The scientific evidence that ETS causes adverse health effects is inconclusive at best. In fact, no studies have shown conclusively an elevated risk of disease from passive smoking. Furthermore, the overwhelming majority of studies or groups of studies, no matter how manipulated or combined together, has failed to prove the existence of any considerable risk.

These very, very low risks are established by using epidemiological techniques. Epidemiology is only effective when there is one cause and one effect (disease). With multi-factorial diseases such as the ones attributed to primary or passive smoking, the accuracy of epidemiology decreases in proportion of the factors, to the point of being virtually useless beyond a small number of concomitant factors. The "tobacco-attributed diseases" have typically 40 to 300+ concomitant factors.