MIDLAND, Mich. (AP) _ Dow Chemical Co. will halt use of ozone-damaging fully halogenated chlorofluorocarbons in its Styrofoam building insulation by next year, a spokesman said today.

Dow will substitute non-fully halogenated CFCs for the fully halogenated CFCs linked in studies with damage to the world's ozone layer, spokesman Steve Reiher said.

The ozone layer of the atmosphere, about 15 miles above the ground, helps protect the Earth from harmful rays of the sun.

The fully halogenated CFCs also are used in some Dow packaging and industrial foams, and substitutes also will be sought for the manufacture of those materials, Reiher said.

''We don't have a substitute that is as far along as we do in the Styrofoam, so we can't give a timetable,'' Reiher said. But he added that the company is giving top priority to finding alternatives.

Styrofoam is a brand name for Dow's blue sheet insulation for buildings, and though the name often is used for other products such as plastic foam cups or food packaging, Dow does not manufacture those, he said.

Dow purchases the CFCs used in its insulation and other products from various producers, Reiher said.

''So we are also depending on DuPont and Pennwalt and the other CFC producers to come up with and produce these alternatives,'' he said.