“It is well established that extreme heat events are on the rise, and this trend is projected to continue in response to changing climate,” explained Amir Sapkota,PhD, associate professor in the School of Public Health’s Maryland Institute for Applied Environmental Health and senior author of the study published in the Journal of Allergy and Clinical Immunology in Practice. “Our study is the first to provide evidence of how such increases in extreme heat events contribute to allergic diseases such as hay fever at a national level.”
The study, “Exposure to extreme heat events is associated with increased hay fever prevalence among nationally representative sample of US adults: 1997-2013,” linked National Health Interview Survey Data (1997-2013) with extreme heat event data. The research team, which included five UMD researchers as well as scientists from the Centers for Disease Control and Johns Hopkins University, identified extreme heat events during 1997-2013 at the county level by comparing the daily maximum temperature (Tmax) to the county and calendar month specific thresholds (95th percentile of Tmax values) that were calculated based on 30 years of baseline data (1960-1989).
The research team used statistical analysis to investigate the association between increased frequency of extreme heat events and hay fever prevalence, taking into account other factors such as age, race/ethnicity, sex, educational level, poverty status, and urban/rural residence. They found that adults in the highest quartile of exposure to extreme heat events had a seven percent increase in hay fever compared to those in the lowest quartile of exposure.
“While the exact mechanisms by which long-term exposures to extreme heat events increase the risk of hay fever remain unclear, one potential explanation is changes in plant phenology,” said Crystal R. Upperman, PhD, lead author of the study. “Higher frequency of extreme heat events, particularly in the winter and spring may lead to a longer pollen season as warmer temperatures contribute to earlier onset of greening and flowering of plants -- including trees-- which are major sources of pollen.”
Researchers found the timing of extreme heat events also made a difference in hay fever rates. The association between exposure to extreme heat events and hay fever was more pronounced for extreme heat events that occurred during the spring season, with evidence of a clear exposure-response relationship. This was not the case for extreme heat events that occurred during the fall season.
Sapkota and Upperman say the seven percent increase in prevalence of hay fever is modest, but they argue it could have significant implications for millions of people in the United States who suffer from allergy, including decreased quality of life and additional medical expenses.
According to Raghu Murtugudde, a leading UMD Earth system scientist and study co-author, U.S. chronic disease burdens like asthma and other respiratory diseases can cost tens of billions per year in terms costs of treatment and of lost labor. "Climate change and atmospheric emissions directly play into changes in vegetation and pollen combining with heat and pollution to exacerbate these morbidities. Thus this [study] is another cautionary tale on the need to factor in such impacts on health as we evolve new emission and climate policies for the country," Murtuggude said. Read Murtugudde's LiveScience climate op-eds here.
The study was funded by National Institute of Environmental Health Sciences (NIEHS) grant 1R21ES021422-01A1 to researchers Chengsheng Jiang (UMD SPH), Frank C. Curriero (JHU), and Amir Sapkota (UMD SPH). Publication was also made possible by U.S. Environmental Protection Agency (EPA) Science To Achieve Results (STAR) grant (F13B20312) to Crystal Romeo Upperman (UMD SPH).