Third World countries that tended to regard cleaning up the environment "as the rich man's problem" a decade ago, now are soliciting the World Bank's help, Bank President A. W. Clausen said last night.

In the annual Fairfield Osborn lecture here, Clausen said that the bank, like its Third World clients, places increasing emphasis on the environmental implications of the projects it finances.

He estimated at a press conference that the environmental costs of the typical World Bank project might account for between 3 percent and 5 percent of the total, which he described as "minimal" and less costly "than to ignore them and pay the penalties at some future time." He added that "not a single bank project" of any kind goes into effect until there is a review of its environmental impact, including measures built in to protect public safety.

Clausen noted that the cost of environmental safeguards is much less when starting a project in a developing country than when retrofitting existing facilities, an expense that has been criticized by some persons within the U.S. private-business sector.

World Bank aides estimated that in contrast to the 2 1/2 percent of gross national product that the United States devoted annually to environmental costs in the 1978-80 period, none in a group of 40 to 60 less-developed countries devoted more than 0.3 percent of GNP to the same effort.

The Osborn Lecture commemorates the work of Fairfield Osborn, a Wall Street investment broker and naturalist who publicized environmental problems throughout his life. He also organized the Conservation Foundation, sponsor of last night's lecture.

Clausen emphasized that Third World countries not only had become increasingly aware of the compatibility of economic growth and environmental regulations, but acknowledge that it's impossible to have one without the other.

Clausen cited World Environment Center data showing that 102 developing countries now have ministries or agencies dealing with environmental problems compared with only 11 in 1972. "It isn't that Third World countries are better able to afford environmental protection than they were 10 years ago," Clausen said. The reason for their interest comes from an understanding that there is a long-run payoff from it, Clausen said.

"It costs a lot less, for instance, to protect the forested watershed above a new dam than to deal with a silted reservoir later," he added. "Similarly, the benefits of an irrigation project can be diminished if, for lack of proper planning, it leads to an increase of schistosomiasis."

Bank officials concede that many Third World countries used to be hostile to spending money on environmental regulations. The bank's vice president for external relations, Munir Benjenk, said that attitudes changed as the public became aware of some of the effects of pollution.

For example, Benjenk said fish used to be "the poor man's food" 30 years ago in his home city of Istanbul. But now, because fishing with the help of explosives had polluted the waters, fish had become expensive.

Clausen said that some environmental problems, such as urban air pollution, are often worse in poor countries "that can't yet afford even minimal controls." Other problems, such as deforestation, are concentrated in the Third World, where the great dependence on firewood as a fuel has resulted in forests being cut down 10 times faster than trees are being planted.

He cited water supply and sanitation, energy, and population planning as three areas where "the goals of economic development and environmental enhancement coincide."