Small children could be ingesting up to 0.12 micrograms per kilogram of body weight, so an average one-year-old who weighs 11 kilograms (24 pounds) could be eating 1.32 micrograms of the lethal substance every day.
This amount falls outside the safe amounts set by the US Environmental Protection Agency, but its calculations are based on an indirect exposure from the mother's womb to the fetus.
The truth is that nobody really knows what the effects are on a growing child exposed to these levels, say scientists from the American Chemical Society. But we do know that methylmercury can cause neurological and reproductive problems in adults, and developmental problems in infants and small children.
The scientists tested levels of methylmercury in 119 infant cereal samples, and discovered that almost all of those that were rice-based had much higher levels of methylmercury. The rice had come from polluted areas in America and China, with average concentrations of methylmercury of 2.28 micrograms per kilogram.
Fish such as albacore tuna are another common source of methylmercury.