KNOXVILLE, Tenn. (AP) _ Federal safety officials conducting a special hearing this week will be looking for ways to prevent fatal chain-reaction accidents on fog-shrouded highways.

Central to the hearing are four crashes in Utah, California and Tennessee.

''We've got four major accidents that have occurred in the past five months that have involved 238 vehicles and 21 deaths and 90 injuries,'' said James L. Kolstad, chairman of the National Transportation Safety Board.

''It seemed to us that this would be a good opportunity to address ... what can be done by both the states and the federal government to deal with this problem.''

The NTSB will hold a hearing Wednesday and Thursday in Knoxville to hear from U.S. and European experts on fog conditions, fog-detection equipment, driver reaction and governmental policy.

The hearings are tied to investigations into fatal, fog-related accidents - Jan. 2 near North Salt Lake, Utah; Jan. 7 in Buttonwillow, Calif.; Feb. 7 near Fresno, Calif.; and Dec. 11 in Calhoun, Tenn.

NTSB spokesman Mike Benson said conclusions drawn from the hearings may be applied to foggy highways throughout the country.

Although the four accidents occurred in different parts of the country, there were ''some common threads,'' Kolstad said in a telephone interview Friday from Washington.

All were on limited-access highways and involved chain-reaction collisions when motorists slowed after entering the fog. Perhaps most significantly, all were in known, fog-prone areas with water nearby.

In the case of the Calhoun accident, there were six previous multivehicle collisions due to fog in the same location since 1974. The rural stretch of Interstate 75 is marked only by a warning sign equipped with flashing lights that must be manually activiated.

''The question is - have we done enough in alerting motorists to the existence of fog so that preventive measures can be taken in advance of a chain-reaction kind of collision?'' Kolstad said.

The Calhoun accident in December involved 99 vehicles, 12 deaths and more than 50 injuries. It happened 66 miles south of Knoxville.

In the other accidents: Three people were killed and 19 injured in the 50- vehicle crash near North Salt Lake; two died and two were injured in an 18- vehicle wreck in Buttonwillow; and four people were killed and 31 injured in a 75-vehicle collision near Fresno.

''Our mission is to find out what happened in these accidents and try and make the recommendations, based upon the information that we gather, to prevent others,'' said Kolstad, who will lead the hearing.