Today’s chemical measurements are 1,000 times more sensitive than in the 1970s. Analyses at the parts per billion range — about one drop in a swimming pool — are commonplace.

Arsenic is a naturally occurring metallic element. Like other chemical elements, its atoms seep into water supplies, travel on the wind from plumes of volcanic ash, accumulate in soil and spread through industries.

About one-tenth of a gram of certain forms of arsenic is immediately lethal, a fact responsible for its historical fame as a homicidal poison.

Formerly, scientists thought that low doses were not a concern, but recent studies have revealed that long-term exposure to doses in the ppb range poses various threats.

In 2001 the Environmental Protection Agency lowered the limit for arsenic in water to 10 ppb from 50 ppb, but there are no regulations for arsenic in food.

Arsenic is present in many foods due to absorption through the soil and water. While most crops do not take up much arsenic from soil and water, rice takes it much more readily than other grains.

Linking arsenic levels to outbreaks of disease began in the 1960s in Taiwan when epidemiologists traced an outbreak of gangrenous, blackfoot disease to well water that was contaminated by arsenic.

In the 1970s in Bangladesh, villages began drilling wells to prevent the spread of infectious diseases that lived in sewage-tainted surface waters. It worked, but by the 1990s lung and bladder cancers, cardiovascular diseases, diabetes and skin lesion cases had grown insidiously.

Geologic testing revealed that arsenic leaching into groundwater had driven levels in some wells to exceed 500 ppb. Two decades later the World Health Organization recommended declaring a public health emergency for what has been called “the largest mass poisoning of a population in history.”

Antofagasta, Chile, switched from well water to cheaper runoff from the Andes between 1958 and 1970 when scientists discovered up to 800 ppb of arsenic in the city supply from mountain mineral deposits.

Residents during that period had notably higher rates of bladder and lung cancers.

Some laboratory studies suggest that arsenic exposure as low as 10 ppb can compromise immune systems and cause higher rates of skin cancer from UV exposure.

With the effects of arsenic poisoning mounting up, scientists began to wonder about crops grown in arsenic-­tainted water.

Tests on rice paddies in Bangladesh revealed that rice plants up took arsenic from water and soil at 10 times the rate of other grains.

Arsenic is concentrated in the outer layers of the rice hulls, which are removed from white rice but are left on in brown rice.

Most baby foods in 2008 contained rice, or rice milk, were thickened with rice starch or sweetened with brown rice syrup, which was considered a healthy alternative to corn syrup.

Two organic toddler formulas in the U.K. contained up to 60 ppb, six times the EPA safety limit for water.

At this point no one knows how much rice is safe to eat. There is much variation in rice from different regions, and rice used in cereal, energy bars and the like can come from just about anywhere in the world.

Richard Brill is a retired professor of science at Honolulu Community College. His column runs on the first and third Fridays of the month. Email questions and comments to brill@hawaii.edu.