NEW YORK (AP) _ Respiratory diseases have become the leading cause of death for infants in their first year of life, the American Lung Association said Friday.

In its 1984-1985 annual report, the association said the disorders account for 38.9 percent of infant deaths. The figure is from 1982, the latest year for which statistics are available, said association spokeswoman Michelle Kling.

Part of the reason for the increase in percentage is that deaths from other causes have declined, Kling said.

One disease, respiratory distress syndrome, is the leading cause of death in the first 28 days after birth, claiming about 5,000 infants last year, the association said.

Also known as hyaline membrane disease, the syndrome strikes premature babies. It results from deficiency of a bodily chemical that normally coats tiny sacs within the lung where blood picks up oxygen. The sacs are supposed to expand and contract with the lung, retaining a little air between expansions. But without the chemical, called surfactant, the sacs collapse when the baby exhales, and the infant must struggle to open them up again.

Respiratory distress syndrome entered the news last year when it struck the Frustaci septuplets in Riverside, Calif. Three of them survive. The disease also killed John F. Kennedy's son, Patrick, in 1963.

Doctors can save most victims with mechanical ventilators, and they are experimenting with ways to supply surfactant.

Other respiratory diseases contributing to the infant deaths include influenza, pneumonia, sudden infant death syndrome, whooping cough, cystic fibrosis, a lower respiratory tract infection called bronchiolitis and a chronic condition called bronchopulmonary dysplasia, Kling said.

Sudden infant death syndrome, also called crib death, claims 7,000 to 10,000 American infants a year and accounts for one-third of infant deaths from lung-related diseases, the association said. Its cause is not completely understood.

The association also said that in the general population, death rates for chronic obstructive pulmonary diseases, chiefly emphysema and chronic bronchitis, rose 25 percent from 1970 to 1982, while the rate for lung cancer jumped 33 percent.

Smoking was the major reason for the increases, Kling said. The association blames cigarettes for 80 percent of deaths from the obstructive pulmonary diseases and 85 percent of deaths from lung cancer.

The 1982 mortality rate for emphysema was 3.6 deaths per 100,000 population, and for chronic bronchitis, 0.9 deaths per 100,000 population, Kling said. In chronic bronchitis, glands excrete too much mucus into the airways because of repeated expsure to inhaled irritants such as cigarette smoke.

The lung cancer death rate was 37.9 deaths per 100,000 population, Kling said. The disease killed about 126,000 people last year, the association said.