The main finding of this study was that environmental irritants are often triggering factors in chronic unexplained cough and cough sensitivity to inhaled capsaicin was higher in chemical-sensitive patients than in non-sensitive patients. We also found that 22 (69%) of the patients in the chemical-sensitive group had a positive capsaicin inhalation test, and met the diagnostic criteria for SHR [12, 14]. In the non-sensitive group, only 3 of 7 patients (43%) had a positive capsaicin inhalation test. Further, in comparison to previous studies the results indicate higher capsaicin sensitivity in the non-sensitive group than in earlier tested healthy controls [28, 29], but the present non-sensitive group is too small to draw any major conclusions. The study included twice as many women as men, and the study results are in accordance with previous reports of women being over-represented in cough clinics  and being more sensitive to inhaled capsaicin [30, 31].
A total of 13 patients had been excluded from participating because they had recovered from their cough. Eleven belonged to the non-sensitive group, constituting 48% of the non-sensitive individuals. This indicates a possibility of recognising a group of cough patients who have symptoms induced by environmental irritants and who are at risk of having the problems last for several years. However, although increasing evidence suggests environmental irritants are important factors in chronic cough [10, 32–34], larger groups of patients need to be studied to find out whether chemical sensitivity is essential for long-lasting symptoms to occur.
The use of two devices, the Pari Boy and Maxin MA3 could be perceived as a limitation. However, an earlier study showed that the Pari Boy and the Maxin MA3 device, which we used in the present study, can be used interchangeably to estimate levels of neural sensory reactivity, and there was good agreement between the cough results of capsaicin with the two devices . Each provocation method had also a good ability to distinguish patients with SHR from healthy controls, although the Maxin MA3 device showed even higher degree of discriminative ability between patients with SHR and healthy control [11, 15].
The capsaicin inhalation test is non-specific because of the huge variation in capsaicin cough sensitivity among healthy individuals and in the different conditions affecting the airways [18, 35]. The patients in the present study were carefully examined and other causes of cough were excluded. To avoid bias, international guidelines recommend capsaicin concentrations be given randomly, and that saline be randomly interspersed between incremental concentrations of capsaicin . Previous studies have, however, shown that the order in which capsaicin concentrations are given is of importance for the cough outcome of capsaicin provocations [14, 18] and that in patients with SHR even inhalation of saline induced coughing [11, 37]; these findings influenced our choice of method, with first saline and then low and high concentration of capsaicin being given. Furthermore the participants had not previously been tested with capsaicin and they were not told that the capsaicin concentrations would increase during the provocation. In our experience, the capsaicin inhalation test used herein with tidal breathing is a stable and reproducible method, and represents an objective test to measure sensory reactivity in the airways. The limits used for a positive capsaicin inhalation test were in accordance with those used previous studies in patients with SHR [12, 14].
Researchers have sharply disagreed on the cause of chronic unexplained cough [38, 39], which included post-nasal drip syndrome  and GERD [40, 41]. During the last decade, however, a more common view seems to have developed  and the post-nasal drip syndrome is now often replaced with a more general description, ‘upper airway cough syndrome’ . The upper and lower airways are viewed as being closely related and complementing each other with regard to reflexes [44–46]. This close connection is also evident in regard to chronic cough. The cough hypersensitivity syndrome is a new paradigm that accounts for unexplained cough and includes several groups of chronic cough patients, both those with symptoms that may indicate a reflux disease and those with a general hypersensitivity towards, for example, environmental irritants [32, 33, 47]. The current study demonstrates that an association exists between the upper airways and chronic cough, because many patients also had rhinitis symptoms after the capsaicin provocations. It also shows a general airway hypersensitivity linked to increased capsaicin cough sensitivity.