Legislation exists that should prohibit the simple sending of old PCs to be dumped - but the problem is that Nigeria's booming second-hand computer industry gives ample scope for computer waste to be brought in.

Most used machines are not tested for functionality before they are exported to Nigeria, and according to John Oboro, assistant general secretary of the dealers' association Capdan, there are far more bad computers coming in than good.

"The systems coming in are junk," he says.

"They are no good for us. The things are not serviceable; obsolete, they are of no economic value. Honestly speaking, 75% is junk."

Offloading responsibility

The import trade in used PCs is flourishing - one warehouse complex in Port Apapa in Lagos is handling up to 40 container loads each month.

But Kitan Ogungbuyi, a senior scientist at the Nigerian Environment Ministry, explains that it is easy to disguise the condition of what is coming in.

"We have more than 500 tonnes coming in daily, most of the time shipped as used electronics for which the documentation is not clear," she says.

"Sometimes it comes in with used cars put in the same container, so it is very difficult to track what's coming in. That's the big problem."

In customs documents, the computers are described as being "shipped for re-use."

But Jim Puckett of environmental NGO the Basel Action Network says that in practice, dubious exporters exploit the re-use category to increases their profits and offload their environmental responsibilities.

"Unscrupulous exporters from the North are intentionally mixing bad with good so that they are able to avoid disposal costs," he says.

"Usually when you bring a computer to a recycler, you pay a fee. But brokers will take this fee, and instead of recycling will mix in some good equipment and trade it... exporters say it is working equipment to help the poor to bridge the digital divide, but what we've observed is not bridging the digital divide but the creation of a digital dump."

In fact, it is several dumps, spread around the city. Lagos has no computer recycling facilities itself, and so the waste computers build up in huge piles.

On these dumps children scavenge for the contents - they can earn around US $2 a day by collecting components - but are also putting their health seriously at risk.

Studies by the Nigerian Ministry of Environment suggest that basic components such as lead are being recovered and then smelted in people's back yards, which poses a huge risk of lead poisoning.

And research at Nigeria's University of Ibadan warned of a "chemical time bomb scenario", with children highly susceptible to toxic substances which could lead to long term cancers affecting the lungs and all parts of the body.

Toxins in the water

Meanwhile, the tips sit on swamp land, which increases the environmental risk as chemicals seep into the high water table.

Old computers can contain mercury, and heavy metals like nickel, cadmium and chromium. Plastic casings use flame retardant chemicals and monitors contain lead.

Professor OladDele Osibjano of the University of Ibadan warns that overall, dumping e-waste is creating a toxic legacy.

"We've found excess heavy metals in the soil, as well as in plants and people who eat vegetables," he says.

"That has a lot of social health implications. You have grazing animals, people picking vegetables and eating them, and then the drinking water containing [these toxins]."

The international Basel Convention is meant to regulate and control the movement of hazardous waste from developed to developing countries - but it can be difficult to enforce.

For a start, the US - where many of the second hand computers come from - has not signed the convention.

European countries have - but a recent study revealed 48% of spot checks on exports of various waste shipments showed they were illegal.

Ultimately, while some international manufacturers now run their own take-back schemes and have pledged to produce "greener PCs" with fewer harmful ingredients, the UN has now called for an end to Western countries using Africa as a landfill for useless electronics.

And Professor Osibanjo says it is a call that needs an urgent response.

"E-waste is an emerging issue in Africa," he says.

"Developed countries should try to love their neighbour as themselves, and not give to their neighbour the things they don't want."