E-cig only users have fewer carcinogens in their bodies than cigs, but not most people, who are dual users with cigarettes

A new well-done study, “Nicotine, Carcinogen, and Toxin Exposure in Long-Term E-Cigarette and Nicotine Replacement Therapy Users: A Cross-sectional Study,” published in Annals of Internal Medicine, has been attracting a lot of attention from e-cigarette enthusiasts because it shows that levels of carcinogens in e-cigarette users’ bodies is much lower than when they smoke cigarettes. 
This result is exactly what one would expect because it is well-established that e-cigarettes deliver much lower levels of most carcinogens than conventional cigarettes.  E-cigarette advocates are using this paper to stress the value of switching from smoking cigarettes to e-cigarettes.
Everyone – including me – agrees that switching entirely from cigarettes to e-cigarettes (assuming no effects on cessation) would be a good thing. 
The problem is, as this paper notes, that almost all e-cigarette users keep smoking cigarettes, so-called dual use.  The paper (in their Figure 2) shows that the levels of carcinogens in dual users’ bodies is as high as if they smoke cigarettes.
So, for the great majority of e-cigarette users as they are actually used in the real world, there is no health benefit of e-cigarettes.
In addition, as I recently noted, e-cigarettes have similar effects on the cardiovascular system as smoking cigarettes and cardiovascular disease kills about as many smoker as cancer does.
Here is the abstract of the paper:
Background: Given the rapid increase in the popularity ofe-cigarettes and the paucity of associated longitudinal health related data, the need to assess the potential risks of long-term use is essential.
Objective: To compare exposure to nicotine, tobacco-related carcinogens, and toxins among smokers of combustible cigarettes only, former smokers with long-term e-cigarette use only, former smokers with long-term nicotine replacement therapy (NRT) use only, long-term dual users of both combustible cigarettes and e-cigarettes, and long-term users of both combustible cigarettes and NRT.
Design: Cross-sectional study.
Setting: United Kingdom.
Participants: The following 5 groups were purposively recruited: combustible cigarette–only users, former smokers with longterm (≥6 months) e-cigarette–only or NRT-only use, and longterm dual combustible cigarette–e-cigarette or combustible cigarette–NRT users (n = 36 to 37 per group; total n = 181).
Measurements: Sociodemographic and smoking characteristics were assessed. Participants provided urine and saliva samples and were analyzed for biomarkers of nicotine, tobacco specific N-nitrosamines (TSNAs), and volatile organic compounds (VOCs).
Results: After confounders were controlled for, no clear between-group differences in salivary or urinary biomarkers of nicotine intake were found. The e-cigarette–only and NRT-only users had significantly lower metabolite levels for TSNAs (including the carcinogenic metabolite 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanol [NNAL]) and VOCs (including metabolites of the toxins acrolein; acrylamide; acrylonitrile; 1,3-butadiene; and ethylene oxide) than combustible cigarette–only, dual combustible
cigarette–e-cigarette, or dual combustible cigarette–NRT users. The e-cigarette–only users had significantly lower NNAL levels than all other groups. Combustible cigarette–only, dual combustible cigarette–NRT, and dual combustible cigarette–ecigarette users had largely similar levels of TSNA and VOC metabolites.
Limitation: Cross-sectional design with self-selected sample.
Conclusion: Former smokers with long-term e-cigarette–only or NRT-only use may obtain roughly similar levels of nicotine compared with smokers of combustible cigarettes only, but results varied. Long-term NRT-only and e-cigarette–only use, but not dual use of NRTs or e-cigarettes with combustible cigarettes, is associated with substantially reduced levels of measured carcinogens and toxins relative to smoking only combustible cigarettes.
The full citation is Lion Shahab, PhD; Maciej L. Goniewicz, PhD; Benjamin C. Blount, PhD; Jamie Brown, PhD; Ann McNeill, PhD; K. Udeni Alwis, PhD;  June Feng, PhD; Lanqing Wang, PhD; and Robert West, PhD Nicotine, Carcinogen, and Toxin Exposure in Long-Term E-Cigarette and Nicotine Replacement Therapy Users A Cross-sectional Study Ann Intern Med. 2017; doi:10.7326/M16-1107.  The paper is available here.
Here is some of the press coverage:
E-cigarettes users have lower carcinogen and toxin levels than smokers, study finds
Study of cancer-causing toxins finds e-cigarettes much safer than smoking
Are e-cigarettes safer than tobacco? New study fires up debate
New study comes the closest yet to proving that e-cigarettes aren’t as dangerous as smoking
Long-term vaping 'far safer than smoking' says 'landmark' study