Nick Wilson and two others blog at Otago University:
The studies we found suggest a very diverse range of results as shown in Table 1, but all suggest lower levels of risk for vapers compared to tobacco smokers. In particular, the risk associated with carbon monoxide seems likely to be close to 0% or a few percent at most. However, preliminary evidence (ie, one study by Carnevale et al (7)) suggests that the effect of vaping on four other inflammatory markers of likely relevance to cardiovascular disease (CVD) and respiratory disease may be at least half that of tobacco smoking. The results for cancer-related toxicants were variable, from 0% to 23% of the levels observed for tobacco smokers, with most studies reporting between 14% and 23% – a substantial level of exposure. But it is plausible that some of these toxicants could be due to unreported dual use with smoked tobacco (and even exposure to secondhand smoke).
So almost no carbon monoxide impact, and reduced levels of other toxicants.
It is important to know that e-cigarettes are a reduced harm product – that is not the same as harmless. You don’t want people going from not being a smoker to using e-cigarettes. But you do want people who are already smoking to start using e-cigarettes as a substitute for tobacco.
The academics conclude:
It seems likely that if smokers shift entirely to vaping their risk of chronic disease would be expected to decline. But if they stay vaping long-term – then they may still be exposed to some notable level of toxicants that are hazardous in terms of cancer, cardiovascular disease and possibly long-term respiratory disease. The safest option for smokers using vaping to reduce their health risk would be to limit the duration of dual use with cigarette smoking (ie, switching completely to vaping as soon as possible) and to also limit the total duration of vaping with a goal of reaching abstinence from both smoking and vaping, wherever possible without relapsing to smoking, which represents the greater risk to health.