'I know' seems to describe a state of affairs which guarantees what is known, guarantees it as a fact. One always forgets the expression, 'I thought I knew.'
Smoking is seriously bad for your health. Smokers die an average of 14 years earlier than nonsmokers, from causes ranging from lung cancer to heart attacks; tobacco-related diseases are the leading cause of premature death in developed nations. Many of these deleterious effects have been known for over a century – and yet, until the 1960s, smoking was routinely advertised as good for you, and doctors were recruited to champion the health benefits of smoking. Lucky Strike and Camel both advertised themselves as doctor-recommended, and the Journal of the American Medical Association ran cigarette ads until the 1950s. In fact, advertising the health benefits of smoking was so common that at least one brand went on the counterattack: Old Gold billed itself as “a treat, not a treatment.” Today all tobacco products sold in the United States (and much of the rest of the world) are required to carry a warning about the health risks of smoking.