[1] SW Texas State Univ, Div Pulm & Crit Care Med, Dallas, TX 75390 USA
[2] Natl Jewish Med & Res Ctr, Dept Med, Div Environm & Occupat Hlth Sci, Denver, CO 80206 USA
[3] Univ Colorado, Hlth Sci Ctr, Dept Prevent Med & Biometr, Denver, CO 80262 USA
[4] Univ Colorado, Hlth Sci Ctr, Div Pulm Sci & Crit Care Med, Dept Med, Denver, CO 80262 USA