Every spring, as pollen season arrives, many people find what should be pleasant weather transforming into a challenging time filled with sneezing, itchy eyes, and persistent congestion. It’s frustrating, to say the least.
Recent studies suggest that the pollen season is becoming longer each year, and interestingly, artificial light—specifically, the illumination from cities at night—plays a significant role in this extension.
City Lights and Pollen Season
City lights can actually prolong pollen season, keeping allergens present in the air for extended periods. This change affects the timing of allergen exposure for urban dwellers, making life more uncomfortable for many.
Dr. Lin Meng from Vanderbilt University researched the relationship between nighttime outdoor lighting and the delayed ending of pollen activity. Across Northeastern cities, it was noted that artificial light is linked to longer pollen seasons, as streetlights and other outdoor lighting disrupt the natural cycle of darkness and light that plants follow.
Dr. Meng noted, “The impact of artificial light at night on the end of the pollen season is larger than its effect on the beginning.”
Plants Follow Day Length
Plants depend on light and dark to know when to flower. They react to the length of the day, and this natural clock can be influenced by artificial lighting, which tricks plants into thinking the day is longer than it is. This can delay the cues that signal the end of their flowering period.
Even a small amount of light can disrupt these signals, particularly in areas near bright streetlights.
Why Fall Ends Later
The persistence of late-season pollen can be attributed to trees and weeds continuing their activity longer when nights are bright. Artificial light can delay natural aging processes in leaves, allowing growth to extend as well.
While warm temperatures often trigger early blooming in spring, it’s the darkness that signals dormancy in the fall. This means that artificial light can exert a considerable influence on these natural processes.
Counting Pollen in the Air
Daily pollen counts are conducted through a certified network, which samples the air and analyzes pollen grains. In this study, researchers utilized data from 2012 to 2023 collected from 12 locations in the Northeast, comparing it to satellite data on night brightness.
The results indicated a strong correlation between artificial light and pollen season length, though further research is needed for definitive conclusions.
When Exposure Turns Severe
The impact of longer seasons is felt more acutely on days with high pollen counts. The study found that in areas with artificial light at night (ALAN), 27% of seasonal days reached severe pollen levels, compared to only 17% in darker areas. Breathing in these particles can lead to histamine reactions, causing discomfort that can last hours.
The increased prevalence of high-pollen days also places a demand on healthcare facilities as individuals seek relief.
When Millions Share Exposure
A report from the CDC estimated that 25.7% of U.S. adults experienced seasonal allergies in 2021. Global studies indicate that over 80% of the population lives under light-polluted skies, particularly in densely populated neighborhoods where symptoms can persist for weeks as streetlights continue to disrupt plant cycles.
Heat and Rain Still Matter
While warmer years still move up the start of pollen season, nighttime light remains a relevant factor even when accounting for weather conditions. Warm temperatures promote faster plant growth, while rain can wash pollen away, yet the influence of artificial light persists across varying climatic circumstances.
Streetlight Choices Can Help
Lighting principles suggest using only the necessary amount of light, directing it downwards, and opting for warmer tones. Shielded fixtures can limit stray light exposure, potentially decreasing the effect on plants after dark. Timers and motion sensors can further enhance this by minimizing unnecessary nighttime brightness.
Pollen Season, Lights, and Urban Planning
City planners often select fast-growing trees for urban environments, but many of these species produce allergenic pollen. Using lower-pollen varieties near schools and residences can reduce exposure, particularly when arranging for less illumination in certain areas. While changing street lighting may not fully solve the issue, it can certainly help mitigate pollen levels impacted by urban lighting.
Overall, the findings suggest that the city’s light impacts the timing of plant activity and ultimately extends pollen exposure, resulting in more difficult allergy seasons. Future research can aim for a clearer understanding of the causes, while planners can begin to cut down on excessive nighttime lighting and monitor its health effects.
This study is detailed in the journal PNAS Nexus.
