The Third World, challenging industrial nations on yet another environmental issue, is slowing efforts to eliminate chemicals that are eating away the Earth's crucial ozone layer.

Prodded by increasing scientific evidence that chlorofluorocarbons are destroying the ozone shield protecting Earth from cancer-causing ultraviolet rays, the West has promised to ban or significantly reduce their use by the end of the century.Third World countries do not dispute the facts and figures, but they are just embarking on large-scale expansion of refrigeration, air conditioning, plastics and electronics industries, most of which depend on chlorofluorocarbons.

Australia's science minister, speaking an an international conference in London this week, wondered aloud if it is fair to ask for equal measures in both kinds of countries.

"This raises a major moral problem in the West," said the minister, Barry Jones.

The dispute recalls the 1960s debate over the insecticide DDT. The West discovered it was harmful to both plants and animals and imposed bans or restrictions in 1970-71 against the wishes of developing nations, which feared a resurgence of disease-carrying insects.

Another environmental issue dividing the West from the Third World is the dumping of toxic waste.

President Daniel arap Moi of Kenya told the London conference that recent dumpings in Africa of toxic chemicals from industrial countries were "unfriendly actions . . . equivalent to declaring war on the Earth's ecosystem."

On the ozone issue, developing countries demand Western help in financing any switch from chlorofluorocarbons to safe substitutes.

The chemicals, developed in the United States more than 50 years ago as the working fluid for refrigerators, also are used in aerosols, fast-food packaging and computer solvents. About 1.2 million tons are produced annually, more than 76 percent in the United States and European Economic Community.

When chlorofluorocarbons reach the stratosphere 15-25 miles up, as much as 100 years after emission, they break apart and their chlorine atoms deplete the ozone. Chlorofluorocarbons also trap heat, increasing Earth's temperature in a process known as the greenhouse effect.

Washington now says it will ban all chlorofluorocarbons by 1999 if safe substitutes become available. The 12-nation EEC has agreed to a similar timetable, but its environment commissioner, Carlo Ripa di Meana, wants an earlier target.

Thirty-one countries have ratified the 1987 Montreal Protocol requiring the amount of chlorofluorocarbons to be cut in half by 1999.

Another 20 countries have agreed to sign up, but China, India and Brazil, with rapidly growing populations, are not among them. They have urged Western nations to create a fund to help them acquire technology to replace chlorofluorocarbons, but insist the money not be deducted from existing aid.

"Any reduction in these resources for whatever reasons would mean that the poor of these countries will have to wait longer for the promised freedom from hunger and poverty," said Z.R. Ansari, India's environment minister. "The poor are no more prepared to wait, and there will be social upheaval if they are asked to wait any longer."

China's environment commissioner, Liu Ming Pu, told the London conference his nation of 1.1 billion people uses less than 2 percent of the world's chlorofluorocarbons.

Western nations say the potential for chlorofluorocarbons use in the developing world is enormous and could wipe out any cuts made by industrial countries.