For unknown reasons, death rates from asthma among American children and young adults have been increasing steeply since the late 1970s, according to a study published today.

The rise in death rates is greatest among children between the ages of 5 and 14 and is affecting both whites and non-whites, according to the study, which is reported in today's issue of the Journal of the American Medical Association.

People with asthma suffer attacks during which the bronchi, the tubes through which air reaches the lungs, become narrowed and inflamed. Attacks are often triggered by allergies or colds and can be fatal if breathing becomes so difficult that not enough oxygen reaches the bloodstream.

The study suggested that several factors may be contributing to the trend, including decreasing access to medical care that could prevent asthma deaths, increasing prevalence of the disease or complications related to asthma treatments.

Researchers at George Washington University and at the National Institutes of Health used government figures to examine asthma death rates among Americans between the ages of 5 and 34. They found that between 1968 and 1977, the death rates in that age group were declining by almost 8 percent per year.

But beginning in 1978, the pattern mysteriously reversed itself, with asthma death rates increasing by an average of more than 6 percent per year for the age group between 1978 and 1987, the most recent year for which data are available. Among 5 to 14 year olds, the average increase was 10.1 percent a year.

Significant increases have occurred in several regions of the country but were most severe in New York City, Chicago, Phoenix and Fresno, Calif.

In 1987, the asthma death rate for Americans between 5 and 34 years old was 4.2 per 1 million people, higher than the 1968 rate. A total of 481 Americans in the age group died of asthma in 1987, according to the report.

Asthma deaths in the New York and Chicago areas may be partly driving the national trend, the study said. In 1985, those two areas accounted for 21 percent of all asthma deaths in the age group studied.

The pattern of increasing death rates was seen among both whites and non-whites, but non-whites had much higher death rates from asthma than whites throughout the 20-year period. In 1987, the asthma death rate for non-white males was almost five times as high as the death rate for whites of both sexes, the study said. The researchers did not break down death rates for non-whites according to ethnic group.