Inclusion Criteria:  E-Cigarette group:  21 years of age or older Used e-cigarettes (≥3 times/week for ≥3 months)  Non e-cigarette group  ● 21 years of age or older  Exclusion Criteria:  20 years old and younger Use of cigarettes for 15 days or more in the past 60 days Use of other tobacco products (cigars, hookah, smokeless) weekly or more frequently in the past 60 days Use of marijuana or any illicit or prescription drugs for non-medical use weekly or more frequently in the past 60 days Known allergy to propylene glycol or vegetable glycerin Evidence of cardiovascular, pulmonary, renal, hepatic, metabolic, or cerebral diseases Disorder or use of medication that affects cardiopulmonary health Evidence of pregnancy or current nursing