Many containers were corroding, adding spillage to the list of contamination problems.
Toxins that have seeped into soils and groundwater have contaminated food, drinking water, and the air.
Betlem has often found dead cats, birds, snakes, goats, and sheep inside and around buildings where corroded containers have started leaking. It is not unusual for children to play in the vicinity of the stockpiles.
The meat of animals grazing in such areas is sold in public markets, adding to the buildup of toxins in people.
Betlem says pesticides are normally obsolete for their intended use after two years, but remain hazardous.
More than a decade ago, the UN Food and Agricultural Organization began warning African nations and others about the dangers of negligent pesticide use.
But the Africa Stockpiles Program springs mainly from an initiative started in 2000 by the Worldwide Fund for Nature (WWF).
Together with the Pesticide Action Network, the environmental nonprofit lobbied governments, the United Nations, and pesticide manufacturers to start ASP.
Mwandla, who is with the WWF's Nairobi office, says ASP is a joint effort that involves the Global Environment Facility (GEF), governments, non-governmental organizations, and pesticide manufacturers through their international federation called CropLife.
ASP's major donors are the World Bank, on behalf of the GEF, and European governments. It has also been endorsed by the African Union's ministerial conference on the environment.
John Aston, representing CropLife at the media briefing in Nairobi, said there were no facilities in Africa capable of destroying the chemicals to internationally required standards.
Pesticides need to be incinerated at temperatures of at least 1,650°F (900°C) to limit harmful emissions. It was therefore necessary to export the chemicals to facilities in places such as Wales and Finland, where they can be safely destroyed.
He said the pesticide manufacturers were closely involved in the cleanup operation and in training people to properly handle and use the chemicals.
In Ethiopia pesticide manufacturers have already helped dispose of about 800 tons of obselete material, even though many of the mountainous country's approximately 900 stockpiles were held in practically inaccessible areas.
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