Medical radiation, the source of 90 percent of all manmade radiation, should be cut to half of present levels to protect the public from possible future ill effects, John C. Villforth, director of the government's Bureau of Radiological Health, said yesterday.

He made the recommendation in an interview after the release of reports urging "early and concerted" federal, state and private action to reduce medical-dental radiation as "by far" the most effective step possible to prevent future radiation-induced harm.

Health, Education and Welfare Secretary Joseph A. Califano Jr. told a news conference that elimination of all "unnecessary" medical and dental radiation is "critically important," and said he is ordering a speedup in such efforts by HEW.

A task force work group on public information urged patients to take part in their own radiation protection -- by questioning doctors about the benefits and risks of any radiation ordered, by asking for shielding of their sex organs, and by keeping a record of their X-ray exposure.

Both Califano and the Interagency Task Force on the Health Effects of Ionizing Radiation -- the body issuing yesterday's reports -- made it clear that medical-dental radiation literally dwarfs all the other ways, military and industrial, in which man creates rays that can both heal and harm.

Califano estimated that medicaldental X-raying, radiation therapy and nuclear medicine account for "nearly 90 percent" of all the manmade radiation to which the general population is exposed.

The task force's work group on radiation exposure reduction put the medical figure at 17 million, or 89.5 percent, of the 19 million "rems" (units of "radiation exposure to man") produced annually by all A-bombs, nuclear reactors, and the mining, manufacture, transport and use of nuclear materials.

Califano yesterday ordered HEW officials to work with state governments and medical groups to "expand and expedite" present efforts to:

"Set specific guidelines" for doctors' and other healers' uses of common X-ray procedures -- for example, guides to help doctors determine whether or not to order a skull X-ray in a specific case, or chest or stomach X-rays.

Create similar guidelines to improve now spotty training, testing and certification of the technicans and others who administer most medical and dental radiation.

The radiation exposure work group tentatively suggested an even broader set of measures. But it urged that the "feasibility and cost effectiveness" of each be evaluated first.

For example, said this group, there should be a review -- a step implying a possible tightening -- of existing medical exposure standards or permissible levels, in the same way that current job exposure standards are being reviewed.

The task force also urged exploration of ways to change health insurance reimbursement rules to encourage less use of radiation.

A major step in this direction was taken in early February by Blue Cross-Blue Shield, the nation's largest health insurer. It recommended that hospitals stop giving all medical (as contrasted with surgical) patients routine admission tests -- and that local "Blues" stop paying for such tests unless specifically ordered by the admitting doctor.

The recommendation was made mainly to cut medical costs and applies both to X-rays and laboratory testing.

The District of Columbia Medical Society board of trustees recently recommended that both surgical and medical admission testing be covered by such a rule to help cut both costs and unnecessary radiation.

Califano and the task force declared that medical-dental radiation can have huge benefits both in diagnosis and treatment, including treatment of many cancers. The need, said Villforth, is to use radiation only when the benefit exceeds the potential harm, and to use only as much radiation as is needed for each patient. The harm otherwise can include possible cancer -- usually appearing only after many years -- and damage to the genes, which may harm future generations.

He said his agency -- lead agency at HEW in radiation control -- believes "approximately 30 percent of all medical exposures are unnecessary," and often done for nonmedical reasons, like supposedly protecting doctors from malpractice suits.

And the radiation dosage used in "necessary" medical procedures could be cut by another 30 percent, he said, by all the newest X-ray films and machines.

Califano said that in the last five years HEW has "already" helped reduce by 60 percent the average dose paitents get from dental and breast X-rays (mammography) -- proof, he said, that such cuts are possible.