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. Author manuscript; available in PMC: 2023 Dec 1.
Published in final edited form as: Drug Alcohol Depend. 2022 Nov 12;241:109700. doi: 10.1016/j.drugalcdep.2022.109700

Electronic Nicotine Delivery Systems (ENDS) Dependence Among Texas Young Adults, 2014–2019: Increasing Trajectory after the 2017 Surge in Vape Pod Popularity

Alexandra Loukas 1,*, C Nathan Marti 2, Melissa B Harrell 3, Keryn E Pasch 4, Anna V Wilkinson 5
PMCID: PMC9806720  NIHMSID: NIHMS1852889  PMID: 36434881

Abstract

Introduction.

Use of electronic nicotine delivery systems (ENDS) increased significantly after 2017. The increase is attributed to a surge in popularity of vape pods containing nicotine salts, which are high nicotine concentration ENDS that may heighten risk for dependence. However, little is known about changes in ENDS dependence before and after the 2017 surge. We examined the trajectory of ENDS dependence among young adults from 2014–2019.

Methods.

Participants were 1,700 18–25-year-olds (57.6% female) from 24 Texas colleges who reported past 30-day ENDS use in at least one of eight study waves. ENDS dependence was assessed at all waves with one item, use of ENDS within 30 minutes of waking. A discontinuous growth curve model was fit to test the hypothesis that the ENDS dependence trajectory would increase only after 2017, from 2018–2019. The model included socio-demographic and cigarette dependence covariates.

Results.

The proportion of young adults reporting ENDS dependence ranged from 2.3% in 2014 to 8.2% in 2019. Results from the discontinuous growth curve model indicated that the ENDS dependence trajectory was flat from fall 2014 to spring 2017. However, the post-2017 trajectory, from spring 2018 to spring 2019, was positive and significant indicating that the proportion of young adults reporting ENDS dependence increased after 2017.

Conclusions.

The increase in ENDS dependence after 2017 likely resulted from the surge in popularity of vape pods containing nicotine salts. These vape pods are a disruptive technology that may result in more young adults developing nicotine dependence symptoms than do earlier generation devices.

Keywords: Vape pods, e-cigarettes, longitudinal models, young adults

1.1. INTRODUCTION

Data from the National Health Interview Survey indicate that current use of electronic nicotine delivery systems (ENDS) among 18–24-year old young adults in the United States (US) increased significantly from 2017–2018, after a period of decline from 2014–2017 (Creamer et al., 2019). Data from the young adult cohort of the Population Assessment of Tobacco and Health (PATH) study also showed an increase in past 30-day ENDS use among 18–24-year-olds from 2016/2017 to 2018/2019, with the rate almost doubling from 15.1% to 26.2% in the one year period (United States Department of Health and Human Services et al., 2021). The increase in ENDS use has been attributed to the surge in popularity of vape pods that contain nicotine salts, particularly JUUL, a sleek USB-shaped device that dominated retail sales of ENDS products from late 2017–2019 in the US (Gaber et al., 2022; Huang et al., 2019; King et al., 2018). Sales and popularity of JUUL began to fall in late 2019 due to voluntary actions and regulatory requirements that resulted in the removal of flavors, except menthol and tobacco, from their products (Gaber et al., 2022). However, other vape pod “knock-offs,‖ like Puff Bar, gained popularity, and vape pods remain among the most popular ENDS devices used by young people (Gaber et al., 2022; Hammond et al., 2022).

Electronic nicotine delivery systems are often marketed as alternatives to cigarettes (Collins et al., 2019) and adults report using ENDS to try to quit smoking cigarettes (Osibogun et al., 2022). Although ENDS and cigarette use co-occur (Osibogun et al., 2017), some young adults who use ENDS have never smoked cigarettes (Bandi et al., 2021) and most college student young adults report using ENDS for reasons other than quitting or cutting down on smoking (Leavens et al., 2019). Young adults report initiating and using ENDS for various other purposes including socialization, belonging, stress reduction, and appetite suppression (Clendennen et al., 2021; Robertson et al., 2022). Use of ENDS by young adults is concerning because these products contain nicotine, the substance that contributes to addiction/dependence and to long-term, regular use (Benowitz, 2010).

Vape pods, like JUUL, contain particularly high concentrations of nicotine (Goniewicz et al., 2018). High concentrations are possible because these vape pods contain nicotine salt liquids rather than the freebase nicotine liquid used in earlier generation ENDS products. Nicotine salts facilitate the inhalation of high concentrations of nicotine that are otherwise too harsh to inhale when in freebase form. As such, earlier generation ENDS products contain nicotine liquids with concentrations that range from 3–24 mg/mL in comparison to a 5% JUUL pod that contains 59 mg/mL (Benowitz et al., 2021). Moreover, one 5% JUUL pod delivers approximately as much nicotine as one pack of cigarettes (Prochaska et al., 2022). In part because of the high nicotine concentrations, more young adults report a greater nicotine hit as a reason for using vape pods with nicotine salts than they do for earlier generation ENDS (Case et al., 2020). Similarly, there is evidence that youth and young adults who use vape pods report more symptoms of nicotine dependence than those who use earlier generation ENDS products (Boykan et al., 2019; Tackett et al., 2021) though some studies find mixed evidence for differences in dependence symptoms across device type (Case et al., 2020; Lin et al., 2022). Moreover, data from repeated cross-sectional surveys conducted in Canada, England, and the US indicate that the prevalence of ENDS dependence symptoms among 16–19-year-olds increased from 2017–2019 (Hammond et al., 2021), when vape pods with nicotine salts were gaining popularity.

One symptom of dependence reported by adults who use ENDS is use of their device upon waking (Sidani et al., 2019; Simpson et al., 2021; Soule et al., 2020). Qualitative research indicates that 18–25-year old young adults keep their device close while sleeping, such as under their pillow, so that they can use it when they wake (Simpson et al., 2021). Time to first cigarette is a validated and widely used measure of nicotine dependence (Heatherton et al., 1991), and there is evidence that time to first use of ENDS is correlated with adolescent and young adults’ reported frequency of ENDS use and the concentration of nicotine in their ENDS devices (Pienkowski et al., 2021). Some research indicates that adults who use ENDS daily report a longer time to first use upon waking than adults who use cigarettes daily (Liu et al., 2017). However, more recent research with young adults who are using later generation devices indicates that a greater proportion of those who use ENDS exclusively report ENDS use within the first 30 minutes of waking (54%) compared with those who use cigarettes exclusively who report cigarette use within the first 30 minutes (18%) (Jankowski et al., 2019). These findings underscore the disruptive nature of later generation ENDS devices and suggest that there may be a new generation of nicotine dependent young adults who are using ENDS rather than cigarettes. Despite evidence that those who use later generation devices may be more likely than those who smoke cigarettes and those who use other ENDS products to report symptoms of nicotine dependence, relatively little is known about changes in nicotine dependence after the 2017 surge in popularity of vape pods, like JUUL. Moreover, longitudinal studies tracking ENDS dependence before and after the surge are particularly lacking.

The purpose of the present study was to examine the longitudinal trajectory of ENDS dependence across a 4.5-year period from 2014–2019 among young adults. Dependence was assessed with one item, using ENDS within 30 minutes of waking, and we assessed if the proportion of young adults who reported ENDS dependence increased after the 2017 surge in popularity of vape pods containing nicotine salts. Participants were 18- to 25-year-old college students involved in a longitudinal study who reported past 30-day ENDS use in at least one of eight study waves. Vape pods are popular among college students (Roberts et al., 2022) and national data indicate that college students were more likely than their same-aged peers to report ENDS use from 2018 to 2020 (Schulenberg et al., 2020); thus, this population may be at elevated risk for developing symptoms of dependence. We hypothesized that the proportion of young adults reporting ENDS use within 30 minutes of waking would increase only after 2017, from 2018–2019, consistent with limited evidence showing increased prevalence of ENDS use (Creamer et al., 2019; Loukas et al., 2021; United States Department of Health and Human Services et al., 2021) and dependence (Hammond et al., 2022) among youth and young adults after the 2017 surge in popularity of vape pods containing nicotine salts. Given evidence that ENDS and cigarette use co-occur among young adults (Osibogun et al., 2017), we controlled for cigarette dependence (using the analogous measure of smoking within 30 minutes of waking), when examining the ENDS dependence trajectory. We also controlled for five socio-demographic variables of participant sex, type of college attended, race/ethnicity, sexual and gender minority identity, and age.

2.1. MATERIAL & METHODS

2.2. Participants

Participants were from the Marketing and Promotions across Colleges in Texas (M-PACT) project. Project M-PACT is a longitudinal, rapid response study that tracked the tobacco use of 5,482 college students from fall 2014 to spring 2019. Eligible students were 18–29 years old and full- or part-time degree or certificate-seeking undergraduate students at one of 12 4-year or 12 2-year Texas colleges. Of the 13,714 eligible students, 5,482 provided consent and completed the baseline survey. The present study included only students who were 18–25 years old at baseline and who reported past 30-day ENDS use on at least one of eight study waves; 1,833 students met these criteria, but 133 were excluded due to missing data on covariates resulting in a final sample of 1,700 participants. Difference tests indicated the 133 participants who were not included in the present study due to missing data did not differ from the 1,700 that were included on any of the Wave 1 variables of age, sex, sexual or gender minority (SGM) identity, 4-year (versus 2-year) college attendance, race/ethnicity, ENDS dependence, or cigarette dependence. At Wave 1, the 1,700 participants were 18–25 years old (M=20.70; SD=1.87); 57.6% were female; 27.9% reported an SGM identity; 93.6% attended a 4-year college versus 2-year; and 35.6% were non-Hispanic, white, 33.8% were Hispanic/Latino, 14.8% were non-Hispanic, Asian, 7.4% were non-Hispanic, black, and 8.5% were another race/ethnicity or reported two or more races/ethnicities.

2.3. Procedure

Students were recruited from 24 colleges located in the five Texas counties surrounding Austin, Dallas, Houston, and San Antonio to participate in a longitudinal, web-based study. Eligible students provided informed consent and completed a baseline online survey in fall 2014early spring 2015. There were five follow-up waves that occurred every six months until spring 2017, and two annual follow-ups in spring 2018 and spring 2019. In between the spring 2017 and spring 2018 follow-up waves, an abbreviated survey to maintain contact with participants was conducted (fall 2017) that did not include items pertaining to tobacco dependence. In total, students submitted up to nine waves of data from fall 2014 to spring 2019: eight full study waves (Waves 1–6 and Waves 8–9) and one abbreviated wave (Wave 7). For purposes of this study, data were drawn only from the eight full study waves. Retention rates for the eight study waves ranged from 70% (Wave 9) to 81% (Wave 4) of the original 5,482 participants.

2.4. Measures

ENDS dependence.

ENDS dependence was assessed at all eight waves with one item modeled after Heatherton et al. (1991) “How soon after you wake up do you typically use your first ENDS product?” Responses were dichotomized so that participants reporting using an ENDS product within 30 minutes of waking were coded 1 to indicate ENDS dependence. Participants reporting using an ENDS product 30 minutes or more after waking or reporting they do not use ENDS daily were coded 0.

Socio-demographic covariates.

Time-invariant covariates, assessed at baseline/Wave 1, included sex (0=female; 1=male), type of college attended (0=2-year; 1=4-year), and race/ethnicity (dummy coded as Hispanic/Latino; non-Hispanic black; non-Hispanic Asian; and non-Hispanic other; non-Hispanic white served as reference group). Sexual and gender minority identity also was included as a covariate and coded 1 for participants reporting gay or lesbian, bisexual, queer, transgender, or other identity at any study wave and 0 for those reporting heterosexual/straight at all waves. Participant exact age at the time of the assessment was centered at age 18 and treated as a time-varying covariate.

Cigarette dependence covariate.

Cigarette dependence was assessed at all eight waves with one item from Heatherton et al. (1991) “How soon after you wake up do you typically smoke your first cigarette of the day?” Similar to the scoring of the ENDS dependence item, response options were dichotomized. Participants who reported smoking within 30 minutes of waking were coded 1 and all others were coded 0.

2.5. Data Analysis

The ENDS dependence trajectory from 2014–2019 was assessed using a generalized linear mixed model with a logit link function for a binary distribution. The analysis was conducted with the R glmer function from the lme4 package, version 1.1.29 (Bates et al., 2019) in R version 4.2.0 (R Core Team, 2021). The model was fit using maximum likelihood estimation, which allowed us to make use of all available complete cases. Thus, all eligible participants who had at least one wave of complete data were included in the study. Each model contained five socio-demographic covariates (sex, type of college attended, race/ethnicity, SGM identity, time-varying age) and the time-varying cigarette dependence covariate. The hypothesis that the proportion of young adults reporting ENDS dependence would increase only after 2017 due to the surge in popularity of vape pods containing nicotine salts was assessed using a discontinuous growth model (Singer and Willett, 2003). The discontinuous model segmented the 2014–2019 trajectory into three distinct components/parameters: change in dependence prior to the 2017 surge in vape pod popularity (fall 2014 to spring 2017); change in dependence after the 2017 surge in vape pod popularity (spring 2018 to spring 2019), and elevation change beginning in spring 2018, representing the shift when vape pods increased in popularity. For brevity, we refer to these three parameters as the pre-2017 trajectory, the post-2017 trajectory, and post-2017 elevation change, respectively.

3.1. RESULTS

Prior to testing the study hypothesis, we examined the proportion of young adults reporting ENDS and cigarette dependence at each wave. As shown in Table 1, the proportion of young adults reporting ENDS dependence was relatively consistent across the first six study waves from 2014–2017, ranging from a low of 2.3% in fall 2014 to a high of 2.9% in spring 2017. The proportions became increasingly greater after 2017, with 4.9% and 8.2% of young adults reporting ENDS dependence in the final two waves, spring 2018 and spring 2019, respectively. In contrast, the proportion of young adults reporting cigarette dependence was consistent across all eight waves, ranging from a low of 3.0% to 3.6%. Closer examination of the spring 2019 data indicated that the proportion of young adults reporting ENDS dependence (8.2%) was almost triple the proportion reporting cigarette dependence (3.3%).

Table 1.

Proportion of young adults reporting cigarette and electronic nicotine delivery systems (ENDS) dependence at each study wave, 2014 to 2019.

Wave 1 Wave 2 Wave 3 Wave 4 Wave 5 Wave 6 Wave 8 Wave 9
Fall 14 Spring 15 Fall 15 Spring 16 Fall 17 Spring 17 Spring 18 Spring 19
na 1697 1394 1457 1499 1449 1487 1398 1275
ENDS dependence 2.3% 2.7% 2.5% 2.8% 2.6% 2.9% 4.9% 8.2%
Cigarette dependence 3.2% 3.2% 3.6% 3.2% 3.0% 3.0% 3.6% 3.3%
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Note. Wave 7 data were not used in this study because it was an abbreviated survey that collected only tobacco use, not dependence, data. Dependence assessed at each wave with one item regarding use of each product (cigarettes or ENDS) within 30 min of waking (1 = yes; 0 = no).

a

The number of participants at each wave; 1700 unique participants contributed data to one or more waves.

Results from the discontinuous growth curve model testing the study hypothesis (see Table 2 and Figure 1) indicated that the pre-2017 trajectory and the post-2017 elevation change were not significant. The ENDS dependence trajectory was flat between fall 2014 and spring 2017 and there was no sharp increase in ENDS dependence. However, the post-2017 trajectory was significant and positive indicating that the proportion of young adults reporting use of ENDS within 30 minutes of waking increased after 2017, from 2018–2019. Regarding the covariates, male sex, SGM identity, and cigarette dependence were positively associated with ENDS dependence and Hispanic ethnicity was negatively associated with ENDS dependence. Males, participants reporting an SGM identity, and those reporting using cigarettes within 30 minutes of waking were more likely than their peers to report using ENDS within 30 minutes of waking whereas those reporting Hispanic ethnicity were less likely to do so than their peers.

Table 2.

Parameter estimates for the discontinuous growth curve model predicting electronic nicotine delivery systems (ENDS) dependence from 2014–2019 (N=1,700)

Odds Ratio (95% CI) z p
Age in years 1.07 (0.91, 1.25) 0.78 .435
Male sex 2.16 (1.15, 4.08) 2.38 .017
Sexual and gender minority identity 2.33 (1.20, 4.51) 2.50 .012
Non-Hispanic, white Reference
 Hispanic 0.44 (0.20, 0.97) −2.04 .041
 African-American/black 0.80 (0.21, 2.96) −0.34 .733
 Asian-American 1.05 (0.43, 2.57) 0.12 .908
 Other race or ethnicity 1.16 (0.39, 3.45) 0.27 .784
Four-year (versus two-year) college 0.92 (0.26, 3.28) −0.13 .894
Cigarette dependence 3.03 (1.78, 5.17) 4.07 <.001
Pre-2017 trajectory 1.12 (0.87, 1.43) 0.85 .395
Post-2017 elevation change 0.99 (0.43, 2.27) −0.03 .975
Post-2017 trajectory 2.59 (1.60, 4.20) 3.87 <.001
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Figure 1.

Figure 1.

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Discontinuous growth curve model showing the proportion of young adults reporting ENDS dependence, defined by use of ENDS within 30 minutes of waking, before and after the late 2017 surge in popularity of vape pods with nicotine salts (N=1,700)

4.1. DISCUSSION

Results from this longitudinal study extend existing research by showing the proportion of young adults reporting ENDS dependence was consistent from 2014–2017 and then increased only after late 2017 when vape pods containing nicotine salts, such as JUUL, surged in popularity. It is important to note that the increase in ENDS dependence among the current sample occurred even after controlling for cigarette dependence, suggesting that the increase was not likely due to increases in dual use of cigarettes and ENDS. Rather, descriptive findings indicated that the proportion of young adults reporting cigarette dependence remained constant across the entire 4.5-year study period, from 2014–2019, and that the proportion of young adults reporting ENDS dependence was almost three times higher than those reporting cigarette dependence in 2019. Findings provide support for the impact of the surging popularity of vape pods with nicotine salts on ENDS dependence among young adults. Findings also contribute to the growing body of research indicating that these vape pods are a disruptive technology, suggesting that there may be a new generation of nicotine dependent young adults who are using ENDS.

This study is unique in its examination of the longitudinal trajectory of ENDS dependence before and after the 2017 surge in vape pod popularity. Consistent with expectations and findings from limited research on adolescents and young adults (Hammond et al., 2022; Pierce et al., 2022), the proportion of young adults reporting ENDS dependence increased only after 2017. These findings provide evidence that the increase in the proportion of young adults reporting use of ENDS within 30 minutes of waking is therefore likely due to the surge in popularity of vape pods with nicotine salts, such as JUUL. These vape pods are popular among college students (Roberts et al., 2022), who report initiating and using these products for various reasons, including for the nicotine hit (Case et al., 2020). Vape pods containing nicotine salts have high levels of nicotine (Goniewicz et al., 2018), higher than earlier generation ENDS devices with freebase nicotine (Benowitz et al., 2021), and college students report a greater nicotine hit from JUUL as a reason for use than from earlier generation devices (Case et al., 2020). Although young adults who use vape pods are aware that dependence may result from use (Katz et al., 2019), many report not always knowing how much nicotine is in their vape pod (McKeganey et al., 2020) and/or they have difficulty understanding the meaning of nicotine concentrations on ENDS labels (Morean et al., 2021). Lack of knowledge regarding nicotine content in ENDS products may result in continued use and an elevated risk for developing dependence. As such, additional research is needed that examines how to communicate to young adults about nicotine strength and its implications for dependence and potential long-term use.

The increase in the proportion of young adults reporting using ENDS within 30 minutes of waking, the indicator of dependence assessed in the present study, has implications for long-term use of ENDS by young adults. A shorter time to first use of JUUL upon waking has been associated with less confidence in quitting JUUL among college students (Pulvers et al., 2021). A review of the research with adults who smoke cigarettes indicates that those reporting a shorter time to their first use of a cigarette have less success quitting smoking and maintaining abstinence (Branstetter et al., 2020). Although research on ENDS dependence is not as well developed as that on cigarettes, it is possible that a shorter time to first use of ENDS also will be associated with a lower likelihood of cessation success among young adults and thus, a higher likelihood of maintaining ENDS use. Given that youth and young adults who use JUUL, and similar devices, report more symptoms of nicotine dependence than those who use earlier generation ENDS products (Boykan et al., 2019; Tackett et al., 2021) and potentially those who smoke cigarettes too (Jankowski et al., 2019), use of vape pods containing nicotine salts may result in more youth and youth adults who use regularly over the long-term than did earlier generations of ENDS devices and cigarettes. These findings suggest that there may be a new generation of young adults who are dependent on ENDS. These findings are concerning because although ENDS may be less harmful than cigarettes, they are not without harm and contain potentially toxic substances (National Academies of Sciences and Medicine, 2018). Findings point to the need for interventions tailored to young adults that aim to prevent and decrease or eliminate ENDS use. Programs that prevent the initiation and escalation of ENDS use that are delivered early in young adulthood are particularly important as ENDS are often used for the first time during this developmental period (Perry et al., 2018) and there are no prevention programs tailored to this population.

This is the first study to examine the longitudinal trajectory of ENDS dependence among young adults before and after the 2017 surge in popularity of vape pods containing nicotine salts. Among the study’s strengths are the large cohort of young adult college students followed across a 4.5-year period from 2014–2019 and the repeated assessments of ENDS dependence every six months for the first 2.5 years and then annually for the last two years. However, the study has limitations. First, our convenience sample was drawn from Texas colleges and thus findings may not be generalizable to the population of young adults. However, participants were drawn from 24 colleges located in the four largest metropolitan areas of Texas and are racially/ethnically diverse, and data from the Monitoring the Future study indicate that college students were more likely than their non-college peers to use ENDS products from 2018–2020 (Schulenberg et al., 2020). Second, we assessed dependence using only one item, use of ENDS within 30 minutes of waking. Although there are other symptoms of ENDS dependence (Simpson et al., 2021), time to first use is a validated symptom of nicotine dependence (Heatherton et al., 1991) and recent research indicates that time to first ENDS use predicts youth and young adult’s reported frequency of ENDS use and of nicotine concentration in their ENDS products (Pienkowski et al., 2021). Nonetheless, subsequent research with samples of non-college student young adults from other parts of the US and assessing other symptoms of dependence should be conducted.

5.1. CONCLUSIONS

Vape pods with nicotine salts are a disruptive technology that may result in more young adults developing symptoms of dependence than do earlier generation ENDS devices. Young adults are aware that ENDS use may result in dependence (Katz et al., 2019), but they are not always aware of how much nicotine is in their devices (McKeganey et al., 2020) and/or do not understand nicotine concentration (Morean et al., 2021). It is important therefore to communicate to and educate young adults about nicotine concentrations, and implications for dependence and potential long-term use of ENDS. This communication should be delivered within programs aimed at preventing and reducing or eliminating young adults’ ENDS use. There are an increasing number of interventions tailored to young adults for ENDS cessation (Graham et al., 2020), but no programs are focused on preventing ENDS initiation and escalation among this population. Colleges are one venue where ENDS prevention (and intervention) programs can be implemented, especially given that 2/3rds of 12th graders enrolled in college in 2018 (Hussar et al., 2020), and colleges have the infrastructure and necessary expertise for the successful implementation of prevention and intervention programs.

In addition to the implementation of prevention and intervention programs, there is a need for additional regulation of ENDS products. Regulations that require consistent labelling/packaging across ENDS products and that are displayed prominently, in easy-to-understand terms and/or images should be a priority for the Food and Drug Administration given young adult’s limited knowledge regarding nicotine concentrations. Regulations that limit the nicotine concentration in all ENDS products, like those enacted in England and European Union countries (Nyakutsikwa et al., 2021; Snell et al., 2021), also may be helpful in curbing the number of young adults who develop symptoms of dependence, and in turn, result in fewer young adults who use these products regularly and over a long-term.

HIGHLIGHTS.

  • No longitudinal studies track changes in ENDS dependence among young adults.

  • We examined ENDS dependence from 2014–2019, capturing the surge in vape pod sales.

  • ENDS dependence was consistent from 2014–2017, but increased from 2018–2019.

  • The increase is likely due to the late 2017 surge in sales of vape pods with nicotine salts.

Footnotes

CONFLICT OF INTEREST STATEMENT

MBH is a consultant in litigation involving the vaping industry

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Contributor Information

Alexandra Loukas, Kinesiology & Health Education, University of Texas at Austin, 2109 San Jacinto Blvd. D3700, Austin, TX 78712.

C. Nathan Marti, Kinesiology & Health Education, University of Texas at Austin, 2209 San Jacinto Blvd. D3700, Austin, TX 78712.

Melissa B. Harrell, Epidemiology, Human Genetics, and Environmental Sciences, The University of Texas Health Science Center at Houston School of Public Health, in Austin, 1616 Guadalupe Street, Suite 6.300, Austin, TX 78701.

Keryn E. Pasch, Kinesiology & Health Education, University of Texas at Austin, 2109 San Jacinto Blvd. D3700, Austin, TX 78712.

Anna V. Wilkinson, Epidemiology, Human Genetics, and Environmental Sciences, The University of Texas Health Science Center at Houston School of Public Health, in Austin, 1616 Guadalupe Street, Suite 6.300, Austin, TX 78701.

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