* Abbreviations: BAC — : benzalkonium chloride NaCl — : sodium chloride UD — : unit dose Benzalkonium chloride (BAC), a quaternary ammonium compound, is a common antimicrobial agent used in many everyday household products. It kills on contact most bacteria, fungi, and some viruses, with a relatively long duration of action. It has surfactant qualities and can penetrate bilipid membrane layers, allowing for deeper penetration of medicines delivered dermally. In high concentrations (>7.5%), BAC can be caustic and, if ingested, can cause severe gastrointestinal injury, respiratory depression, and neurologic toxicity.1 Even at lower concentrations, BAC can be a skin, eye, and oral mucosa irritant. However, when used as a preservative in medicinals, BAC is so dilute that such effects are usually not an issue. Children’s airways have a limited repertoire of physiologic mechanisms (eg, edema, inflammation, and bronchoconstriction) to deal with exogenous chemicals, resulting in coughing, wheezing, and difficulty breathing. Introducing any chemical into a child’s airways should entail careful scrutiny, weighing both risks and benefits. For many years, BAC has been employed as a preservative in some inhalant albuterol solutions used in the management of wheezing in asthma despite previous reports of its ability to precipitate bronchospasm.2–5 In this month’s issue of Pediatrics , Pertzborn et al6 further investigate the risk … Address correspondence to Alan D. Woolf, MD, MPH, Pediatric Environmental Health Center, Boston Children’s Hospital, 300 Longwood Ave, Boston, MA 02115. E-mail: alan.woolf{at}childrens.harvard.edu