Cigarette smoking causes immediate and damaging changes in teenager's blood, the kind of changes that can lead to early artery problems and heart disease.

University of Cincinnati scientists reported this finding to the American Heart Association yesterday.

They said the news may help convince teen-agers that smoking is harmful. Increasing numbers of teen-agers, especially girls, are smoking, and studies have shown that an immediate health effect will make more people change a habit than some distant threat.

Other research has shown that smoking also harms teen-agers' lungs. Studies of thousands of youngsters, some as young as 10 years old, have shown that they suffer coughs and wheezes two or three times more frequently than nonsmoking classmates. Breathing tests and examinations of lung tissue confirm the damage.

The main effect in the young smokers' blood is a decrease in the blood fat known as HDL (high-density lipoprotein, or cholesterol). Blood fats in general help clog arteries and cause heart disease. But a high HDL level, though all the evidence isn't in, seems to have the opposite effect, actually helping to prevent heart attacks.

In studies financed by the National Institutes of Health, Dr. John Morrison and his colleagues in Cincinnati examined 965 schoolchildren aged 12 to 19. The 144 smokers among them smoked an average of eight cigarettes daily -- only 53 smoked more than 10 a day -- yet their average HDL level was 11 percent lower than nonsmokers'.

Another report, to a heart association meeting in Anaheim, Calif., showed that smoking reduces adults' HDL levels, too -- by 11 percent in men and 14 percent in women, on the average. These reductions are "large enough to significantly increase" a persons's heart disease risk, said Dr. Michael Criqui, an epidemiologist at the University of California in San Diego.

Exercise seems to increase HDL levels, say a set of reports in the Journal of the American Medical Association this week. This effect has been found in marathon runners, cross-country skiers and most recently in heart attack patients on moderate exercise programs -- walking, jogging and doing calisthenics for 45 minutes three times a week.

Whether high HDL levels definitely protect against heart disease still must be proved by long-term studies, Dr. William Castelli, head of a federal heart study program warned. But he and other specialists increasingly advise a "prudent" life style, including exercise and a moderate, low-fat diet, as a possible preventive.