THE JUST-RELEASED Surgeon General's report on smoking should be required reading for smokers. Previously known associations between smoking and cancer are now clearer; a few new ones turn up, and there is stronger evidence that non- smokers sharing the same air with smokers may share the ill effects.

Smoking was officially recognized to be the country's chief preventable cause of death three years ago. That it is a causal factor in cancer of the lung, larynx, mouth and esophagus was also a finding of the 1979 Report of the Surgeon General. Based on additional evidence, the new report concludes that smoking is not just a cause but a major cause--in the case of lung and larynx cancer, the major cause --of these diseases. Similarly, the known connection between smoking and bladder, kidney and pancreatic cancers is upgraded from a "significant association" to a "contributory factor." For the first time in this series, the report cites evidence of an association between smoking and cancers of the stomach and cervix.

All in all, smoking accounts for 30 percent of cancer deaths. But though the Surgeon General's report deals only with this one risk, cancer is not the chief cause of death from smoking. That distinction belongs to coronary heart disease, which is also the chief cause of all deaths in this country (cancer is No. 2). Smokers also suffer higher rates of dozens of chronic and acute diseases ranging from emphysema and peptic ulcer to influenza. Smoking is the chief avoidable risk during pregnancy, and it vastly increases the risk of most occupational exposures. In short, it is such a pervasive health risk that no epidemiological study would be conducted today without detailed questions on smoking history.

Still, more than 50 million Americans smoke. Most of them know there are risks, though few understand how great they are. Maybe a few, though we hope not very many, believe the Tobacco Institute when it asserts, as it did this week, that "the question is still open" on whether smoking causes cancer.>

The real reason there are still so many smokers is that it is so hard to stop for good. Scientists disagree on whether smoking is an addiction comparable to heroin or alcohol addiction, but it is clear that it is more than just a habit. Studies now under way may turn up better methods of helping people to quit permanently. Until then, the best therapy is a healthy dose of good, old-fashioned fear.