The short-term effects of different cold spell definitions on asthma outpatient visits in Lanzhou, China

With global warming, most studies have focused on the health impacts of heat waves, while the health effects of cold spells, especially in developing countries, still need to be explored. Additionally, existing research on temperature extremes and asthma primarily targets severe asthma cases requiring hospitalization or emergency care, neglecting outpatients with mild symptoms. This study aimed to identify the optimal definition of cold spells in Lanzhou, China, and examine their association with outpatient asthma visits, identifying potentially vulnerable populations.

This study collected daily asthma outpatient records, along with meteorological and air pollutant data, from January 1, 2017, to December 31, 2020, in Lanzhou, Gansu Province. Twenty-four cold spell definitions were developed using four temperature indices (daily mean, daily minimum, daily mean apparent, and daily minimum apparent temperatures), two thresholds (P₁₀ and P₅), and three durations (2, 3, and 4 days). A time-series fitted poisson generalized linear model (PGLM) and distributed lag nonlinear model (DLNM) were applied to estimate the short-term effects of cold seasons (November to March) on outpatient asthma visits, controlling for confounding factors such as humidity, air pollutants, time trends, holidays, and weekdays. Stratified analyses by sex and age were conducted to identify vulnerable populations and examine the influence of cold spell duration on asthma clinic visits.

Various definitions of cold spells influenced asthma outpatient visits, with similar trends observed. The model fit was best when the daily minimum apparent temperature was below the 10th percentile, and the duration was more significant than or equal to 4 days. Based on this optimal definition, for the total population, the main effect of the cold spell on asthma occurred at Lag0, Lag1, Lag6, and Lag7, with Lag7 producing the most significant effect (RR = 1.208, 95% CI: 1.052 ± 1.388). In the subgroup analyses, the cumulative effect of lag 0-7 days (Lag0-7) was higher for females and those in the 0-18 age group than for males and other age groups, respectively. In addition, the longer the duration of the cold spell from lag 3 days (Lag3) onwards, the greater its effect.

Cold spells in Lanzhou City can notably increase asthma outpatient visits, with females and individuals aged 0-18 particularly affected. Moreover, the longer a cold spell persists, the greater its impact, especially in the latter days of the event.