Montgomery County Executive Sidney Kramer, calling the radioactive gas radon a potential health threat to county residents, urged all property owners yesterday to test their buildings to see whether corrective action is necessary.

Such voluntary testing would be part of a county program Kramer is establishing to educate the public and collect information about the colorless, odorless gas and its prevalence in Montgomery.

Radon is produced in the decay of uranium, found in small quantities in soil. Health experts say long-term exposure is associated with a risk of lung cancer.

Kramer's recommendation came in a letter to Council President Rose Crenca, accompanying the report of a radon study task force.

David G. Sobers of the county's Department of Environmental Protection and a member of the task force, said tests should be made to protect health and to learn more about the problem in the county.

The task force, using records of 3,596 tests performed by private firms in county homes, found that "at least some part of up to one third of all dwelling units may contain radon levels above" federally recommended guidelines.

The group cautioned however that the tests were not made at random, or by a single entity.

The tests indicated, the task force said, that the highest concentration of radon is in northern sections from Barnesville through Clarksburg, Damascus and Laytonsville.

The lowest, the tests indicated, were in central and southern areas around south Rockville, Garrett Park, Kensington, Bethesda, Chevy Chase and Silver Spring.

The task force stressed action is necessary but not on an emergency basis.

It called for a "careful and systematic approach" in which government should inform . . . educate, and generally . . . assist citizens and businesses in evaluating and responding to the radon problem."

Stewart McKenzie, the council's environmental analyst who served on the task force, called radon Montgomery's "most serious natural environmental health hazard" and said he was "surprised and somewhat alarmed" at the problem's size.

Kramer said the county program would use current resources this year, but more money may be needed next year.