Short-term exposure to ambient ozone is associated with a modest increase in the odds of outpatient visits for allergic rhinitis, and that association grows stronger as temperatures rise, according to a time-stratified case-crossover study conducted at a tertiary hospital in Henan Province, China. The analysis, published in Frontiers in Public Health, drew on 43,418 outpatient visits for allergic rhinitis recorded between April and October of 2013 through 2023, and found that each 10 microgram per cubic meter increase in ozone corresponded to a 1.2 percent rise in the odds of a visit, with the estimate climbing from essentially null at cooler temperatures to a 2.1 percent increase at warmer ones.
The research team, led by investigators from Zhengzhou University and the First Affiliated Hospital of Zhengzhou University, used a time-stratified case-crossover design, in which each patient serves as their own control. For every outpatient visit, the visit date was treated as the case day, and control days were selected from the same year and calendar month, matched by day of the week. This strategy controls for long-term trends, seasonality, and day-of-week effects, as well as for time-invariant individual characteristics such as baseline health status and stable lifestyle factors. Because ozone pollution in Henan is most prominent during the warm season, the analysis was restricted to visits occurring from April through October of each year.
Exposure assessment relied on gridded environmental datasets rather than ground monitoring stations alone. Daily maximum 8-hour average ozone concentrations came from the ChinaHighAirPollutants dataset, which provides spatially continuous estimates across China at a resolution of 0.1 by 0.1 degrees, roughly 9 to 11 kilometers per grid cell in Henan. Meteorological data, including 2-meter air temperature and dew-point temperature from which relative humidity was calculated, came from the ERA5-Land reanalysis dataset produced by the European Centre for Medium-Range Weather Forecasts. Detailed residential addresses were geocoded and matched to the nearest grid cell, while records without detailed addresses were assigned county- or district-level averages. All exposures were averaged over the current and previous day, a lag 0-1 moving average intended to capture short-term cumulative effects.
In the main statistical model, ozone was entered as a linear term, and the analysis adjusted for temperature, relative humidity, and fine particulate matter (PM2.5) using natural cubic splines for the meteorological variables. The central result was an odds ratio of 1.012 per 10 microgram per cubic meter increase in ozone, with a 95 percent confidence interval of 1.006 to 1.017. Expressed on an alternative scale, an interquartile range increase in ozone of 53.4 micrograms per cubic meter was associated with an odds ratio of 1.064. Mean ozone concentrations on case days were slightly higher than on control days, 127.3 versus 124.7 micrograms per cubic meter, while PM2.5 levels were nearly identical between the two day types, at 41.4 and 41.9 micrograms per cubic meter respectively.
The study also examined how the ozone association varied across the exposure range using a non-linear spline model. The lowest predicted odds occurred at an ozone concentration of approximately 110 micrograms per cubic meter. The authors note this pattern suggests the association was more pronounced at higher ozone concentrations, particularly above roughly 150 micrograms per cubic meter, rather than increasing at a constant rate across the observed distribution.
Temperature itself showed a non-linear relationship with outpatient visits. The minimum-risk temperature in the full-sample model was estimated at 15.5 degrees Celsius. Relative to that reference, the odds of a visit rose with increasing temperature, peaked at approximately 24.3 degrees Celsius with an odds ratio of 1.336, and then declined at higher temperatures. The authors caution that this decline at very high temperatures should not be read as evidence of a protective effect; it may partly reflect behavioral adaptation, reduced outdoor activity, air-conditioning use, changes in healthcare-seeking behavior, or simply fewer observations at the hottest extremes.
The central novelty of the study lies in its test of temperature modification. When an interaction term between ozone and a spline function of temperature was added to the model, the fit improved significantly, with a likelihood-ratio chi-square of 31.2 on three degrees of freedom and a P value below 0.001. In the overall population, the ozone-related odds ratio per 10 micrograms per cubic meter was 1.000 at the 25th percentile of temperature, 1.014 at the 50th percentile, and 1.021 at the 75th percentile, with the first estimate’s confidence interval spanning the null and the latter two excluding it. Similar increasing patterns appeared in both sexes, with odds ratios at the 75th percentile of temperature of 1.020 in males and 1.022 in females.
Age-stratified results revealed the most striking heterogeneity. Among patients younger than 18 years, the ozone-related odds ratio climbed from 1.006 at the 25th percentile of temperature to 1.065 at the 75th percentile, a substantially steeper gradient than in any other group. Patients aged 18 to 64 showed a weaker increase, from 1.001 to 1.014 across the same temperature contrast, while no consistent positive pattern emerged among those aged 65 and older. A formal three-way interaction among ozone, the temperature spline, and age group was statistically significant, with a chi-square of 34.55 on six degrees of freedom and a P value below 0.001, indicating that temperature modification of the ozone association differed across age groups. The overall age heterogeneity test was also significant.
The authors offer several possible explanations for the stronger estimates among children and adolescents, emphasizing that these are interpretive considerations rather than demonstrated mechanisms. Young people have developing respiratory and immune systems, higher ventilation rates relative to body weight, and potentially more outdoor activity during the warm season, which could raise the relative inhaled dose at a given ambient concentration. Parents may also be more likely to seek medical care when children develop acute nasal symptoms, which could amplify associations observed in outpatient data. For older adults, the absence of a clear association should be interpreted cautiously: the subgroup was small, limiting statistical power, and rhinitis symptoms in older patients may be more heterogeneous, with greater contributions from non-allergic rhinitis, comorbidities, medication use, and differing healthcare-seeking patterns.
Biologically, the findings are considered plausible within the framework the authors describe. Ozone is a highly reactive oxidant gas formed through photochemical reactions involving nitrogen oxides and volatile organic compounds in sunlight, and inhaled ozone reacts with airway lining fluid and epithelial cell membranes, generating reactive oxygen species and lipid oxidation products that may trigger oxidative stress, epithelial barrier dysfunction, increased mucosal permeability, and local inflammation. Because the nasal cavity is among the first contact sites for inhaled ozone, the nasal mucosa may be particularly vulnerable. The authors also cite animal studies of allergic rhinitis models and a panel study of patients suggesting links between air pollution exposure and nasal oxidative stress, while noting these lines of evidence are supportive context rather than direct demonstrations in this study population.
Several limitations frame the interpretation. Exposure was assigned by residential address without information on personal mobility or indoor exposure, so misclassification is possible. Data on pollen, bioaerosols, viral infections, indoor allergens, and other irritants were unavailable, and because pollen is a major trigger of allergic rhinitis that varies with temperature and season, residual confounding by pollen may have partly influenced the observed temperature-dependent pattern. PM10 was excluded because it was highly correlated with PM2.5, and the gaseous co-pollutants nitrogen dioxide, sulfur dioxide, and carbon monoxide were not included in the main model. The data came from a single large hospital, which may limit generalizability. Sensitivity analyses, including alternative spline specifications, dropping PM2.5 adjustment, and restricting the period to May through September, yielded consistent results, with the restricted-period odds ratio at 1.016.
The authors suggest that as warm-season ozone pollution becomes increasingly prominent in China, health risk assessments based solely on pollutant concentrations may not fully capture high-risk conditions, and that ozone-related rhinitis visits may be more likely under higher-temperature conditions, particularly among children and adolescents. They frame these observations as support for considering ambient temperature alongside ozone levels in warm-season health warnings, while the study’s observational, single-hospital design means the estimates describe associations in this population rather than established causal effects.
Subject of Research: Temperature-dependent association between short-term ambient ozone exposure and allergic rhinitis outpatient visits in Henan, China
Article Title: Temperature-dependent association between ambient ozone exposure and allergic rhinitis-related outpatient visits: a time-stratified case-crossover study in Henan Province, China
Article References: Ding, D., Lu, W., Wang, H., Wang, L., Gao, J., Wei, T., & Liu, H. (2026). Temperature-dependent association between ambient ozone exposure and allergic rhinitis-related outpatient visits: a time-stratified case-crossover study in Henan Province, China. Frontiers in Public Health, 14, Article 1952329. https://doi.org/10.3389/fpubh.2026.1952329
Image Credits: AI Generated
DOI: 10.3389/fpubh.2026.1952329
Keywords: ozone, allergic rhinitis, air pollution, temperature, case-crossover study, outpatient visits, Henan, China, effect modification, children, warm season, public health
News Source: Russell Cooper. (October 27, 2026). Ozone Raises Allergic Rhinitis Outpatient Visits, Stronger in Warmer Weather. Scienmag.
