Why you should use exclusively denicotea silica gel filters in your cigarette holders: Recently, manufacturers of pipe filters offer the 9 mm activated charcoal filter as suitable for cigarette holders. It is true that the activated charcoal filter can be inserted in the cigarette holder but it does not fit perfectly that means its diameter is a bit smaller than that of the denicotea silica gel filter and therefore draws disturbing unguided air while smoking. An optimal effect filtration is not guaranteed by 9 mm activated charcoal filters. Additionally, the degree of saturation of the filter can not be seen, so you would not know exactly when the filter should be changed.