For example, we have performed detailed studies investigating factors influencing the formation of a number of toxicants, including the influence of cigarette design parameters on the following toxicant yields[2-8];
The generation of carbon monoxide from a burning cigarette is mainly through the following three types of reactions[9 ];
The dominant temperature ranges and reactive species for each of the three reactions have been revealed by detailed pyrolysis studies using isotopic gases (18O, 13CO2, 18CO2)[ 9 ]. In addition, the relative contributions from these reactions have been quantified: approximately 30% of the total carbon monoxide was formed by thermal decomposition of tobacco, 36% was produced by tobacco combustion, and at least 23% was produced via reactions of carbon dioxide. The remainder was produced by an interaction of the carbon dioxide reduction and further oxidation. The multiple sources and the generic and interdependent nature of these reactions imply that it is extremely challenging to achieve significant carbon monoxide reduction in cigarette smoke.
Various studies have been carried out to investigate catalysts that can be added to cigarette filters and that can oxidise carbon monoxide at room temperature. However, it has been difficult to find catalysts that maintain a high catalytic activity after a lengthy period of storage (months or more), and more importantly able to resist “poisoning” by other contituents present in the smoke. In practice, effective reduction of carbon monoxide in cigarettes is mainly achieved through the use of highly permeable cigarette paper and the filter ventilation, both of which encourage the preferential diffusion of carbon monoxide away from mainstream smoke.
Effective reduction in smoke toxicants may also be achieved through new tobacco growing and curing practices, which can lead to the reduction of precursors for some toxicants.
It has been established that the TSNAs in smoke are mainly generated via two routes: distillation of TSNAs already present in the cured tobacco, and pyrolytic formation of TSNAs during smoking. It may therefore be feasible to at least reduce the part of TSNAs formed during the curing process and already present in the tobacco, and to assess the effects of such reduction on the final smoke TSNA levels.
For the pyrolytic formation, a number of nitrogenous tobacco leaf constituents (e.g., alkaloids and nitrate) are known precursors for the TSNAs in smoke. These leaf precursors are influenced by the type of tobacco (Virginia, Burley or Oriental), agriculture practices such as fertiliser, the curing process, and finally the combustion and pyrolysis conditions. All these factors and their interactions may be further affected by cigarette designs and smoking regimes applied. A general outline of the multiple factors affecting the TSNA levels in cigarette smoke is shown in the schematic below:
Among other studies, we have investigated factors contributing to TSNA formation during the curing of Canadian flue-cured tobacco. In this study we found that indirect heating during flue-curing significantly reduced the accumulation of TSNAs in tobacco in comparison to direct heating. Under direct heating conditions TSNA levels were positively correlated with temperature. Measurements of the microbial population on tobacco showed no difference between direct and indirect heating, and also demonstrated a negative correlation between microbial populations and TSNA levels in cured leaf.
We have investigated the generation of formaldehyde in cigarettes in two ways.
The first approach was to review over 30 years of research into formaldehyde generation in cigarettes and its subsequent behaviour in smoke. This review highlighted the complex smoke chemistry related to this compound.
In addition, we examined the effect of saccharides on smoke formaldehyde yields. We reported that sugars added on US blend cigarettes were associated with an increase on formaldehyde yields under a variety of pyrolysis and machine smoking conditions[16-19]. We found different sugars increase formaldehyde yields to different extents, and possible mechanisms to explain this effect were advanced. Other studies carried out showed that a significantly higher proportion of the formaldehyde measured in mainstream smoke is produced in the first puff, highlighting that this compound is also sensitive to the cigarette combustion conditions.
Tobacco absorbs various metals from the environment during growth. Some of these metals, such as arsenic (As) and chromium (Cr), are transferred into cigarette smoke. A total of six metals (As, Cd, Cr, Pb, Ni and Se) are routinely measured. Cr in cigarette smoke is classified as “carcinogens to humans” by the International Agency for Research on Cancer (IARC). However, we know very little about its exact oxidation state or speciation in cigarette smoke. This matters, as the same metal of different oxidation states, e.g. Cr(III) vs. Cr(VI), or speciation may have very different toxicities.
In the case of arsenic, the most toxic arsenic species are considered to be the inorganic species, e.g., arsenite, (As(III)O3 )3- , and arsenate, (As(V)O4 )3- ; the trivalent species being more toxic than the pentavalent ones. In contrast, organic arsenicals in food and plant materials (e.g. arsenobetaine, monomethyl arsenic acids, arsenosugars, etc.) have little or no reported toxicity. We have conducted a preliminary assessment on the arsenic speciation in cigarette smoke by using synchrotron X-ray absorption spectroscopy. The technique is capable of directly probing smoke samples without any sample preparations. This is particularly desirable because metal speciation analyses on redox sensitive elements such as As and Cr are prone to alterations by conventional sample preparations.
Our study provides the first information on the oxidation states of As in cured tobacco leaf, cigarette ash and smoke particulate matter. It also reveals aspects of dynamic redox equilibrium between As(III) and As(V) in the total particular matter (TPM). Unfortunately, the levels of arsenic species present in these samples are too low to ascertain speciation, even for some of the most advanced synchrotron facilities available today. We will continue to pursue other experimental techniques to gain further understanding on the exact metal species and their interaction with the smoke matrix.