TABLE 2.

Epidemiological studies of e-cigarette use and respiratory disorders

First author [reference]	Subjects n (age group)	E-cigarette measure	Respiratory measure	Covariates	Findings	Smoking control	Additive effects	Interaction
Wang [43]	39 747 (≥18 years)	30-day use	Ever Dx with COPD by doctor or nurse	Demographics, smoking, CHD	aOR=1.53 for current exclusive e-cigarette users	Yes	Yes	n.a.
Wills [44]	8085 (≥18 years)	Ever-use, current use	Ever Dx with COPD by doctor, nurse, other health professional	Demographics, smoking, SHS, BMI, stress	aOR=2.58 for whole sample, 2.98 for nonsmokers	Yes	Yes	Inverse#
Strong [47]	32 320 (≥18 years)	Current established user (cigarette or e-cigarette)	Ever Dx by doctor, other health professional with COPD, chronic bronchitis, asthma or emphysema	Demographics, other tobacco product use, marijuana use	aOR=1.39 for solo e-cigarette users, aOR=2.07 for dual users	Yes	Yes	Equal¶.+
Perez [48]	2727 for case– control (18–64 years)	Current e-cigarette user	Ever Dx by health professional with bronchitis, emphysema or COPD	SHS, BMI, other tobacco product use, health measures	aOR=1.47 for total sample, aOR=2.94 for nonsmokers	Yes	n.a.	Inverse#
Li [49]	28 171 (≥18 years)	Current established e-cigarette user	Wheezing, whistling, coughing, past 12 months (7 items)	Demographics, BMI, SHS, asthma, mental/physical health	Solo e-cigarette users at more risk than nonusers, aORs 1.37–1.78; for dual users, aORs 2.32–3.58	Yes	Yes	n.a.
Osei [50]	705 159 (≥18 years)	Current e-cigarette use	Ever Dx by health professional with emphysema, bronchitis or COPD	Demographics, poverty status	aOR=1.75 for all cases, aOR=2.64 for never-smoker/daily user	Yes	Yes	Inverse#
Hedman [51]	6519 and 23 753 (20–75 years)	Use daily or sometimes	Long-standing cough, phlegm or wheeze in past 3 months, 12 months	Demographics, age, survey	For any respiratory symptom, aOR=1.46 for exclusive e-cigarette users; aOR=4.03 for dual users	Yes	Yes	n.a.
Bhatta [46]	23 760 (18–65 years)	Ever-use, current use	Dx by doctor, other health professional with emphysema, bronchitis or COPD ever (wave 1), past 12 months (wave 2, wave 3)	Demographics, smoking, poverty status, clinical variables	aOR=1.29 for incident respiratory disease for current e-cigarette use at wave 1	Yes	Yes	n.a.
Bowler [52]	3536 smokers (45–80 years)	Ever-use, current (monthly, weekly, daily) use	Repeated measures of spirometry, bronchitis, COPD exacerbations	Demographics, smoking, baseline clinical variables	E-cigarette users had more bronchitis, more COPD exacerbations, decline in lung function over time	Yes	Yes	n.a.

Dx: diagnosed; CHD: coronary heart disease; aOR: adjusted odds ratio; n.a.: not available (data not available or test not performed); SHS: second-hand smoke exposure; BMI: body mass index. ^#: effects of e-cigarettes greater among nonsmokers; ^¶: effect of e-cigarettes equal in smokers and nonsmokers; ⁺: in this study, interactions of marijuana with e-cigarette use were tested.