Six years after the insect killer DDT was globally outlawed on grounds of environmental damage, two researchers say there are new reasons for doubting the chemical is harmful and are urging its use against malaria.

In a book launched on Wednesday, Donald Roberts, professor of tropical medicine at the U.S. military's Uniformed Services University of Health Sciences, and Richard Tren, head of lobby group Africa Fighting Malaria, argue that DDT is the only effective weapon against the deadly mosquito-borne parasite.

Environmental group Greenpeace defended the United Nations' aim of eventually eliminating DDT use worldwide and said evidence that it harms wildlife and human health was sound, even if not conclusive.

DDT's unprecedented power to kill insects won its inventor a Nobel prize in the 1940s and it was considered a wonder chemical until evidence emerged of its toxicity to wildlife and people, leading Western nations to ban it in the 1970s.

A treaty to forbid its use worldwide along with a dozen other industrial chemicals came into effect in 2004, but some countries like South Africa and Ethiopia still take advantage of tightly limited exemptions allowing indoor spraying.

Dichloro-diphenyl-trichloromethylmethane (DDT) has been blamed for birth defects in humans and threatening endangered birds such as the bald eagle by thinning their egg shells.

"There are an almost endless list of claims that DDT causes one kind of harm or another but ... with each claim, the evidence that the DDT is the cause is simply not there," Roberts told Reuters in a telephone interview.

NETS, INSECTICIDES

"The Excellent Powder" claims new evidence shows DDT is harmless because it is similar to organic chemicals found in nature that animal life can deal with.

The book also tackles the issue of resistance to the poison, saying DDT is a good repellent, not just killer, of mosquitoes.

Malaria kills roughly a million children a year, mostly in Africa, according to the World Health Organization.

In the tropical West African nation of Ivory Coast, malaria kills 176 children under five each day, the government's top malaria official, Dr Sam Koffi Moise, told Reuters.

"The challenge is to give access to better prevention. We need mosquito nets but also insecticides like DDT," he said.

Roberts and Tren's book examines a 2009 study linking DDT in South Africa to birth defects and argues the data doesn't support it.

"Millions of malaria deaths ... occurred during ... decades of environmental activism (against) DDT," the book concludes.

Tren, a free market lobbyist who has also criticized tobacco control, said bird species harmed by DDT were already under threat and that DDT was "a minor source of harm compared to the hunting, shooting, poisoning and land use changes."

Greenpeace scientist David Santillo told Reuters greens approved use of DDT where there was no alternative, but evidence of it accumulating in birds and polar bears was clear, and evidence of harm to humans worrying enough to urge caution.

"If we're to wait until we have absolute confirmation that (health problems are) a direct result of DDT exposure that's something we'll probably never have because you can't expose humans deliberately to DDT to measure the effect," he said

"There's a need to develop a broader range of malaria controls to break this reliance on DDT ... as a silver bullet."