The study, which appears in the journal Environmental Geochemistry and Health was supported by IAFNS Food & Chemical Safety Committee and focuses on mitigating arsenic uptake in rice, which often grows in flooded fields.
A focus on arsenic is important as high levels of arsenic exposure can cause acute symptoms like vomiting, diarrhea and nerve damage. And long-term exposure to low levels of arsenic can lead to skin problems, increased cancer risk (particularly skin, bladder and lung cancer), and cardiovascular disease.
Rice is typically grown under flooded conditions that support anaerobic microbial metabolism that promotes generally small amounts of plant uptake of arsenic. Some amount of this arsenic ends up consumed by humans and in some populations and diets, rice is one of the largest sources of inorganic arsenic intake.
To identify arsenic mitigation opportunities, the researchers conducted a literature search using an extensive database spanning from 2008 through 2024. The review covered 120 studies in 14 countries and included 44 field studies, 68 potted plant studies, and 8 studies on postharvest practices.
Rice uptake of arsenic can be limited by selecting varieties that accumulate less arsenic in the grain. However, the difference in arsenic accumulation between subspecies can impact yields, as high-yield varieties typically have higher total arsenic accumulation than lower-yielding varieties.
Of all the mitigation methods surveyed, alternative water management techniques are perhaps the most effective in limiting arsenic concentrations in rice. These irrigation management strategies include sprinkler irrigation, alternate wetting and drying, furrow irrigation, aerobic cultivation and rainfed irrigation. However, there may be barriers to implementing alternate wetting and drying irrigation in place of flooding due to several reasons including — but not limited to — a lack of control of irrigation water due to the pump and canal distribution networks at some farms.
Despite differences in soil type, climate and type of irrigation method applied, every study in the review found that irrigation methods with dry periods lowered total grain arsenic anywhere from 10 to 98% in rice when compared to continuous flooding.
However, one drawback of employing irrigation methods with dry periods is the likelihood of higher cadmium concentrations — another toxic heavy metal — in some soils because that metal is most bioavailable under dry conditions.
Soil Amendments
Additives intended to improve the soil called “soil amendments” containing silicon compounds have been demonstrated to lower arsenic uptake in plants. The majority of studies reviewed found that silicon amendments decreased grain total arsenic 7% to an estimated 78% at various levels of application.
Studies in this review that measured grain arsenic also found that application of the metal selenium as an amendment lowered grain total arsenic an estimated 7–100% depending on factors such as arsenic level in the soil and application amount. Higher rates of application were generally more effective, but overdoses of selenium can be toxic to the plant and runoff from fields can pollute the environment.
The majority of reviewed studies found that sulfur amendments lowered total arsenic 27–72% in rice, though not under all management conditions.
Combining different soil amendments can be effective as well. Studies show that combinations of amendments were able to lower grain total arsenic up to 61% while also lowering grain cadmium up to 93%.
Postharvest Techniques
Parboiling, or partially cooking and drying the rice in the husk, is known to reduce arsenic in the grain. On an industrial scale, it requires more energy and labor than milling raw rice. Alternatively, rinsing, soaking and modifications in cooking require minor amounts of water and energy and are viable methods of reducing arsenic for home cooks. For example, cooking methods that involve briefly parboiling rice before cooking and discarding the parboiling water lowered levels of arsenic up to 83%.
The article also addresses data needs and the high cost of testing crops for arsenic. According to the authors, “Investment in accurate, high-throughput testing would also enable farmers who grow low-arsenic rice to contract with mills to provide low-arsenic rice and rice products intended for sensitive populations…”
The researchers add that “Our recommendation is to create regional, public-sector laboratories … for high-throughput arsenic speciation analysis that could facilitate creating a pipeline for low-arsenic rice streams.”
According to lead author Marguerita Leavitt from the University of Arkansas — now an associate consultant and analyst at SES, Inc. in Kansas City, “We hope that this paper will provide a roadmap for growing healthier rice using research-based methods.”
Co-author Dr. Benjamin Runkle, also at Arkansas, adds “this review and perspective really highlights the way that water-saving practices can provide multiple benefits — not only are they more environmentally sustainable, they can also improve grain quality and reduce some health risks too.”
Photo credit: Beth Biros
