Published by City Farmer, Canada's Office of Urban Agriculture


Lead poison: How safe is sukumawiki (kale)?

Daily Nation on the Web
Friday, December 12, 2003

Concern over high concentrations of lead in kale (sukumawiki) sold in Nairobi has evoked mixed reactions from various quarters. While authorities at Kenyatta National Hospital (KNH) have no reports of lead poisoning in among the clients, other commentators have drummed up the crucial role of urban farming and the need for policy regulation to make it safe.

Commenting on a United Nations Environment Programme report that indicates that the Nairobi sukumawiki contains 5,000 micrograms of lead per kilo, which is way above the World Health Organisation recommended standard of 300, KNH chief nurse Helen Njoroge says: "We have not heard of any lead-related cases in the wards. But it is something that may be investigated."

Sukumawiki, literally "the week -pusher" in Kiswahili, is the name of the most popular green vegetable in Kenya's urban centres. The vegetable, which is daily fare for the rich and poor alike, got its nickname from its being cheap and readily available, making it a fallback vegetable for especially cash-strapped urban dwellers. Any health hazards linked to its consumption are therefore bound to affect the majority of urban dwellers.

A lot of it is grown along the Nairobi river, which is heavily contaminated with factory effluent and solid waste. Leaded fuel is the commonest source of environmental lead, which also finds its way into the food chain. Repeated consumption of contaminated vegetables could lead to a 4.25- point loss of the child's average intelligence quotient (IQ). Other effects include higher levels of lead in the blood.

According to a leading occupational health specialist, Dr William Sakari, lead per se is not poisonous. It is when it is exposed to the environment that fumes and dust from the smelting process combine with oxygen to form the poisonous lead oxide. Lead levels in the general population are rarely above 40 micrograms and rarely above 55 micrograms among industrial workers. "The highest level I know, and up to now I have not known why, is 140 micrograms. But surprisingly, he (the patient) never showed any signs or symptoms," says

Dr Sakari, raising one of the problems with lead poisoning. And perhaps it is this kind of situation that makes it difficult even for KNH to keep tabs on potential lead poisoning.

Excessive tiredness and loss of appetite are among the symptoms of lead poisoning. For men, high exposure to lead leads to reduced sexual libido, a condition Dr Sakari finds to be quite prevalent among lead-related factory workers. "Fortunately, it is temporary because once you take off for one week, the body gets rid of it and the (sexual) function becomes normal," he says. Signs of lead poisoning include anaemia. An exposed person might have a haemoglobin count of 12, compared to the normal 15. "The metal suppresses the formation of haemoglobin," Dr Sakari explains.

Lead poisoning rarely warrants hospital admission, the doctor says. Admission is further discouraged by the fact that whatever medication might be used to reduce the lead content in the body is in itself so hazardous that it is rarely the choice for medical practitioners. That is why taking off for one or two weeks to allow the body to get rid of the lead through urine is the preferred option.

Although the joint Unep and Jomo Kenyatta University of Agriculture and Technology study noted that there was no danger in eating thoroughly cleaned sukumawiki, therein lies the rub. Residents of parts of the capital city often go for weeks without water. The situation is worse for the low-income groups who, because they lack their own standpipes, often pay many times peer unit than their connected counterparts to meet daily needs.

In tackling the lead menace, policy orientation is geared towards making urban farming safe. At a recent meeting of the Consultative Group on International Agricultural Research (CGIAR), Nairobi deputy mayor Lawrence Ngacha said the city council was developing projects to address urban waste management, including the link to urban agriculture. Mr Ngacha specifically mentioned city collaboration with NGOs and partners such as the Westenet Coalition. "We are having problems managing our solid waste production. Since over 60 percent of this waste is organic and can be turned into compost, we need to investigate that production," he said.

He drew linkages to urban farming and household food security, which probably override the potential dangers of lead poisoning. Contributing to the lead debate, Dr Diana Lee-Smith, the regional coordinator for Africa of Urban Harvest, stresses the need for regulation of urban farming to enhance food security. Urban Harvest is a CGIAR programme.

Dr Lee-Smith, a pioneer in urban farming research, regrets that urban food policy has been ignored in national planning. "When people make agricultural policies, they generally leave out urban. It is a kind of blind

spot. But now people doing he food security policy analysis for Kenya want to include urban."

While the policies are not yet in place, Dr Lee-Smith stresses the need for them to be cross-sectoral in order to involve people from the Local Government and Planning ministries, because it is the cross-sectoral nature of urban farming policies that make them difficult to handle singlehandedly. So far, in the whole of sub-Saharan Africa, the only local authority dealing with food and agriculture in its administration is Kampala, according to Dr Lee-Smith. About a third of the people in Nairobi and Kampala do urban agriculture, calling for regulations and policies that tell them what to do about it to make it safe.

Dr Lee-Smith sees urban food policies as adding to a win-win situation. She says that if people address this aspect of policy, then there is potential for benefiting waste management strategies," which is one of the routes through which sukumawiki is contaminated. Safe handling of urban waste will make it possible to use organic wastes as a resource. Because such waste can be used for soil nutrition, and many people in developing countries recognised waste as a resource, then policy will help them recycle waste to compost and soil nutrient input that would be beneficial to them.

Leaden Gardens

Janet Raloff
Science News
Week of Dec. 6, 2003; Vol. 164, No. 23

Soils in many cities of the United States carry a poisonous legacy: heavy concentrations of lead. The metal was deposited for years as fallout from flaking leaded house paint and the emissions of cars burning leaded gasoline. Recognizing the threat posed by tainted soil, environmental scientists have warned that growing edible plants in soils near streets or within several feet of homes and other painted structures risks extracting lead from the soil and bringing it to the dinner table.

Doing so could have serious health implications, the experts warn, because even low concentrations of ingested lead have been associated in children with impaired brain development, balance problems, heightened risk of tooth decay, hearing loss, and shortened stature. In adults, such lead can boost the risk of high blood pressure.

The good news, engineers at Northwestern University now report, is that even in heavily lead-tainted dirt, most of the metal usually becomes trapped in the parts of plants that few people eat, such as roots and stems. Indeed, the new data indicate that intentionally using inedible plants to sop up lead could prove a low-cost strategy for detoxifying backyards and school grounds, making them safer for children.

Root is the problem

For their new 2-year pilot study, Kimberly A. Gray and her colleagues sampled edible plants that home gardeners had been growing in two densely populated neighborhoods in Chicago. One community was dominated by brownstone apartment buildings; the other, by painted frame homes.

Gray says that few measurements of plants' uptake of lead from soil had ever been reported before this study. Her team collected whole plants, so the researchers could compare lead deposition in roots, stems, leaves, and fruit. Sampled plants included apples, cantaloupe, corn, cucumbers, strawberries, tomatoes, zucchinis, cilantro, mint, peppers, rhubarb, carrots, radishes, and basil. The researchers also tested small quantities of the soil in which each plant was grown.

Since cooks wash garden produce before eating it, Gray's team measured the metal's concentrations only after a plant had been washed-either in a water rinse or a mild detergent solution. The idea was to remove any lead-tainted dirt that had clung to the plant.

Even after washing, many of the plants exhibited notable concentrations of lead-evidence that they had accumulated the toxic metal in their tissues. "From a scientific standpoint, that's interesting, because lead is molecularly sticky," Gray observes. "That is, it's not very mobile. So, to get it from the soil into a plant isn't easy." Yet, once it did enter a plant, her data indicate, it could be transported upward into the shoots, leaves, and, occasionally, the fruit.

Invariably, roots showed the highest lead concentrations, followed by successively lower accumulations in stems, leaves, and the typically edible portions. Root concentrations varied widely, even within a given species. For instance, her team reports that one sample of soil with a lead concentration of 1,600 parts per million (ppm) produced no measurable lead anywhere in a grape plant. In another case, the roots of grapes grown in soil with 944 ppm lead showed lead contamination of 480 micrograms per gram (?g/g)-though there was no measurable accumulation higher in the plant.

Most sampled plants accumulated at least some lead in their roots. The good news: In only a few species did the lead move higher into the plant. In almost every case, these were leafy crops, such as cilantro, collard greens, lemon balm, chard, and mint, or root crops such as carrot, onion, and radish. Overall, lead concentrations in the edible produce ranged from 12 to 60 ?g/g of tissue. Gray and her colleagues report the findings in an upcoming issue of Science of the Total Environment.

Invisible threat

The Environmental Protection Agency considers 400 ppm lead as the upper concentration that might be safe for dirt in which children play. The Northwestern engineers found that more than 75 percent of the garden soils they sampled had contamination exceeding this concentration. In many of the tested gardens, lead tainting ranged from 1,000 to 4,500 ppm.

"You'd have thought that lead levels that are so toxic to humans would stunt the growth of plants," Gray says. "But they don't." Indeed, in many of the most contaminated gardens, plant growth was lush. What this means, she says, is that there's no way a homeowner will know whether the heavy metal is present-short of sending the soil out for testing.

Moreover, the researchers had suspected that soil adjacent to painted, wood-exterior homes would have higher lead concentrations than soils around brick apartment buildings do. In fact, Gray's team found no difference. She now suspects that this is because even brick buildings have substantial painted trim and their yards are subject to lead fallout from fossil fuel combustion.

Children are put at greater risk by lead ingestion than adults are, the Northwestern researchers note. First, lead is neurotoxic and the neural development of children continues for many years after birth. Second, children absorb between 30 and 75 percent of the lead in what they eat, whereas adults typically absorb only about 11 percent.

Gray's team calculated that eating as little as 1 tablespoon (1.75 gram) of cilantro tainted with the lead concentration found in the new study-49 ?g/g-would result in an intake of 85.75 ?g of lead. Ingesting that amount daily, a child could build up a blood-lead concentration of roughly 10 ?g per deciliter of blood, which other studies have linked to several neurological deficits, including reduced IQ.

Therefore, the researchers argue, "it is imperative that people be equipped with the information and knowledge necessary to reduce or eliminate potential risks associated with urban gardening." Toward that end, the team recommends that city gardeners plant their crops in containers or raised beds, using new topsoil. The researchers also recommend that gardeners line the bottom of the new soil with a semipermeable barrier that permits water drainage but prevents root crops from penetrating to potentially tainted soil.

Any crops brought in from the garden should be washed carefully. For leafy produce, outer leaves should be sacrificed, and root vegetables peeled.

However, Gray maintains, the best move would be to clean lead from urban soils, perhaps by trapping it in ornamental grasses or other inedible plants with substantial roots. Then the tissue could be harvested and removed for hazardous-waste treatment as part of a municipal lead-remediation program.

Currently, Gray's group is evaluating garden species for the best candidates for such a lead-removal strategy.


Finster, M.E., K.A. Gray, and H.J. Binns. In press. Lead levels of edibles growing in contaminated residential soils: a field survey. Science of the Total Environment. Abstract available at

Federal information about lead hazards can be found at and at

Further Readings:

Bower, B. 1996. Excess lead linked to boys' delinquency. Science News 149(Feb. 10):86.

Fackelmann, K. 1996. Hypertension's lead connection. Science News 149(June 15):382. Available at

Harder, B. 2003. Traces of lead cause outsize harm. Science News 163(April 26):269. Available to subscribers at

Raloff. J. 2003. Lead delays puberty. Science News 163(June 28):408. Available to subscribers at

______. 2001. Tofu may get the lead out. Science News Online (June 30). Available at

______. 2001. Lead therapy won't help most kids. Science News 159(May 12):292. Available at

______. 2001. Even low lead in kids has a high IQ cost. Science News 159(May 5):277. Available to subscribers at

______. 2001. Leaden calcium supplements. Science News 159(March 31):205. Available to subscribers at

______. 1998. New lead record is no honor. Science News 154(Sept. 19):182. References and sources available at

______. 1997. Caries: Legacy of mom's lead exposure? Science News 152(Sept. 6):149. Available at

______. 1995. Another source of lead in kids. Science News 148(Nov. 25):365.

______. 1995. Lead may foster immune attack on brain. Science News 147(Jan. 14):23.

______. 1992. Why lead may leave kids short. Science News 142(Aug. 29):143.

______. 1986. Low lead levels can harm kids' hearing. Science News 130(Dec. 20&27):390.

Tong, S., et al. 1998. Declining blood lead levels and changes in cognitive function during childhood: The Port Pirie cohort study. Journal of the American Medical Association 280(Dec. 9):1915-1919. Abstract available at


Kimberly A. Gray
Department of Civil and Environmental Engineering
Northwestern University
2145 Sheridan Road
Evanston, IL 60208-3109

Search Our Site

pointer Return to Contents' Page pointer

Revised Tuesday, December 16, 2003

Published by City Farmer
Canada's Office of Urban Agriculture