arsenic in toenail samples and drinking water supplies, which is consistent with the findings of our study31.
According to the results, in the villages exposed to arsenic, 85% of the fingernail samples contained arsenic above the normal level, while in the villages without arsenic exposure, 28.57% of the fingernail samples had arsenic concentration higher than normal value.
The observations of a study by Chakraborti et al., who analyzed the concentration of arsenic in 176 biological samples (hair, urine, and nail), indicated that 69 participants had skin lesions caused by arsenic exposure, and the rest did not have arsenical skin lesions. In this study, normal arsenic levels in hair, nail, and urine samples were reported to be 20–200 µg/kg, 20–500 µg/kg, and 22.
In the multivariate linear regression model adjusted for age, gender, BMI, smoking, and exposure to arsenic, the variables including gender and arsenic levels in drinking water were significantly associated with fingernail sample arsenic (p
In a study by Schmitt et al., human nail samples were used as biomarkers for arsenic exposure in the Mongolia region, China. In this study, the nail samples were collected from 32 participants. A total of 19 individuals were exposed to high concentrations of arsenic through drinking water (264–648 µg/L), and 13 subjects were exposed to low concentrations of arsenic (0.3–9.8 µg/L), representing as the control group. A significant relation was found between arsenic levels in drinking water and the nail samples in this study. Also, based on the findings of this study, the concentration of arsenic in the nail samples in males due to greater water consumption was higher than in the female participants, which is consistent with the findings of the present study32.
In another study, the correlation between arsenic concentrations in drinking water sources and toenail samples in Nova Scotians was investigated by Yu et al. A total of 960 "samples of male and female" aged 35- 69 years were selected for arsenic analysis in toenail clipping samples. In this study, information including age, gender, drinking water source, treatment use, ethnicity, level of education, employment status, household income, smoking status, physical activity, and BMI was asked through a questionnaire from all the participants. Based on the findings of their study, a statistically significant association was observed between the arsenic concentration of drinking water sources and toenail samples (r = 0.46, p 33.
In a study conducted by Liu et al., the association between arsenic uptake and human health risk was evaluated using biomarkers including hair, nails, urine, and saliva. The results of their investigation represented that the concentration of arsenic in biological samples was higher in males, older, and participants with skin lesions than those in others. Also, they reported that females had a higher methylation capacity than males, so the concentration of arsenic in biological samples was lower in females, which is consistent with the findings of the present study26.
The present study results showed that the use of the fingernail samples as a biomarker to evaluate arsenic exposure through ingestion of drinking water was appropriate. Nail samples contain high amounts of scleroproteins (like keratin) and sulfhydryl groups; it should be pointed out that arsenic has a strong affinity for binding to these sulfhydryl groups. Also, due to the feeding of germinal nail matrix from a rich blood source, arsenic deposition in nail samples occurs shortly after consumption. It can be concluded that because of the slow growth rate, less probability of external pollution compared to other biomarkers (like hair samples), easier collection, and non-invasive properties, the use of nail samples (fingernail and toenail samples) as a suitable biomarker to study exposure to environmental pollutants is on the rise18,32. It should be noted that the levels of measured arsenic in nail samples in different studies are reported in Table 4.