Dr Mao pointed out that haze should be the main cause of unexplained cough. In some patients with chronic cough, no cause is identified, leading to the diagnosis of idiopathic cough or unexplained cough, which is often associated with an increased response to tussive agents such as capsaicin. Common cold or influenza are often triggers of chronic cough (including the common causes and unexplained cough) in our experience. It is reported that air pollution exposure clearly exacerbates preexisting bronchial asthma,4 and sulfur dioxide exposure increases cough response and airway inflammation in guinea pigs.5 However, there is no study to identify that air pollutants are able to cause chronic cough or increase cough sensitivity in humans directly so far. As pointed out in our article, it is still unclear whether unexplained cough represents a single entity or includes certain undiscovered causes of chronic cough. We consider that air pollutants may contribute to persistent cough, but it is unknown whether haze is a main cause of the unexplained cough.