Global warming and arsenic in rice

In a series of posts we are going to look at the impact of global warming on food production and the potential for an increase in toxic compounds in our normal diet. First off is rice and higher levels of arsenic found in the rice grain when exposed to higher temperatures during cultivation.

Rice is the world’s most important foodstuff providing nutrients and energy to more than one half of the world’s population. Unfortunately, rice can also contain arsenic, which can cause multiple health conditions and diet-related cancers. In an earlier post we described possible chronic health effects of natural levels of arsenic in food and water.

Here we will cover two issues – the influence of higher global temperatures on arsenic levels in rice and types of arsenic compounds formed in soil under different environmental conditions.

Temperature dependence of arsenic accumulation

Arsenic occurs naturally in soil at different levels across the world. When farmers grow crops like rice under flooded conditions, arsenic is drawn out of the soil and into the water. As rice plants extract water through their roots to its leaves, arsenic follows as it mimics other molecules that rice plants preferentially draw out of the soil.

Now researchers at the University of Washington have found that warmer temperatures, at levels expected under most climate change projections, can lead to higher concentrations of arsenic in rice grains at ranges where they begin to have further health concerns. Arsenic concentrations in the grain more than tripled between the low- and high-temperature treatments.

However, the researchers didn’t have the means to check the type of arsenic compounds found.

Some forms of arsenic are more toxic than others.

It is important to know that not all arsenic is the same as arsenic exists in several different forms. Fish and seafood usually contain high levels of arsenic, but most of this is arsenobetaine, an organic form with little toxicity. It is the inorganic arsenic that can be found in water and rice and a range of other food commodities that has been of particular concern.

However, arsenic speciation is not easy to perform, which has created some confusion. Inorganic and methylated oxyarsenic species have been a focus of research, but thioarsenates, in which sulfur takes the place of oxygen, have largely been ignored.

Now University of Bayreuth researchers, together with scientists from Italy and China, have for the first time systematically investigated under which conditions, and to what extent, sulphur-containing arsenic compounds are formed in rice-growing soils. It turns out that the amounts of thioarsenates formed are linked to the pH-values of the soils and other environmental parameters.

Formation of thioarsenates in soil, their uptake in rice plants and their potential risks to human health urgently require further research as at least one organic sulphur-containing arsenic compound discovered in rice fields is already known to be carcinogenic.

A bad situation potentially made even worse

Arsenic is one of WHO’s 10 chemicals of major public health concern and in particular for the millions of people who rely on rice as their staple food. Young children are also at risk if rice-based products make up a large part of their diet.

Global warming has the potential to make a bad situation even worse. With an increase in global temperatures higher levels of arsenic in rice will follow and the composition of the arsenic compounds may change, for better or worse.

So please be careful in contributing to global warming.