ADVANCEMENT OF THE SCIENCE
the substantial asthma disparities observed among Black and Native American popula- tions compared with Asian and White popu- lations in Wisconsin (Wisconsin Department of Health Services, 2020). All analyses were conducted using SAS version 9.4.
Main Characteristics of Asthma Exacerbations Among Residents From Ashland, Dodge, Forest, Grant, Sauk, Taylor, and Vilas Counties in Wisconsin TABLE 1 continued
Results
Characteristic
# (%) or Mean ± SD
Characteristics of Asthma Exacerbations
Sex
Male
604 (42.4) 820 (57.6)
A total of 1,424 asthma exacerbations were identified during the study period among residents of seven Wisconsin rural counties, including 1,267 (89%) emergency depart- ment visits, 80 (6%) hospitalizations, and 77 (5%) observation stays (Table 1). The mean age at the time of exacerbation was 35 years, and 820 (58%) of the cases were female patients. Most of the patients were White (85%), and 5% were Native American, 5% were Hispanic, 4% were Black, and <1% were Asian. A relatively even distribution of exacerbations was observed across the sea- sons, with spring (32%) having the highest proportion, followed by fall (26%), winter (22%), and summer (21%). The mean PM 2.5 level for all possible days in the study period was 6.7 µg/m 3 (±4.7), and the mean level on exacerbation days was 7.5 µg/m 3 (±4.8).
Female
County
Ashland
139 (9.8)
Dodge Forest Grant Sauk Taylor
449 (31.5)
31 (2.2)
264 (18.5) 336 (23.6)
97 (6.8)
Vilas
108 (7.6)
Unique days (total/possible)
811/1,277 (63.5)
Median exacerbations/day (range)
1 (1–8)
Mean exacerbations/day Exacerbations by season
1.8 ± 0.96
Winter (December–February)
309 (21.7) 454 (31.9) 295 (20.7) 366 (25.7) 57.3 ± 21.2
Spring (March–May)
Summer (June–August)
Fall (September–November)
Association Between PM 2.5 and Asthma Exacerbations
Mean daily maximum temperature (°F)
The number of exacerbations per AQI category of lag0–6 was compared with all days in the study period, which revealed significant dier- ences in lag0–5 ( p < .05; Table 2). There were higher percentages of moderate to unhealthy AQI levels in lag0–5 compared with the whole study period, with lag2 (2 days prior to exac- erbation) having the greatest dierence ( p = .0001). After controlling for maximum daily temperature, conditional logistic regression analysis showed that PM 2.5 levels on lag2 were significantly associated with increased hazard of asthma exacerbations (HR 1.184; 95% CI [1.051, 1.344]; Table 3). Stratified analysis found no eect modifi- cation by sex, race and ethnicity, age, season, or county of residence on lag2 (all p -values >.05; Figure 1). Associations between PM 2.5 and asthma exacerbations, however, were highest among female patients (HR 1.219; 95% CI [1.041, 1.438]); White individuals (HR 1.195; 95% CI [1.041, 1.370]); people
≥65 years (HR 1.524; 95% CI [1.105, 2.119]); exacerbations occurring in spring (HR 1.268; 95% CI [1.020, 1.568]); and residents of Ashland County (HR 1.424; 95% CI [1.062, 1.931]) and Sauk County (HR, 1.331; 95% CI [1.010, 1.757]). Discussion Our case-crossover study aimed to assess the relationship between PM 2.5 and asthma exacer- bations in rural Wisconsin. We identified cases with asthma exacerbations using hospital dis- charge data from the Wisconsin Hospital Asso- ciation among residents of seven rural coun- ties with a PM 2.5 air monitor. We used PM 2.5 data collected by the Wisconsin Department of Natural Resources as the exposure variable, and we included daily maximum temperature data from NOAA in the analysis. When the number of exacerbations per AQI category of lag0–6 was compared with
all days in the study period, significant dif- ferences were revealed in lag0–5, as the per- centage of exacerbations in the moderate to unhealthy AQI levels was significantly higher than the whole study period. After control- ling for maximum daily temperature, PM 2.5 levels on lag2 were significantly associated with an increased hazard of an asthma exac- erbation occurring. The stratified analysis on lag2 found elevated associations among female cases, White individuals, people ≥65 years, exacerbations occurring in spring, and residents of Ashland and Sauk counties. Our findings are consistent with previous studies conducted in urban areas and dem- onstrate that asthma exacerbations are more likely to occur in the days following higher PM 2.5 levels. For instance, a study by Rosser et al. (2022) found a significant association between PM 2.5 and asthma exacerbations on lag days 2 and 3. Similarly, Glad et al. (2012)
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Volume 87 • Number 6
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