Arsenic (As) is a metalloid widely found as various organic and inorganic forms in water, soil, and air. Organic and inorganic As are also present in most foodstuffs. Contribution of each food category to daily total (both inorganic and organic) As intake of the Japanese was estimated. Fish and shellfish contributed the greatest to total daily intake, followed by seaweed and rice.
As compound present in fish and shellfish is mostly arsenobetaine and in seaweed is mostly arsenosugar; both are organic As and considered virtually nontoxic. Meanwhile, inorganic As [arsenate and arsenite] is more toxic than organic As. Two food items— rice and hijiki seaweed, a brown alga Sargassum fusiforme,—were estimated as major contributors to daily inorganic As intake in the Japanese population. Because rice is a globally important food crop, the Codex Alimentarius Commission set the maximum levels for inorganic As in white rice in 2014 (0.2 mg kg-1) and brown rice in 2016 (0.35 mg kg−1).
According to a survey of rice As concentrations by the Ministry of Agriculture, Forestry and Fisheries of Japan, approximately 5 % of brown rice cultivated in Japan collected in 2012 contained inorganic As at the concentration higher than 0.35 mg kg−1, and the average concentration (n = 600) of inorganic As was 0.21 mg kg−1 for brown rice. It was higher than that in brown rice collected from 2004 to 2006 (n=600) by 0.15 mg kg−1, so we examined the cause of the difference.
For twenty years (1996-2015), rice plants had been cultivated in a paddy field at our institute under flooding condition before and after heading. The field experiments show positive correlation between temperature after rice heading stage and inorganic As concentration in rice grain. Global warming will increase temperature in rice growing areas, which could result in higher concentration of inorganic As in rice grain.
To verify the effect of high temperature on As concentration in rice grain, we conducted a pot experiment in temperature gradient chambers (TGC). A chamber was divided into zones of three different temperature (TGC1, TGC2, TGC3). Twenty seedlings per pot were circular-dense-transplanted, and transferred into each TGC one week after heading time and harvested at maturity. Average air temperature from one week after heading to harvest time were 27.0 °C (TGC1), 28.3 °C (TGC2) and 29.3 °C (TGC3). Rice inorganic As concentration were ranked as TGC3 > TGC2 > TGC1. There was a significant negative correlation between inorganic As concentration and thousand‐kernel weight.