Millions of discarded mobile phones and computers are posing a "major challenge" to human health, the chief of the United Nations Environment Programme said Thursday.

Achim Steiner told a UN conference on waste management on the Indonesian resort island of Bali that 20 million mobile phones were thrown away each year in China. Meanwhile, the global number of personal computers was expected to double to two billion by 2015.

"The rapid growth and rapid redundancy of all this equipment ... represents a major challenge to the international community in terms of human health and the environment," he said. He said 20 to 50 million tonnes of electronics waste was produced every year -- enough to load a train that would stretch around the world.

"The growth in electronics is unlikely to abate any time soon, especially as disposable incomes rise in the rapidly developing and developing economies." Much of this waste was ending up in landfills usually in developing countries in Africa and Asia, where it leaks chemicals and heavy metals into the environment, Steiner said. "This is effectively long-distance dumping," he said.