More poor countries may soon be pumping up their arsenal against malaria with DDT, the pesticide long banned in the United States.

The difference: DDT is no longer sprayed outdoors, but to coat the inside walls of mud huts or other dwellings and kill mosquitoes waiting to bite families as they sleep.

New World Health Organization guidelines are poised to expand the small number of malaria-plagued countries already using DDT. The push is for more indoor spraying with a number of insecticides, including DDT, as a safe, effective and cheap option, say officials familiar with the announcement scheduled for Friday in Washington.

“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 WHO will say that “indoor residual spraying, including with DDT, has been underutilized, which has hampered international efforts to effectively combat malaria in Africa,” said a Bush administration official, speaking on condition of anonymity.

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.

WHO officials would not discuss the guidelines on Thursday.

A draft report from the office of WHO malaria chief Dr. Arata Kochi describes indoor spraying as an important but neglected third weapon — along with insecticide-treated bed nets and new medications — in the war on malaria.

WHO “will in the coming year greatly intensify its work to assist countries to make the best use of” indoor spraying, the document says.

Proponents say Friday’s WHO announcement is important in ending mixed signals.

“Whereas before they were kind of discouraging and stigmatizing the use of DDT, now they’re promoting it,” said Aaron Cooper, a spokesman for Sen. Tom Coburn, R-Okla., a physician who has urged stronger anti-malaria programs.

DDT is easily history’s most notorious insecticide. While it isn’t classified a human health hazard, it was banned in the U.S. in 1972 after decades of widespread agricultural spraying led to environmental damage around the globe.

DDT never disappeared in developing countries, although political pressure and lack of funding meant few continued to use it. Then a 2001 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.

Why? 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, explained Clive Shiff, a professor at Johns Hopkins University’s Malaria Research Institute. It has to be applied only about once a year.

Bednets soaked in different insecticides already are used to protect sleeping families. Iif the nets are torn or aren’t used every night, a mosquito can infect someone. Plus, mosquitoes can develop resistance to those nets’ chemicals, Shiff added, pointing to a 2002 malaria outbreak in part of South Africa using bednets. DDT in those houses quelled the outbreak.

“It would be naive to say DDT is a magic bullet for malaria. It isn’t,” stressed Attaran. It won’t work in some places where mosquitoes already are resistant to a range of insecticides, he noted. He suspects DDT will be of most use in eastern Africa, where that problem hasn’t yet emerged.

Attaran called for research “to make sure we’re using insecticides and DDT not in a willy-nilly way but in an optimal way in the right places.”

Nor, scientists cautioned, is indoor spraying alone a solution, as mosquitoes bite everywhere. Countries are being encouraged to adopt comprehensive malaria programs that also include newer, more effective medications, as Bush’s malaria chief, Admiral R. Timothy Ziemer, was to outline Friday.

“President Bush has directed Admiral Ziemer to use the most safe and effective tools available to control and combat malaria in Africa,” said White House spokeswoman Emily Lawrimore. Indoor spraying “programs are an important part of his Presidential Malaria Initiative to save thousands of people from a highly treatable and preventable disease.”