Arsenic in Rice a Persistent Problem and Climate Change Could be Making it Worse

A recent paper published in the Proceedings of the National Academy of Sciences reviews decades of efforts by scientists to get arsenic out of rice. 

Arsenic, in inorganic form, is a pollutant which can stem from industrial processes and pesticides, and chronic exposure can lead to health conditions like diabetes, cardiovascular disease, and cancer. 

A team of researchers first discovered the toxic pollutant was present in rice in the early 2000s, and in 2012 an analysis revealed that nearly all 65 types of rice and rice products produced contained arsenic, many at concerning levels. 

In 2016, the U.S. Food and Drug Administration recommended that arsenic levels in rice-based infant cereals be lowered.

While data shows that the levels of arsenic in rice grains are dropping, the concerns about arsenic levels persist, and may be worsened by climate change, as recent research shows that higher temperatures can increase the uptake of arsenic by rice plants. 

That’s why scientists are scrambling to develop solutions for getting arsenic out of rice, but finding that many of the techniques that decrease arsenic can increase another toxin called cadmium, which is also potentially deadly.

One solution considered by researchers is silicon, a primary nutrient in husks, and an abundant metalloid that is non-toxic when ingested, to outcompete arsenic for entry into the plant. 

To test silicon as a solution, a team of researchers added silica gel to arsenic-contaminated soil and found it could lower arsenic levels in rice grain by as much as 40%.

However, the fertilizers are expensive, leading the researchers to try rice husks, which are high in silicon and low in arsenic. 

In 2016, the team confirmed that adding rice husks to potted rice plants resulted in grains with 25 to 50% less inorganic arsenic.

Other solutions involve manipulating paddy water and soil.

In 2019, a team of researchers found that letting soil moisture drop to 35% volumetric water content twice during the growing season can halve the inorganic arsenic in the field grain compared to existing flooding paddies, but may not be effective for regions where there are high cadmium levels.

Inverting soil can also decrease arsenic in rice grains, but a separate team of researchers in Bangladesh found the solution is only temporary, as irrigation water will continue to deposit more arsenic.

To develop longer lasting solutions to getting arsenic out of rice, scientists are now turning to genetics to understand why certain varieties of rice, like basmati, accumulate less arsenic. 

This past March, a team of researchers revealed a genetic mutation that indirectly reduces arsenic accumulation in rice grain by about a third, and they are now attempting to harness this mutation to breed low-arsenic rice. 

Still, the need for a more permanent and safe solution for getting arsenic out of rice remains, and problem may be made worse by the prospect of a warmer future, as the data shows that doubling atmospheric CO₂ and increasing temperature by 5 °C nearly doubled the inorganic arsenic concentration in rice grains.