In our response, we used the NIOSH arguments to further support the appropriateness of our approach. In particular, we mentioned that the targeted population for e-cigarette use is the smoking population. Whether sensitive or not, these people are exposed to a large number of toxicants on a daily basis for many years. Additionally, we emphasized that a risk level of 1 in 1000 developing lung dysfunction represents a tremendous benefit for smokers who have a 1 in 3-4 risk of developing chronic obstructive lung disease in their lifetime. Additionally, we provided further evidence from the literature on the levels of diacetyl in tobacco cigarette smoke. Studies by Fujioka and Shibamoto and by Moree-Testa and Saint-Jalm found diacetyl in smoke at levels similar to the study by Pierce et al. Finally, we accepted the argument by NIOSH that we underestimated the ventilation rate of workers. That was a conscious decision, because we wanted to use the most stringent criteria in the comparison. Thus, we used the worst-case scenario of resting ventilation rate, which underestimated the safety-limit exposure set by NIOSH. Based on mild and moderate activity, the NIOSH-estimated safety exposure limit would be 86micrograms/day (mild activity) and 302micrograms/day (moderate activity) for diacetyl, and 132micrograms/day (mild activity) and 638micrograms/day (moderate activity) for acetyl propionyl. Our initial calculations considering resting ventilation rate was 65micrograms/day for diacetyl and 137micrograms/day for acetyl propionyl. Thus, the NIOSH-set limits are approximately 50-400% higher than what we considered in our original study, using the worst case scenario.
In conclusion, it is important to provide a comparative measure of exposure to diacetyl and acetyl propionyl from e-cigarette use, considering that the targeted population is smokers who have a high risk of developing disease and are continuously exposed to several toxins (including diacetyl and acetyl propionyl). In any case, we made clear that diacetyl and acetyl propionyl represent an avoidable risk and every effort should be made to remove them from e-liquids. In our opinion, the major impact of our study is that it alerted the industry to take care of this issue. Anecdotal evidence suggests that there is a lot of improvement in this area, and we believe that a future study is needed to evaluate and verify whether diacetyl and acetyl propionyl have been removed from e-liquids.
