Many cigarettes are made with tiny holes around the filter which allow air that has not been drawn through the end of the cigarette to mix with the tobacco smoke during smoking. When smokers place their mouth or fingers over the vents, they keep outside air from diluting the mixture and so take in higher levels of tar and nicotine. It is difficult for a smoker to know if they are covering up the vents.
Many brands have vents that are so tiny they are invisible to the naked eye.
Often the placement of the holes makes it difficult if not impossible for a smoker to smoke a cigarette without blocking some or all of the vents. The cigarettes are designed so that it is 'natural' to cover up some or all of the filter vents, and 'natural' to breathe in heavier amounts of tar and nicotine. Analysis of the filter ventilation in cigarettes shows an extremely high correlation between percent of filter ventilation and type of cigarette. This high level of correlation suggests that filter ventilation in cigarette design defines 'light' and 'ultra-light,' rather than the amount of nicotine in the cigarette itself. Filter vents are more often found in 'light' and 'ultra-light' cigarettes.