Effects of outdoor temperature and humidity on methacholine challenge tests

This study tried to evaluate whether outdoor daily temperature (T) and humidity (H) can influence methacholine test results in outpatients living in temperate climate areas. 4,723 subjects (2391 males; age 35.1±16.15; FEV1 100.36% [IQR: 92.34-108.8]) that performed a methacholine test for a suspected bronchial asthma between 2000 and 2010 were considered. Mean outdoor temperatures (°C) and relative humidity (%), registered when the test was performed, were considered. Patients with bronchial hyperresponsiveness (PD20<3200 µg) were 2,889 (61.2%) and median PD20 were 359 µg [IQR: 160-967]. On receiver operator curve (ROC) analysis, temperature and humidity did not significantly predict hyperresponsiveness (AUC: 0.5); an AUC of 0.55 for temperature was found only in subjects aged <20 and in non-smokers. When we subdivided the subjects into different sub-groups, on the basis of different levels of temperature and humidity (T<10°C or between 10 and 20°C or >20°C and H<60% or between 60% and 80% or >80%), no differences in hyperresponsiveness prevalence and in PD20 were found. A significantly positive relationship between PD20 and T mean was detected in subjects aged from 36 to 51 (r: 0.079; p: 0.031) and in those with severe hyperresponsiveness (PD20<200 µg) (r: 0.110; p: 0.001). The regression logistic model showed how the maximum temperature was a significant lower risk factor for bronchial hyperresponsiveness (OR: 0.995, 95%CI: 0.982-0.998). In conclusion, this study showed that an increase in temperature (excluding extreme values) is associated to a slight, but significant, reduction of bronchial hyperresponsiveness risk.