A minority of epidemiologists have found it hard to understand how secondhand smoke, which is more diluted than actively inhaled smoke, could have an effect that is such a large fraction of the added risk of coronary heart disease among active smokers. One proposed explanation is that secondhand smoke is not simply a diluted version of "mainstream" smoke, but has a different composition with more toxic substances per gram of total particulate matter. Passive smoking appears to be capable of precipitating the acute manifestations of cardio-vascular diseases (atherothrombosis) and may also have a negative impact on the outcome of patients who suffer acute coronary syndromes.