Around 20 years ago in northern Mozambique, a medical student, Hipólito Nzwalo, saw an entire family that could only walk with the help of walking sticks. Their legs were stiff and rigid, moving like a pair of scissors opening and closing, instead of bending at the knee.

They had a neurological disease known as "konzo," which translates to "tied legs." The struggle to walk is irreversible, said Nzwalo, now a neurologist and professor at the Faculty of Medicine and Biomedical Sciences of Algarve in Portugal. It can also lead to sudden paralysis.

Konzo is brought on by exposure to high amounts of a toxin from a starchy root vegetable, cassava—one of the staple foods in the diet of more than 500 million people who live in Africa.

With proper preparation, the toxin, hydrogen cyanide, can be flushed out with water. But in the face of agricultural crisis, drought, and poverty, people are forced to choose between going hungry and adhering to these preparations. A lack of rain can also increase the concentration of hydrogen cyanide in cassava, making the plant even more dangerous to eat. All these factors, and especially drought, are predicted to get worse with climate change and increase the risk of konzo.

Konzo is just one example of how the climate crisis is going to fundamentally change the availability and safety of the foods we eat. In 2019, researchers found that climate change and higher CO2 levels could reduce certain vitamins in foods, like zinc, iron, and protein. But there might be even more dramatic impacts: Instead of just making plants less nutritious, they could also become toxic, like cassava when faced with drought.

This could happen in several ways: Indirectly, through extreme weather, poverty, and hunger pushing communities to eat and rely on underripe or un-prepared food, and creating a scarcity of other options. Or, in more common foods—including barley, millet, flax, maize, sorghum, cherries, and apples—there is the potential for an accumulation of toxins due to loss of water and erratic weather events. Over-exposure to any of these toxins could lead to dramatic effects on health and disorders of the nervous system.

During the food shortages in the Spanish civil war, people had to rely more heavily on grass pea, a hardy legume known for being a "famine food." Grass pea can cause a neurological disease called lathyrism from ingesting too much of two toxic amino acids it contains. The symptoms are similar to konzo—muscular weakness, an inability to walk, paralysis, or tremors. Eating too much grass pea led to an epidemic from 1941 to 1943 of what the Spanish called Azañón's Disease, after one of the villages it afflicted.

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In India, people safely eat small amounts of grass pea as part of their normal diet. But during a drought or extreme weather, if other food crops die off, people might need to consume more of it, on account of it being available and plentiful. “Now you can see the links with climate change,” Peter Spencer, a neurologist at Oregon Health and Science University said.

Many of the plants we eat have toxins in them, but since we eat them at such low levels it doesn’t affect our health. One of the basic tenets of toxicology is that for something to be harmful, it depends on the amount. As the adage goes: “the dose makes the poison.”

People are more at risk for these nutritional neurotoxic disorders when faced with poverty, hunger, a lack of water, and other food options being limited. These pressures will only get worse with climate change, Spencer said, because it will cause water and food insecurity.

In a study from 2019 in the journal Environmental Neurology, Spencer and his colleagues described several other foods that have toxins that can make people sick—like the fruit of the ackee tree, eaten in West Africa and Jamaica, and lychee fruit from South Asia.

Every year, children in India go to the hospital for fever, convulsions, and seizures. One suggested cause is that they are being sickened from toxic chemicals in unripe lychees, which can cause dramatic drops in blood sugar and encephalopathy, or damage from swelling of the brain, Padmini Srikantiah of the Centers for Disease Control and Prevention office in India told CNN. The effect increases if a person who eats them is malnourished; those eating unripe lychees out of hunger or desperation—which could increase with climate change—would be more affected. The children who ate lychees on an empty stomach were found to get more sick. (It's still somewhat of a mystery, though, as recent cases have indicated there might be another factor at play.)

Nzwalo said that there’s no speculation needed about what a changing climate will do to the numbers of people with konzo, because past droughts have already shown it will get worse. In the Kahemba district of Bandundu, almost 2,000 children were crippled during one dry season—a span of a few months from June to September of 2009.

Changing climates can also cause people to migrate to other areas, in search of food or water—and “could lead to introduction of bitter cassava in areas where historically cassava processing methods are not used, exposing locals to the toxicity," Nzwalo said.

If your diet doesn't regularly include foods like lychee or cassava, a 2004 report from the United Nations Environment Program (UNEP) found that some more familiar foods are also at risk for becoming more toxic.

Plants try to protect themselves in the face of a changing climate too, and the ways they do can be harmful to humans. They use a compound called nitrate to grow, and convert it into other molecules like amino acids and proteins. When crops like barley, maize or millet are faced with drought, they slow down or stop this conversion, which leads to a nitrate buildup.

In an interview with German media organization Deutsche Welle, Jacqueline McGlade, chief scientist and director of the Division of Early Warning and Assessment at UNEP, called the result a “poison chalice." If a human eats large amounts of nitrate, it can “stop red blood cells from transporting oxygen in the human body,” Yale360 reported.

In the opposite direction, heavy rains can lead to a toxic buildup of hydrogen cyanide or prussic acid in foods like flax, maize, sorghum, arrow grass, cherries and apples. Hydrogen cyanide is the same ingredient that can be found in some types of chemical warfare, Reuters pointed out. With flooding, there can be an increase in fungal growth and mycotoxins on crops.

All these toxins cause disorders of the nervous system. “They can really make it difficult for people to breathe—it's like asphyxiation [suffocation],” McGlade said in the interview. “If animals or human beings are pregnant, that can cause miscarriage.”

As with many other issues regarding climate, the most vulnerable people are being affected first: subsistence farmers and people in rural areas without access to diverse foods. Around 4.5 billion people in developing countries are already exposed to toxins in their food, the report found.

“We can look at a map of the world of drought conditions and you can pretty much guarantee that somewhere in those areas you are going to find it,” McGlade told Deutsche Welle. “So in sub-Saharan Africa, but increasingly in northern and southern parts of Africa. We see definite trends in Latin America and Brazil. Really, all over the world now.”

If CO2 levels aren't dramatically reduced, in the future we'll need to develop drought-resistant crops that don’t create buildups of those toxins, or crops that are resistant to fungal toxins. “As we look forward and see the effects of climate change, we can really start to see the upper end of this: 70 percent of agriculture production is going to be affected by either too much rain or too little rain,” McGlade said. “So we need to be aware: this exposes potentially billions to toxins.”

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