[1] Palo Alto Med Fdn, Res Inst, Palo Alto, CA 94301 USA
[2] Harvard Med Sch, Massachusetts Gen Hosp, Dept Emergency Med, Boston, MA USA
[3] Stanford Univ, Sch Med, Dept Med, Stanford, CA 94305 USA
[4] Oregon Hlth & Sci Univ, Pulm Crit & Care Med, Portland, OR 97201 USA
[5] San Francisco Med Ctr, Permanente Med Grp, Dept Allergy Asthma & Immunol, San Francisco, CA USA
[6] Univ Illinois, Sch Publ Hlth, Dept Hlth Policy & Adm, Chicago, IL 60607 USA