How safe is your sukuma wiki?

The vegetables you get from the nearest supermarket or your favourite Mama Mboga could be exposing you to heavy metals that are harmful to your health

In Summary

• Intensive testing of vegetable samples from city food markets and two major supermarkets found high levels of mercury and lead

• These could wind up in the greens from sewage water used to irrigate food and inorganic fertilisers

A photo of prepared sukuma wiki
A photo of prepared sukuma wiki
Image: FILE

Before you have your next meal, you may need to ask where the vegetables you have with it are from.

Chances are you got them from your nearest supermarket or from your favourite ‘Mama Mboga’, but do you know you could be exposing yourself to heavy metals that are harmful to your health?

An investigation by the Star has found that you are better off buying sukuma wiki (kales) from your local market than at a supermarket, following intensive testing of samples from various food markets in the city, as well as two major supermarkets.


The one kilogramme kale samples were collected from Gikomba, Marikiti, Korogocho market in Kariobangi, Kangemi and Githurai markets, as well as from Naivas supermarket in Westlands, and Nakumatt supermarket in Lavington.

They were subjected to lab analyses at the Kenya Plant Health Inspectorate Service (Kephis) and at a private laboratory, Analab Limited.

The tests were done in two different labs for verification purposes and also to determine a trend in the level of metals in sukuma wiki in Nairobi markets.

The tests conducted were to determine the presence (and levels) of lead, mercury, iron, copper and cadmium in kales from Nairobi main markets and supermarkets.

The lab analyses done at Kephis indicate that the kales bought from the two local supermarkets contained 0.13mg per kilo and 0.15mg per kilo of mercury respectively.

This is higher than the recommended World Health Organisation limits of 0.1mg per kilo. The samples from the markets had 0.007mg per kilo and a maximum of 0.1 mg per kilo, which were within WHO acceptable limits.

The level of mercury found in kales from Kariobangi market, however, was also higher at 0.11mg per kilo.


Samples from Kangemi, Gikomba, Githurai and Marikiti markets were within limits at 0.06, 0.07, 0.05 and 0.05mg per kilo respectively.



Tests carried out at Analab showed that the samples had mercury levels of 0.01mg per kilo, which is within the stipulated WHO limits. However, there were significant levels of lead in the kales from most markets in Nairobi.

Kales from Kangemi market in Westlands had a level of 0.20mg per kilo of kales, while Korogocho market in Kariobangi had 0.17mg per kilo. Githurai market had 0.08mg per kilo, and kales from Gikomba market had the highest levels of lead at 0.23mg per kilo.

However, Wakulima/Marikiti markets in the Central Business District showed significantly lower levels of lead in comparison to the rest, at 0.01mg per kilo.

While total mercury levels in animals and plants are usually very low, repeated exposure through consumption of contaminated foods can be dangerous, as methylmercury is toxic to the nervous system, kidney, liver and reproductive organs.

According to WHO's fact sheet posted on its website in March 2017, mercury is a major threat to public health because even exposure to smaller amounts poses a threat to human development, especially of children in early stages of life.

Mercury may have toxic effects on the nervous, digestive and immune systems, lungs, kidneys, skin and eyes, severely impacting these functions and compromising the health of anyone exposed to significant amounts of it over time.  

In August 2017, the Minamata Convention on Mercury came into force, committing its 74 Parties to reducing the risks to human health and the environment from the harmful release of mercury and mercury compounds.

Kenya became a party to the convention on October 10, 2013, and is now legally bound to ban new mercury mines, regulate the use of mercury in artisanal and small-scale gold mining, manufacturing processes, and the production of everyday items such as cosmetics, light bulbs, batteries and teeth fillings.

A court case to prosecute eight officials from the Kenya National Bureau of Statistics and two businessmen with attempted murder for allowing the importation of substandard fertiliser containing mercury is currently ongoing. This shows Kenya’s commitment to fighting the harmful use of mercury.

The 10, including former Kebs boss Charles Ongwae, were charged over the importation of 5.8 million kg bags of substandard fertiliser allegedly laced with mercury. An order to retest the fertiliser, which was issued in January 2019, is, however, yet to be honoured to date.

The Star also tested the kales from both the open-air markets and the two supermarkets for cadmium and lead, two heavy metals known to have serious effects on human health.

On lead, a joint report on Food Standards by the UN-Food and Agriculture Organisation and the WHO shows that the mean dietary exposure estimates for children aged about 1–4 years ranged from 0.03 to 9µg per kilo of body weight per day. For adults, it ranged from 0.02 to 3µg per kilo of body weight per day.

Exposure to lead can occur from many sources but usually arises from industrial use. Lead and its compounds can enter the environment during mining, smelting, processing, use, recycling or disposal.

The main uses of lead are in batteries, cables, pigments, plumbing, gasoline, solder and steel products, food packaging, glassware, ceramic products and pesticides.

WHO states that the general non-smoking adult population is mainly exposed to lead through food and water. “Food, air, water and dust or soil are the main potential sources of exposure for infants and young children,” states WHO's International Programme on Chemical Safety.

The main challenge in Nairobi and its environs is that car wash water and industry effluent flow to streams that feed into the rivers used for irrigation. All this waste is absorbed by the plants, which end up on our tables
Kalro director general Dr Eliud Kireger


Agriculture CAS Andrew Tuimur said it is unfortunate that there are laws in Kenya on food safety standards but they are not enforced.

Tuimur warned Kenyans against growing kales at the roadsides to avoid emission of gases that may lead to lead contamination.

“Traders should ensure they transport kales in closed vehicles. Consumers should avoid buying vegetables that are sold by the roadside. Markets that are beside the roads should be enclosed or pushed far from the roads,” he said.  

Weighing on the matter is, Kenyatta University senior lecturer Dr Richard Oduor said lead poisoning is hard to detect, and even people who seem healthy can have high blood levels of lead.

“Signs and symptoms usually do not appear until dangerous amounts have accumulated. The poisoning can cause severe mental and physical impairment,” said Dr Oduor from the Biochemistry and Biotechnology department.

Cadmium is a relatively rare element, mainly used in rechargeable batteries and industrial paints. If not disposed of properly, wastes such as food, soil and grass containing cadmium can release the element into the air, land and water, with harmful consequences.

Plants growing in soil with increased cadmium levels would absorb the heavy metal, and if these plants then enter the human food chain through agricultural crops, they can adversely impact the health of anyone who consumes them.

Cadmium is classified as a human carcinogen. Its accumulation in the kidneys could lead to renal dysfunction. The WHO recommends a maximum limit of 0.2mg per kilo for cadmium.

The test results showed the samples did not have detectable levels of cadmium.

Contamination is likely to happen if the food is grown in places where the soil or irrigation water is contaminated with discharge from factories.
Agriculture ministry's Plant Protection Service head Dr David Mwangi


How do these heavy metals end up in food? Agriculture ministry's Plant Protection Service head Dr David Mwangi said they mainly come from soil and water.

“This is likely to happen if the food is grown in places where the soil or irrigation water is contaminated with discharge from factories,” he said.

Inorganic fertilisers, especially those containing phosphates, have been found to harbour small amounts of heavy metal contaminants, and continued exposure to these fertilisers could lead to the absorption of heavy metals in food crops, which have dire health implications for the human body.

According to a lab technician from Kephis not authorised to speak to the press, the contamination could have occurred from the soil, manure used or during handling of the kales from the farm to the market.

“I recommend frequent testing to ensure the sukuma wiki is safe at all times,” the lab technician said.

There has been a shortage of kales and other vegetables due to prolonged drought around the country. This has led to an increase in vegetables grown under irrigation, which is where a number of supermarkets get their produce.

Irrigation is largely unregulated in Kenya. It could be a potential source of contamination from heavy metals, especially in situations where sewage and wastewater are used.

Anne Waihiga, a greengrocer at Githurai market, said she and other traders in the market get their supplies mainly from Murang'a. Joseph Ngunjiri, a trader in Wakulima market, said they receive supplies from Nyeri, Kiambu and Nyahururu in Central Kenya.

Kenya Agricultural and Livestock Research Organisation director general Dr Eliud Kireger said there are many vegetable farms around Kiambu. They are situated along the rivers and streams in areas bordering Kiambu all the way towards Thika. In the city, urban farmers such as those in Utawala, also supply vegetables to the markets.

“But what happens is that some of these farmers sometimes deliberately puncture the sewer lines. They use the sewage water as fertiliser and yet it has microbes that cause disease,” Dr Kireger said.

“It also has heavy metals because the water from the sewage is combined with chemicals from industrial parks. There are chemicals washed into the drainage system that have paint and affluent from car wash, which has lead. All these are absorbed into the vegetables and we end up eating them.”

He said it is criminal for people to use sewer water for irrigation. “We need to have a traceability system where, if one buys kales or vegetables from a supermarket, they should be able to tell where the product came from,” he said.

"The government should also create a system where the Mama Mboga can trace where she gets her vegetables from and ensure the farm does not use excessive chemicals."

Dr Kireger also pointed out that there are also issues of farmers using agrochemicals known as acaricides (pesticides used to kill mites, ticks and pests).

He said these chemicals are easily absorbed by the vegetables, thus harmful for consumption.

“The main challenge in Nairobi and its environs is that sewerage, consisting of dirty water from car wash and effluent from industries, ends up in the streams that feed into the rivers used for irrigation. All this waste is absorbed by the plants, which end up on our tables,” Dr Kireger said.

From the tests conducted by this newspaper, it appears the leafy kales consumed in Nairobi contain harmful metals that could potentially negate their health benefits.

The samples were bought from the three of the biggest food markets in the city, which supply hundreds of Mama Mbogas, greengrocers and even some of the supermarkets, hotels and restaurants.

These markets are popular for their proximity to major transport hubs and relative affordability. As a result, they have become a preferred source of vegetables for people living in Nairobi and neighbouring towns.

The samples from the markets were bought early in the morning when the markets open for business. The samples were carried with clean, plastic woven bags and taken to the laboratories for testing. The test results were received after 10 working days.


A 2015 consumers and retailers’ survey indicated that sukuma wiki is the third-most popular vegetable after tomatoes and onions. Other popular vegetables reportedly were tomatoes, onions, carrots, cabbage, spinach and potatoes.

“The most consumed vegetable by high-end and middle-class consumers in Kenya’s three main cities Nairobi, Nakuru and Mombasa is the tomato, as is evidenced by 82 per cent of the survey respondents. The second is the onions at 69 per cent, followed by sukuma wiki and spinach at 42 per cent,” read the report.

The report also records supermarkets as the most popular channel for the purchase of both food (including vegetables) and non-food items among the high-end and middle-income earners.

According to the report, Kenyans prefer shopping from supermarkets because of “quality products” (84 per cent) and convenience (76 per cent), while 65 per cent of the sampled households prefer petty traders and street vendors because of low prices.

While over 90 per cent of the respondents go to supermarkets to purchase food and non-food items (95 per cent for food and 96 per cent non-food), for vegetables, only 45 per cent go to supermarkets, the report said.

“Other purchase channels to source for vegetables are kiosks (38 per cent), open market (36 per cent) and dukas (30 per cent). Respondents also buy vegetables from street vendors (22 per cent) and petty traders (13 per cent).”

This story was produced in partnership with Code for Africa’s iLAB data journalism programme, with support from Deutsche Welle Akademie.

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