Toxic Metals Getting into our Food

Heavy metals from industrial pollution and sewage sludge may poison the food we eat. Scientists used to think that living organisms, such as snails and worms, could only ingest heavy metals and other pollutants in the soil if they were dissolved in water, but now they know this isn’t true, meaning they could affect us too. Much of the heavy metal pollution on the West Coast comes from mercury emissions from coal burning in China that end up in rainfall over California.

Most soil pollutants bind tightly to soil particles, and scientists thought this made them inaccessible to organisms that feed on the soil. But when French soil biologist Renaud Scheifler allowed snails to roam inside a container filled with cadmium-contaminated soil taken from land near a lead and zinc smelter, he found that 16% of the cadmium they absorbed was from the supposedly inaccessible cadmium bound tightly to the soil. “Our work is, to our knowledge, the first evidence of this,” he says. This means other heavy metals, such as zinc, copper, lead and mercury, could also be entering the food chain.

Cadmium can cause kidney damage, anemia and a painful bone disorder known as “itai itai,” which means “ouch ouch” in Japanese. Hundreds of people living near Tokyo developed this disease in the 1950s after exposure to cadmium from industrial pollution.

Industrial emissions from Asia are a major source of the mercury in the rain that falls along the California coast. It?s not harmful to us in rain, but when it soaks into the soil, it gets into the food chain. “People are talking about things like removing dental fillings before cremating bodies, but our analyses indicate that this may be a trivial source of mercury compared to the inputs from industrial emissions in Asia,” says researcher Russell Flegal.

Mercury acts as a gas in the atmosphere and is not washed out in rain until it has been oxidized into a form that can be captured by water droplets. “There is a relatively large reservoir of mercury in the atmosphere, and it’s the rate of oxidation that determines how much of it gets deposited in rainfall,” says researcher Douglas Steding. Ozone, a major part of urban and industrial smog, helps create this oxidation.

Mercury pollution comes from coal, and emissions from coal-burning power plants in China produce 10% of the world?s total industrial mercury pollution. Air pollution in China also creates ozone. Air loaded with mercury and ozone moves off the continent into the Western Pacific, where it becomes part of developing storms.

Steding collected rainwater at two sites in central California. Rainwater collected near the coast had mercury concentrations three times the normal level. Rainwater from the inland site had 44% higher mercury levels. The difference is probably due to a difference in each area?s smog (ozone) levels.

Most of the mercury contamination that originates in California is the result of mining. Large amounts of liquid mercury were used in gold mining operations during the Gold Rush, leaving residues that still affect the soil and water today.

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