Discussion

This study presents some of the first information on the substances that US adolescents vape. Nicotine is assumed by many to be the predominant substance that youth vape, although, to the best of our knowledge, this assumption is not based on the scientific data.

‘Just flavouring’—and not nicotine—was by far the most commonly vaped substance in all grades. Among students who had ever used a vaporiser in their life, the portion who used ‘just flavouring’ the last time they vaped was greater than all other substances combined. This response was markedly consistent across grades and was reported by 65–66% of students in 12th, in 10th and in 8th grades. In all grades, the percentage reporting that they had last vaped ‘just flavouring’ was above 57% for males, females, Whites, Blacks, Hispanics, and students with and without a parent who had a college degree. The percentage who vaped ‘just flavouring’ at last use was also high among students who used a vaporiser in the past 30 days, at a prevalence of 59% or higher in all three grades.

Nicotine came in a distant second place among youth who had used a vaporiser. About 20% of 12th and 10th grade students and 13% of 8th-grade students vaped nicotine at last use. Across the demographic groups of gender, race/ethnicity and parental education, nicotine use in vaporisers never exceeded 26% in any of the three grades. In 12th grade, its use was highest among Whites, males and students with at least one parent who had a college degree. This unusual pattern of greater use among more advantaged demographic groups is similar to the early diffusion of other substances such as cigarettes and cocaine, which had been concentrated in advantaged groups in past decades but are now concentrated in disadvantaged groups after the substances developed a reputation as dangerous.10–12

Among the group of students who had vaped in the past 30 days, the portion who last vaped nicotine was 31% in 12th, 27% in 10th and 16% in 8th grade. Among students who had vaped 6+ times in the past 30 days, these percentages were 25–50% higher as compared to those who had vaped 1–5 times. This result indicates that the importance of the cut-off between vaping 1–5 as compared to 6+ times in the past 30 days—a threshold highlighted by previous research13—extends to substances vaped. In no case did the prevalence of nicotine vaping reach 50% or greater.

We note that the percentage who vaped nicotine was higher in the older age groups; longitudinal data are required to determine whether this represents younger vapers progressing to nicotine and/or an influx of new vapers at older ages.

Levels of marijuana vaping were about 6% in each of the three grades among students who had ever used a vaporiser. These levels did not significantly differ by sociodemographic groups, with the exception in 8th grade of higher prevalence of marijuana vaping among Hispanics as compared to Whites. Among students who had used a vaporiser in the last 30 days, vaping of marijuana was highest in 8th grade.

Some youth did not know what substance they last vaped. This percentage was 14% in 8th grade and declined at older age groups, reaching 6% in 12th grade. The lowest percentage was 2% among heavy vapers, who presumably are more intentional in the substances that they vape.

Four major implications follow from the study's main finding that most youth who use vaporisers do not use nicotine. First, these results suggest the need to reconsider the term ‘ENDS’ to denote vaporisers and e-cigarettes, at least among US adolescents. The term stands for ‘electronic nicotine delivery system’, which seems inaccurate for the description of a device that the majority of youth do not use to vape nicotine.

A second implication is the need to reconsider the impact of vaporisers on the estimated national prevalence of nicotine use among US youths. The current assumption that vaporiser use is synonymous with nicotine use leads to a doubling of past 30-day tobacco/nicotine prevalence in 12th grade and a near tripling in 10th and 8th grades (see tables 4 and S3) as compared to estimates based on cigarette use alone. However, the results from this study indicate that many vaporiser users do not vape nicotine. If vaporiser users are considered nicotine users only if they last vaped nicotine in the last 30 days, then national estimates of nicotine prevalence increase by a much smaller percentage of 23–38% across the three grades. These results indicate that while taking into account vaporiser use does indeed increase tobacco/nicotine prevalence, the impact of vaporisers is likely not as large as might appear by their recent, dramatic increase in use among adolescents.

A third implication is that vaporiser use may serve as an indicator for primary prevention programmes aimed at nicotine use. Because many US youth who use vaporisers do not vape nicotine, they are candidates for primary interventions, which are particularly strategic to combat nicotine use because they take place before the need to address nicotine's addictive properties. Furthermore, recent evidence that vaporiser use is a risk factor for future cigarette use14–18 suggests that vaporiser use can serve as an important marker for youth who are at elevated risk for future nicotine use.

A fourth implication is that a different rationale for the regulation of vaporiser devices will be required as compared to the regulation of vaporiser contents. The finding that most US youth do not vape nicotine makes it difficult to ban sales of vaporisers to youth on the grounds that all of them intrinsically deliver harmful substances to children in all circumstances, a primary rationale to ban the sale of cigarettes to youth. In the absence of an established body of evidence that links vaping any substance to impaired health, bans on sales of vaporisers to youth may need to draw on other rationales, such as the potential of vaporisers to desensitise youth to the dangers of tobacco smoking, and/or the argument that children are at high risk to unintentionally vape substances that will harm them.

In contrast, the rationale that children should be protected from harmful substances can be readily extended to the regulation of specific substances marketed for vaporisers. The fact that a substantial portion of youth are vaping nicotine, even if not the majority, underscores the importance of regulations aimed at clearly labelling the contents of these commercial materials and preventing children from access to the ones known to be harmful. Recently proposed ‘deeming’ rules by the US Food and Drug Administration19 provide a cornerstone for regulations along these lines.

It is important to note three limitations of this study. First, it is possible that youth may self-report that they are not using nicotine when, in fact, they are vaping nicotine but do not realise it. This may increase somewhat the estimates of nicotine use among the least experienced users who may not recognise the physiological symptoms associated with nicotine use. The most accurate knowledge of substances vaped is expected to be among the most experienced users, which in this study are the 12th-grade students who vaped six or more times in the last 30 days and presumably are more intentional in the substances they vape. The study's main conclusion that most youth who use vaporisers do not vape nicotine is bolstered by the fact that less than half of this experienced group reported vaping nicotine, a finding difficult to ascribe to inaccurate self-reports.

A second limitation is that the sample of frequent vapers who vaped six or more times in the past 30 days is not large enough to support in depth analysis. This small group is of considerable importance for theory and for policy. On the one hand, it may represent a new class of substance user who uses vaporisers exclusively, a class supported by preliminary evidence in a recent analysis of e-cigarette users in 12th grade.20 On the other hand, it may represent a standard, well-documented class of polysubstance users who have simply extended their use of drugs to a new device.21–23 Furthermore, the heightened use of nicotine in this group warrants future analysis to examine whether their level of nicotine inhaled reaches that of their peers who use cigarettes. A third limitation is that the study lacks information on youth who have vaped multiple, different substances within the past 30 days. Some non-smoking youth who last vaped flavouring in the past 30 days may have also vaped nicotine earlier in the 30-day period, information not currently available in the data because the survey asks about only substance last vaped. Future MTF surveys will collect more detailed information on substances vaped, which will support an alternative measure of nicotine prevalence that takes into account this possibility of youth vaping multiple, different substances These measures will likely increase estimates of the percentage of youth who vape substances other than flavouring.

In conclusion, the majority of US youth who use vaporisers and e-cigarettes do not vape nicotine. This finding challenges many common assumptions and practices, and points to the need for vaporiser-specific research to assess and ultimately regulate the public health threat of vaporisers. Taking into account this finding now, while the field is young, will help ensure that future vaporiser science and regulations are built on a solid footing.