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Roadside cultivation - a heavy metal health hazard?

Kampala is home to nearly three quarters of Uganda's vehicles (Grace Nabulo, Makerere University, Kampala)
Kampala is home to nearly three quarters of Uganda's vehicles
Grace Nabulo, Makerere University, Kampala

Nearly three-quarters of Ugandan vehicles are found in Kampala. Most run on leaded petrol, and each morning and evening the city's main arteries bear a steady stream of traffic. In such an environment, when Kampala City Council took the decision, in 2003, to legalise urban agriculture, they faced a hostile media reaction. How could crops grown in such polluted air be safe to eat?

Research in other cities has found that urban soils often contain high concentrations of heavy metals, including lead, cadmium and zinc. Lead is particularly dangerous, since even small concentrations in the blood can seriously affect children's mental development, with two to three year olds at greatest risk. A variety of sources can contribute to lead contamination of soils, including flaking paints, industrial emissions and incinerators. However, fuel exhausts are the most serious source; a direct link has been proven between distance from roads and heavy metal concentrations in soil.

Collecting the evidence

With the Kampala City Council under strong pressure to reverse its decision, a study* conducted in Kampala revealed that media concerns were not misplaced. Soil, plants and flat surfaces at 11 roadside sites around the city were tested. In three sites - those with heaviest traffic flows - heavy metal concentration in the soil was found to exceed safe limits for agriculture. In addition, leafy vegetables grown near the road at all sites exhibited dangerously high concentrations of lead.

Understanding the routes of plant contamination has been a key focus of the research. Tests across a range of plants revealed heavy metal concentrations to be highest in the plant leaves, followed by roots, fruit, and finally the seed. The research team established that in leafy vegetables, such as Amaranthus, contamination was primarily via atmospheric deposition, rather than uptake through plant roots. Some of the contamination was in the form of a surface film which could be washed off, but most of the contaminants were found in the leaf tissue.

Rules of the roadside

Map showing the 11 roadside sites around Kampala where soil, plants and flat surfaces were tested for contamination (Grace Nabulo, Makerere University, Kampala)
Map showing the 11 roadside sites around Kampala where soil, plants and flat surfaces were tested for contamination
Grace Nabulo, Makerere University, Kampala

The research findings have helped to develop new ordinances for urban agriculture in Kampala, a participatory, multi-stakeholder process initiated by the City Council and organised by Urban Harvest.

Crop choice and site selection have been key issues for discussion. Maize, for example, is recommended as safe for roadside cultivation. The grain is protected from exhaust fumes by the outer leaves, and only insignificant amounts of heavy metals reach the grain via the plant roots. Legume crops - such as peas and beans - are also recommended, although the outer pods should not be eaten. Similarly, tubers such as cassava and sweet potato should be peeled prior to cooking if they are grown on roadside land. Washing is recommended for all vegetables that might be contaminated by atmospheric deposition.

The new ordinances, which include a provision that crops should not be grown along busy highways, have now been formally adopted by the City Council. The research team has made a number of other recommendations to further reduce risks. These include a zoning exercise to delineate areas suitable for cultivation, roadside planting of hedges and trees to minimise the spread of airborne pollution, and that leafy vegetables should not be grown within 30 metres of a road. However, according to Grace Nabulo, a member of the research team who has been heavily involved in creating the new guidelines, such steps will require considerable planning, research and funding to become a reality.

The road ahead

Despite the intimate involvement of urban farmers in reviewing the ordinances for urban agriculture, there has been little change in their farming practice. To address this, an NGO has been established by Nabulo and her colleagues to develop and disseminate information messages on safe urban farming. The Kampala Urban Food Security, Agriculture and Livestock Coordinating Committee (KUFSALCC), is an alliance of public, research and civil society organisations, which includes staff of the Kampala City Council. The committee is working closely with the council's extension staff, and Nabulo is optimistic that they can succeed.

A roadside garden in Kampala (Donald Cole, University of Toronto)
A roadside garden in Kampala
Donald Cole, University of Toronto

The Kampala findings also have wider relevance, particularly for developing countries experiencing rapid industrialisation. Nabulo has been presenting the results of the Kampala study internationally, and has some key messages for urban authorities. "Heavy metals should be acknowledged as a potential health risk in urban agriculture and research on chemical contamination is vital in guiding policy and planning." But she stresses, "Ultimately authorities must find the right balance between benefits and health risks presented by urban farming, if they are to achieve productive and sustainable cities."

*Assessment of lead, cadmium and zinc contamination of roadside soils, surface films and vegetables in Kampala city, Uganda by Nabulo, Oryem-Origa and Diamond, funded by IDRC.

Date published: July 2007


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