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It was the 2016 monsoon season when Jenna Forsyth, then a doctoral student at Stanford University, visited the Chowk Bazaar, a vibrant market in Dhaka, the Bangladesh capital. Forsyth and her collaborators ducked into a shop selling every pigment color under the sun. While a downpour lashed outside, fans inside gently blew clouds of colorful dyes around the room. “It’s an incredible sight to see,” reflects Forsyth, now a postdoc at Stanford.

Her trip was part of an international investigation to explain a puzzling and alarming finding: Why were 30 percent of pregnant women living in the Bangladesh countryside showing up with elevated concentrations of lead in their blood? Lead is a known neurotoxin that increases the risk of miscarriage in pregnant women and can pass from the mother’s blood to her fetus, causing health problems and developmental issues once the baby is born. There is no safe level of lead exposure. The result—part of a broader clinical study into the effect of hand sanitation and clean water on infant health—was surprising to Forsyth and her colleagues when they first learned about it in 2014. Forsyth’s PhD supervisor Stephen Luby was involved in this initial clinical study and asked her to investigate.

Bangladeshi merchants started dyeing their roots to make them more appealing to consumers. Unfortunately, the cheapest yellow dye on the market is lead chromate.

Lead poisoning is usually associated with urban phenomena: vehicle emissions, water pipes, and lead paint. But there aren’t as many vehicles out in the countryside, and families in that region usually don’t paint their homes.

Forsyth’s team started looking for sources of lead. They sampled a wide range of domestic items including metal food cans, spices, and soil. Several samples of turmeric came back with high quantities of lead.

Turmeric contamination with lead has turned up before. A 2014 study from a different group of researchers hypothesized that turmeric was a major source of lead exposure in Bangladeshi children, but the hypothesis was difficult to prove, and there were competing suggestions about where precisely the lead came from. Was lead leaching from contaminated soil where the plant is grown or from turmeric processing machinery? Were manufacturers adulterating the expensive spice to increase its weight?

Forsyth’s collaborators at the International Centre for Diarrhoeal Disease Research in Bangladesh (ICDDR,B) set off to pinpoint the source of lead, working their way through the supply chain. “We relied on our team members’ familial connections,” Forsyth explains. Collaborators with relatives from the major turmeric harvesting and refining regions asked them for local insights and began visiting markets and mills.

“It was really challenging at first,” says Mahbubur Rahman, a project coordinator in the Environmental Interventions Unit at the ICDDR,B. It took time for his team members to build a relationship with local workers who polish the turmeric root to remove the skin and make it look brighter. He says they were afraid of the investigators because of the potential for a regulatory response if they admitted to adulterating the spice. His colleagues assured them they were trying to support local businesses and keep the mill workers safe from lead exposure. The locals then started to open up about their business and manufacturing practices.

The introduction of lead to turmeric

Turmeric owes its bright yellow color to its active ingredient, curcumin. Harvested turmeric roots are dried out and that process brings the curcumin to surface of the root, deepening its yellow luster.

Bangladesh’s turmeric merchants first ran into trouble in the 1980s, when flooding damaged turmeric crops. The wet turmeric looked dull when placed alongside Indian wares, which were just beginning to sell in Bangladeshi markets and were an attractive bright yellow. The Bangladeshi merchants started dyeing their roots to make them more appealing to consumers.

Unfortunately, the cheapest yellow dye on the market is lead chromate. It is only authorized for industrial applications such as paints, but dye vendors turned a blind eye when spice merchants began buying it.

Forsyth and her team needed evidence that lead chromate dye was the source of lead in the rural women’s blood.

“Every lead mine on Earth has a unique fingerprint,” says Joseph Graziano, an environmental health sciences researcher at Columbia University, who did not participate in the work. Lead has four stable isotopes, and ratios between those four isotopes vary between sources.

So the researchers took lead isotopic ratios from turmeric, food containers, and soil. They calculated the closest “fingerprint” match to the lead in the women’s blood was indeed the lead in yellow turmeric dyes. The findings were published September 6 in Environmental Research and September 17 in Environmental Science and Technology.

Graziano describes the latter paper, for which he was a reviewer, as a “tour de force,” adding that “the issue of looking at stable isotopes of lead is not trivial.”

“They made this connection between lead in the spice and the blood in a compelling way,” says Maitreyi Mazumdar, a pediatric neurologist at Boston Children’s Hospital who wasn’t involved in the study.

Mazumdar, a coauthor on the 2014 paper, cautions against treating this study as the final, definitive word on lead poisoning in Bangladesh. “We know there’s many sources of lead exposure for the population in Bangladesh and I think this is a very important source of exposure, but there are many threats.”

It’s still too early to know if the findings will promote long-term change in the turmeric industry. Preventing contamination of turmeric with lead is not a trivial issue, in part because of consumer preference for bright yellow roots. Long-term solutions could include selectively breeding turmeric strains with higher curcumin content, says Forsyth, or developing a method to dry the turmeric roots more effectively, as this would create naturally brighter turmeric that doesn’t need coloring.

Although the US and other countries have quality control measures to ensure imported spices are not contaminated with lead, there is nevertheless some concern that products can be adulterated. Forsyth is starting a collaboration with the New York City Department of Health and Mental Hygiene to investigate lead poisoning within the South Asian community in the city.

In Bangladesh, the researchers gave food safety authorities advance notice of the findings so they could formulate a response. Rahman describes how, on the day their research findings were published online, the Bangladesh government issued a public notice in the print newspapers, warning they would take action against producers who continued to adulterate turmeric with lead chromate. The Prime Minister’s Office of Bangladesh took notice and added its support to stopping lead contamination, a development that encouraged the team. “It really needs the support of the government,” Rahman says, explaining that strong evidence is usually needed to prompt long-term government action and gives food safety authorities the strength to take action against powerful businesses.

J.E. Forsyth et al., “Sources of blood lead contamination in rural Bangladesh,” Environmental Science and Technology, 53:11429–36, 2019.

J.E. Forsyth et al., “Turmeric means ‘yellow’ in Bangladesh: Lead chromate pigments added to turmeric threaten public health across Bangladesh,” Environmental Research, 179:108722, 2019.