The Air Force is preparing a defensive line against attacks by civilian workers claiming they have suffered abnormally high incidences of cancer and psychiatric disorders due to toxic chemicals, and military indifference to their problems.

The battle, so far, centers on Hill Air Force Base, one of the biggest employers in Utah. But it could affect the health and working conditions of hundreds of thousands of federal workers in cleanup and maintenance jobs.

At Hill, an aggressive union team has compiled masses of documents that union leaders believe proves that Air Force has ignored dangerous working conditions and endangered the health of thousands of present and former workers. The national office of the American Federation of Government Employees has taken up the fight first begun by local leaders-and also Air Force employes-in Utah.

Union officials have photocopies of internal Air Force communications indicating high-level officials knew of toxic chemical exposure problems several years ago. The union contends tht Air Force officials denied knowledge of some problems, or downplayed them in answering queries from members of Congress in an attempt to cover up the problems.

AFGE officials have gotten medical authorities from the University of Utah to state that toxic chemical exposure at Hill is a major problem, and they are pushing for federal health and inspection agencies to come on the base to see working conditions, and collect medical data.

The issue has come to a head in recent days since the House Post Office-Civil Service Committee has begun hearings on the matter. Air Force officials are due to testify next week before the committee and its investigations subcommittee, both headed by Chairman James M. Hanley (D-N.Y.).

Committee members say that on the record, they are withholding judgment until Air Force brass testify. But privately some say that data passed on to them by the union-much of it official, internal Air Force correspandence-indicates either incompetence, indifference or what looks like a high-level cover-up of medical problems.

The committee has also been spurred on by the recent nightmare of the hydrogen gas problem at the Three Mile Island Nuclear Power Plant near Middletown, Pa. Although totally unrelated, it has made both the public and politicians aware that official assurances in past that all is well can, to be charitable, sometimes be off a bit.