[Effects of Temperature and Relative Humidity on the Number of Outpatients with Chronic Obstructive Pulmonary Disease and Their Interaction Effect in Lanzhou, China]

Objective: To understand the relationships of daily average temperature and relative humidity with outpatient visit frequency of patients with chronic obstructive pulmonary disease, and whether temperature and relative humidity have a lag effect.

Methods: The effects of daily average temperature, relative humidity, and their interaction in Lanzhou between January 2013 and December 2017 on the outpatient visit frequency of chronic obstructive pulmonary disease patients were analyzed using Poisson generalized linear regression model combined with distributed lag non-linear model.

Results: There was a non-linear relationship between the daily average temperature and the outpatient visit frequency of chronic obstructive pulmonary disease patients. Between -12 °C and -8 °C, the outpatient visit frequency increased gradually with the decrease of the daily average temperature, and the outpatient visit frequency of chronic obstructive pulmonary disease patients increased by 11.60% per 1 °C of temperature drop. The daily average relative humidity also presented a non-linear effect on the outpatient visit frequency chronic obstructive pulmonary disease patients. When the daily average relative humidity was in the range of 15%-28%, the outpatient visit frequency increased gradually with the decrease of relative humidity, and the outpatient visit frequency of COPD patients increased by 37.05% for every 1% decrease of relative humidity. A synergistic effect was found between air temperature and relative humidity on chronic obstructive pulmonary disease, that is, under different relative humidity, the effect of air temperature was different. When the daily average relative humidity ≤ 50% and the daily average temperature≤11 °C, the effect of air temperature was the most obvious. For every 1 °C drop in temperature, the daily out-patient visit frequency of the whole population increased by 12.68% (5.62% in males and 7.56% in females; 5.24% in population < 65 years and 14.74% in population ≥ 65 years). When the daily average relative humidity > 50% and the daily average temperature ≤ 11 °C, the daily outpatient visit frequency of the whole population increased by 9.00% for every 1 °C drop in temperature (< 65 years, 7.11%; ≥65 years, 10.93%). When the daily average temperature > 11 °C, the temperature had no effect on the daily outpatient visit frequency of chronic obstructive pulmonary disease patients under different relative humidity.

Conclusion: The presence of a certain extent of interaction is observed between daily average temperature and relative humidity. Low-temperature and dry environment (relative humidity ≤50%, temperature ≤11 °C) as well as low-temperature and high-humidity environment (relative humidity > 50%, temperature ≤11 °C) can both increase the risk of outpatient visit in chronic obstructive pulmonary disease patients.