About 2 million people under the age of 65 are totally or partially disabled by job-related diseases but only 5 percent of the severely ill get workmen's compensation payments, according to a new Labor Department study.

The diseases include skin ulcers, circulation problems, heart conditions, permanent back and spine ailments and a variety of lung and blood conditions resulting from exposure to asbestos, cotton dust, fine sands in construction work, chemicals, cadmium, metallic fragments, chromium, arsenic, pesticides and the like.

Tom Brown, a Labor Department official, said in an interview yesterday that the study doesn't cover persons who have been disabled by accidents and injuries on the job, only those disabled by severe and chronic diseases resulting from their work. Figures are for 1978.

The study said about one-third of the 2 million suffer from long-term total disability but only 5 percent of these 700,000 get workmen's compensation, because it is extremely difficult to prove that a long-term disease like emphysema or lung-cancer results directly from work conditions.

In some cases, a job-related disease may have an onset time of 15 to 20 years, leaving a workmen's compensation claim open to challenge that the condition resulted from nonwork causes or from employment at some place other than the firm against which the claim is filed.

Such challenges and the existing structure of state workmen's compensation administration make it much harder for a worker disabled by job-related disease to get compensation than for a worker disabled in an accident, where evidence that the disability came on the job is much easier to produce.

The report said it usually takes a year before a worker with an occupational disease gets his first benefit, but only two months for a worker disabled in an accident. Three-fifths of disease claims are initially denied, but only 10 percent of injury claims, and momre than half disease claims are settled by a small lump-sum payment.

Although occupational disease victims have difficulty getting workmen's compensation, the report said many are able to get on Social Security Disability Insurance rolls; for these benefits, the source of the disease is irrelevant as long as the condition is severe enough.

Of the 700,000 persons with severe long-term job-related diseases, about one-quarter get no support from public or private programs. Overall, based on a 1974 survey, about a half get Social Security disability, 16 percent public welfare, 5 percent workmen's compensation and 1 percent private insurance benefits. In some cases, the same worker gets support from several of these sources.

Social Security disability and public welfare outlays for those disabled total about $2.2 billion a year excluding special payments to black lung victims under a separate government program.

To broaden income protection for workers suffering job-related diseases, the report said the government should consider making it easier to claim job-relatedness of a condition under workmen's compensation laws; streamlining their administration of claims; easing "recency of work" or other tests for Social Security disability benefits, and setting up special programs or compensation for specific diseases, similar to black lung benefits. p