Treatment response is the result of a complex interaction between host factors, environmental factors and drug-related factors. Those factors were studied separately in order to obtain the best possible picture of the optimal response to inhaled corticosteroids (ICS). First, we characterized the type of lung injury of World Trade Center (WTC)-exposed firefighters, identifying individuals and their characteristics. Host factors associated with prolonged ICS-containing treatment were body mass index, lower post-9 September 2001 (9/11) spirometry and first post-9/11 modified Medical Research Council (mMRC) dyspnea score. Worse baseline forced expiratory volume in 1 second (FEV1) and respiratory symptoms are host factors that predict response to ICS. Regarding biomarkers, a higher blood eosinophil count and increase in IL- 4 were associated with an increased risk of irritant-induced airflow obstruction. Furthermore, low immunoglobulin A (IgA) soon after WTC exposure was associated with lower longitudinal FEV1 measurements, increased risk of airflow obstruction and increased antibiotic treatment. Moreover, low IgA tends to predict response to ICS. Second, the environmental factor associated with prolonged ICS/LABA treatment was high intensity WTC exposure defined by arrival morning of 9/11. Last, drug-related factors associated with the response to ICS treatment were assessed. In a longitudinal study, ICS treatment improved lung function decline in individuals with WTC-related lung injury. This improvement was more pronounced when WTC-exposed firefighters were more adherent to ICS. Pharmaceutical counseling interventions, analyzed through nation-wide pharmacy data, led to a marked improvement in the adherence of asthma patients to ICS. This also resulted in an improved controller-to-total-ratio, indicating an improvement in asthma control. In conclusion, improved adherence to ICS treatment leads to reduced lung function decline and better asthma control
