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Health or hazard?

Soil and water - bare necessities for a farming life - bear insidious and hidden hazards for urban farmers in developing countries and the consumers of their produce. People need fresh fruit and vegetables for vitamins and minerals, but they do not need an added supplement of heavy metals or the constant challenge of coliform bacteria. Along roadsides, railways and drainage ditches, on waste dumps and unwanted scraps of land, vegetables absorb pollutants from soil, water and air. People have no choice but to grow their vegetables in polluted areas and it is hard to believe they will abandon unsafe farming practices in the face of more immediate hardship. Finding ways to control and confine the hazards has been the subject of much research, but one is reminded of the song which begins, "There's a hole in my bucket" and continues with an over-long process for repairing the bucket that inevitably leads to the need to fetch water. The challenges of producing fresh, safe food in some urban settings may indeed seem insurmountable.

Sorting leafy vegetablesSpinach is good for you?

A study* in Delhi of spinach beet, a popular but perishable leafy vegetable known locally as palak, shows worrying levels of heavy metal contamination. About 70% of the palak traded in Delhi's Azadpur market is grown within the city's boundaries. Tests were carried out between May 2001 and February this year at a number of sites where palak is grown and sold. Lead, which affects children's mental development, was found at levels in excess of the Indian permissible limit in 72% of samples. When applying the more stringent CODEX limit, 100% of samples exceeded safe limits. In some samples, high levels of cadmium (a carcinogen) and zinc were found.

Where do the heavy metals come from? It seems that small-scale industry, brick kilns, vehicle emissions, road dust, diesel generators and coal burning are important sources. Irrigation water contaminated by sewage and industrial effluent is another. The study also showed that much of the contamination is taken up by the roots into the plant tissue rather than simply deposited on the surface of the leaves. Although contamination by lead can be reduced by 50% with two separate washings in clean water, further washing has no effect, and washing does not reduce the level of zinc and cadmium.

With what shall I wet it?

In most developing countries, urban growth far exceeds the capacity of water supply and sewerage to meet citizens' needs. Kumasi, Ghana's second city, is no exception. Fewer than 4% of its residents have access to sewerage, 40% depend on public toilets and fewer than 10% have improved pit latrines. It is not surprising that the water used by growers to irrigate vegetables is contaminated by faecal coliform bacteria nor that high levels remain at point of sale to threaten the health of the consumer.

Tank for soaking poultry manure, Kumasi, GhanaResearch** by scientists in Kumasi has identified another source of bacterial infection. It seems that vegetable growers have recognised the value of manure from the many poultry farms that have sprung up around the city. In the past this chicken litter was simply dumped or burnt along roadsides but it makes good fertiliser, or would do so if coliform contamination could be controlled. Research shows that older manure that has been left in heaps to compost at high temperatures is much safer than the fresh litter that vegetable growers are now clamouring to use. Some even supply wood shavings to the poultry houses in return for the enriched litter which they then sprinkle over growing crops. As expected, farmgate samples of lettuce, cabbage and onions from poultry manure treated fields contain higher levels of coliform than those from untreated fields. Samples from the market show similar levels of contamination except where the market traders regularly wash produce in clean water with a view to extracting a higher price from more discerning customers.

Regulate and educate

So what can be done to make urban-farmed produce safer for the consumer? Proper composting to kill coliform bacteria is one answer. If heavy metals are likely to be present in soil or irrigation water, it is better to grow crops that do not absorb them into the parts that people eat. Cabbages and tomatoes are therefore preferable in such conditions than spinach or lettuce. Adding in plenty of organic material helps to fix heavy metals in the soil, reducing uptake by crops. The Delhi authorities have plans to take food safety into account when siting industrial development in future. Airborne particles can, however, travel far and emissions of pollutants should therefore be controlled at source. Waste - human and livestock - must be managed more safely. Monitoring, regulating and educating are equally essential. Vegetable growers need to be educated on safe practices and consumers must be educated to wash produce in clean water. But, with the song about the bucket still in mind, remember how few people in urban slums have access to ample supplies of clean water.

* Fiona Marshall, Imperial College London. Ravi Agarwal, ToxicsLink, New Delhi, and others. DFID Crop Post Harvest Research Programme R7530. The views expressed are not necessarily those of DFID.
** E. Mensah and R.C. Abaidoo, Kwame Nkrumah University, Kumasi. P. Amoah and P. Drechsel, IWMI.

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1st September 2003