• filter vent blocking
• smoke yields

In their recent article Kozlowski and O'Connor¹ criticise a 1997 review² on cigarette filter ventilation blocking and claim it is in error because it (1) relies on saliva based estimates, (2) does not consider degree of ventilation, (3) does not address brand-to-brand variation, and (4) omits certain tobacco industry studies. We disagree and stand by our conclusions.^2,3

In their criticisms Kozlowski and O'Connor refer only to the 1997 review² presented at a conference and not a peer reviewed article published in early 2001.³ In the latter review, Dr Baker and I considered measurement techniques, effects of vent blocking on machine smoke yields, effects of vent blocking on human smoke yields, and simultaneous determination of vent blocking and smoke yields. We concluded that vent blocking among smokers has only a relatively minor effect on human smoke yields compared to other smoking behaviour factors.³ The large effects observed with smoking machines are misleading because people do not smoke like machines.

Concerning the allegation that we erred because of our reliance on saliva based estimates, the facts are that we discussed the pros, cons, and limitations of all techniques used to estimate the extent of vent blocking.³ We reported that four studies by Kozlowski and colleagues, using the “tar”' stain technique, indicate that 50–59% of the 14 to 158 filters examined in each study showed some degree of vent blocking. Two other studies,^4,5 using the same technique but each based on over 1000 filters, indicate that 21–30% of the filter vents examined were blocked, and most were only partially blocked.⁴ These latter studies are in reasonable agreement with large studies conducted by industry scientists using the saliva stain technique,³ which indicate that up to 24% of filters examined were blocked by …