An experimental approach to confirm that an occupational agent causes asthma Jean-Luc Malo, Hôpital du Sacré-Coeur de Montréal 31 July 2013 I read with great interest the article by Baur in the J Occup Med Toxicol. 2013; 8: 15. A compendium of causative agents of occupational asthma. I fear that the method of assessment used by the author, an evidence-based approach, is not appropriate. Most of the agents that cause occupational asthma have been identified by an experimental approach through specific inhalation challenge in which the subject is exposed to a control substance and the active agent with objective testing (serial assessment of airway calibre). Using such an experimental approach, even if an agent has been found to cause occupational asthma in only one subject, the evidence is sufficient to incriminate this agent in the list of agents causing occupational asthma. Many thanks. Jean-Luc Malo MD Department of Chest Medicine Hôpital du Sacré-Coeur de Montréal 5400 West, Gouin Blvd., Montréal, Canada H4J 1C5 Competing interests I have no competing interest.