McDONALD'S HAS taken the constructive step of telling its suppliers to stop using CFCs in the foam containers in which it puts its fancier hamburgers. CFCs or chlorofluorocarbons are compounds much used around the world in air conditioning, insulation, foam products and aerosol sprays and for other purposes. They have the great virtue of being neither toxic nor flammable. Their offsetting defect is that they last. When finally released, as almost all eventually are, they rise into the atmosphere and mix with and deplete the ozone layer that shields us from ultraviolet radiation. Scientists say a thinner ozone layer may eventually mean more skin cancer, poorer crops and an increase in the greenhouse effect, in which all the world turns to August.

The Carter administration banned use of CFCs as propellants in aerosol cans. Now the Reagan administration has taken the lead in seeking limits on production worldwide. An international protocol to that effect may be signed in Canada next month. The administration has been helped to its position by the fact that the chemical industry thinks it can produce acceptable substitutes for CFCs. It won't be that painful to convert.

McDonald's has set an example. Only a small percentage of CFCs is used in the packaging of food, and McDonald's represents a small percentage even of this. But its familiar containers had become a favorite symbol of the problem; the publicity was bad. Conversely, as Sen. Robert Stafford wrote the company last spring, its influence was likely to be strong.

Now a different gas will be used to produce the bubbles in the foam from which the burger boxes are made. There continues to be pressure on the company to stop using foam entirely, because it is not biodegradable, and to wrap all the burgers in paper, not just the lesser ones as now. But that's tomorrow's issue. On this, McDonald's has been a good citize