A review and comparative analysis of arsenic, cadmium, lead,and mercury in Thai rice, soil, chemical fertilisers and pesticides

Arsenic (As) and other heavy metals such as cadmium (Cd), lead (Pb) and mercury (Hg) have adverse health implications as human carcinogens. Many studies have shown high concentration of some of these elements in rice. Their presence can be attributed to the application of chemical fertilisers and pesticides in the cultivation of the rice and residues from the soil. Thus, consumers that depend on rice as a staple may face potential health risks. There are regulatory controls in various countries to limit the amount of some of these metals in rice. In the European Union (EU) the Annex to Regulation (EC) No 1881/2006 as amended, sets the maximum residue limit for arsenic and cadmium in white rice at 0.20 mg/kg. There was no limit for lead and mercury. In this research, rice, rice husk, soil, chemical fertilisers and pesticides were collected between January and March 2015 from Thailand. The levels of arsenic, cadmium, lead and mercury were compared with international and national standards. 43 rice, 41 rice husk and 42 soil samples were selected from paddy fields in the north of Thailand where the rice was grown in industrially uncontaminated soil. The heavy metal content of the rice samples was analysed using inductively coupled plasma–mass spectrometry (ICP-MS). The results showed that the means of these heavy metal in rice were 1.8 ppb (range 0.5-4.0, n=43) for arsenic, 0.4 ppb (range 0.1-2.2) for cadmium, 1.0 ppb (range 0.4-2.0) for lead, and 0.1 ppb (range -0.03-0.3) for mercury. It also found that the average level of arsenic in rice was lower than the guideline limit of 200 ppb (equivalent 0.2 mg/kg) set out by the Codex Alimentarius for arsenic in white rice. Although cadmium, lead and mercury were detected in the rice, the levels of these trace elements were not significant. The means of the main trace elements in rice husk were as follows: arsenic: 2.0 ppb (range 0.5-4.6, n=41), cadmium: 0.3 ppb (range 0.1-0.8), lead: 0.7 ppb (range 0.76.1) and mercury: 0.1 ppb (range -0.0-0.7). The average concentration of heavy metals in soil was 17.9 ppb, (range 5.2-46.0, n=42) for arsenic, 0.8 ppb (range 0.3-3.0) for cadmium, 83.5 ppb (range 35.6-130.1) for lead, and 0.2 ppb (range 0.1-0.8) for mercury. In Thailand, the Pollution Control Department (PCD) specified a maximum limit of 3.9 mg/kg for arsenic in agricultural soil. At present, there is no record of heavy metal control in rice husk. The results indicated that the arsenic content (mean 17.9 ppb or 0.018 mg/kg) in the soil samples was relatively low compared to the permissible limit. The highest amount found was lead, with a concentration of 83.6 ppb, (equivalent of 0.08 mg/kg) although by reference to PCD (2016), this was well below the Thai acceptable limit of 400 mg/kg for lead in soil. A similar analysis of the 5 synthetic chemical fertilisers showed that the mean of arsenic was 56.1 ppb (range 0.0-158.7 ppb), cadmium: 2.5 ppb (range 0.1- 7.1 ppb), and lead: 4.1 ppb (range 0.8 -9.6), respectively. Mercury was not found in any of the synthetic fertilizers. Similarly, five pesticides were analysed and neither arsenic nor mercury was detected in any of the pesticide samples. However, the average value for cadmium was 0.1 ppb with no range and 0.7 ppb (range 0.5-0.8 ppb) for lead. It was found that both arsenic and lead concentrations were significantly lower in the rice than in the soil samples (p < 0.001). Furthermore, the levels of arsenic and lead found in the chemical fertilisers and pesticides were not significant or high enough to cause alarm. In conclusion, the analysis showed that arsenic was found in all rice, husk, soil and chemical fertiliser samples but it was not detected in pesticides. As expected, the level of arsenic in the soil was higher than in the rice and husks. From the results, the maximum level of arsenic to the least in descending order was: soil > husk > rice. Regarding a risk assessment, it can be concluded from this study that a normal daily intake of selected Thai rice would not be a health concern for humans.