Officials said they hoped their endorsement of the controversial insecticide would influence countries in southern and eastern Africa to allow its application in moderation. They also anticipated that environmental groups would be alarmed by the announcement.
"Help save African babies as you are helping to save the environment," pleaded Dr. Arata Kochi, director of the WHO's malaria department. "Help us advocate for careful limited use of indoor spraying."
DDT is history's most notorious insecticide. It was banned in the U.S. in 1972 after decades of widespread agricultural spraying led to environmental damage around the globe.
But health officials said Friday that there is a distinct difference when it comes to using DDT for agricultural purposes and using it to coat once or twice a year the inside walls of mud huts or other dwellings.
"The dosage is completely different than when the U.S. used DDT," Kochi said.
A small number of malaria-plagued countries already use DDT, backed by a 2001 United Nations treaty that set out strict rules to prevent environmental contamination. But the announcement from the WHO makes clear that it will push indoor spraying with a number of insecticides — and that DDT will be a top choice.
"We must take a position based on the science and the data," Kochi said. "One of the best tools we have against malaria is indoor residual house spraying."
"It's a big change," said biologist Amir Attaran of Canada's University of Ottawa, who has long pushed for the guidelines and described a recent draft. "There has been a lot of resistance to using insecticides to control malaria, and one insecticide especially. ... That will have to be re-evaluated by a lot of people."
The U.S. government already has decided to pay for DDT and other indoor insecticide use as part of President Bush's $1.2 billion, five-year initiative to control malaria in Africa. The disease kills more than 1 million people a year.
"Spraying is one of the tools that must be deployed as robustly and as strategically as possible in our fight against malaria," said Timothy Ziemer, coordinator of the U.S. effort.
Ziemer said the WHO's guidance is critical because the insecticide has not been used in much of tropical Africa for a long time, and in some cases, never. The health organization will need to help those nations use DDT properly.
Kochi portrayed indoor spraying as an important but neglected third weapon — along with insecticide-treated bed nets and new medications — in the war on malaria.
While some well-known environmental groups have signed on to WHO's decision, it has generated some concern from other groups including the Pesticide Action Network, which says there are questions about effects on developing children.
Beyond Pesticides, an advocacy group based in Washington, rejected the argument that low dosages inside homes would make the applications safe. It said even minuscule amounts can wreak havoc on biological systems.
WHO officials said science was on their side.
"There is no credible evidence that DDT is harmful to human health," said Richard Tren, director of Africa Fighting Malaria, who attended the WHO's press conference and was called on by WHO officials to answer technical questions.
DDT never disappeared in developing countries, although political pressure and lack of funding meant few continued to use it. It's primarily manufactured by state-owned companies in China and India.
In 2001, a United Nations treaty that aims to wipe out a dozen of the world's most dangerous chemicals carved out one exception for DDT: indoor anti-malaria spraying, under strict conditions to prevent environmental contamination.
When small amounts are sprayed on interior walls, DDT forms a residue that both repels mosquitoes — discouraging them from flying into the house — and kills those that rest on the walls.
Bednets soaked in different insecticides already are used to protect sleeping families. But if the nets are torn or aren't used every night, a mosquito can infect someone. Plus, mosquitoes can develop resistance to chemicals.
"It would be naive to say DDT is a magic bullet for malaria. It isn't," said Attaran, the Canadian biologist.
It won't work in some places where mosquitoes already are resistant to a range of insecticides, he noted.