If you live in a big, brightly lit city and you feel like allergy season just never ends, you might be right: New research shows that light pollution prompts plants to shed pollen longer, increases the growth of notoriously allergenic ragweed and makes our bodies more prone to allergic reactions, from runny noses to asthma.<\/p>\n
The study, published in the journal PNAS Nexus<\/a>, focused on the U.S. Northeast and found that trees in cities like New York and Philadelphia start producing pollen earlier in the spring and finish later in the fall compared with places in the same region with low light pollution. The difference could add up to 130 days per year to the allergy season, the researchers found.<\/p>\n Light pollution is widespread: Almost 80 percent of North Americans can\u2019t perceive the Milky Way<\/a> because of sources like streetlights, car headlights and illuminated billboards. Beyond washing out the stars, all that light appears to change the physiology of living organisms, humans and ragweed included.<\/p>\n \u201cWe\u2019ve basically tricked plants into making decisions they wouldn\u2019t ordinarily make,\u201d Dr. Katz said.<\/p>\n Dr. Katz and his colleagues used nighttime light data from satellites and airborne pollen counts spanning more than a decade and found that, in places like New York City, the allergy season often starts before March 1, compared with darker areas like rural Connecticut, where the season usually begins a month later. In New York, the allergy season drags on until early November, while in darker places, it tends to be over by October.<\/p>\n What\u2019s more, the study found, pollen counts in light-polluted areas are classified by scientists as severe on about 27 percent of days during the pollen season, compared with roughly 17 percent in darker areas.<\/p>\n The urban heat island effect \u2014 a phenomenon in which sunbaked buildings, roads and sidewalks absorb and then radiate heat \u2014 can also stretch the allergy season. So the researchers incorporated temperature and precipitation data into their model, which allowed them to show that bright night skies had independent effects on pollination. This aligned with a 2025 scientific finding that light pollution has more influence than air temperature<\/a> in lengthening the urban growing season.<\/p>\n Lewis Ziska, a plant physiologist at Columbia University who was not involved in the study, called the findings \u201cseminal,\u201d and said they \u201cshould give us pause in deciding what tree species may be best to plant in the future to reduce pollen exposure in cities.\u201d<\/p>\n Not all plants are equally sensitive. Some, like lime trees, rely more on temperature<\/a> for pollination, while others, like plane trees, are particularly affected by light. Plane trees also have what Dr. Katz called the \u201cdubious distinction\u201d of being among the most allergenic trees in cities like New York.<\/p>\n \u201cIt was planted in vast amounts in cities because it\u2019s quite hardy and does very well in urban conditions,\u201d he said. \u201cHowever, it produces a large amount of allergenic pollen.\u201d<\/p>\n And while we shouldn\u2019t cut down plane trees, because \u201cthey provide cooling, they help with stormwater infiltration,\u201d Dr. Katz said, in the future, we should plant trees that produce less pollen and are less affected by light, like sugar maples and magnolias.<\/p>\n Ragweed, a plant that triggers allergies in up to 20 percent of Americans<\/a>, is also highly sensitive to light. And so is its natural enemy, the earthworm. Experiments have shown that earthworms, which eat ragweed seeds, stay hidden in the soil under typical streetlight conditions<\/a>. At the same time, ragweed may grow twice as high under bright skies as it does under dark ones.<\/p>\n In addition, some research has found that our bodies are more prone to allergies, and not only pollen allergies, if the night skies are bright. That may be because artificial light can trigger inflammation<\/a> and disrupt the circadian clock<\/a> upon which allergic reactions run. A recent meta-analysis found that living in light-polluted areas is associated with a 62 percent higher risk of asthma<\/a> and an 89 percent higher risk of allergic rhinitis, irrespective of air pollution.<\/p>\n \u201cIf you mess up the clock, then some of these pathways will be out of kilter,\u201d said Hannah Durrington, a professor of respiratory and circadian medicine at the University of Manchester and an author on the study on circadian rhythms.<\/p>\n The good news is, it\u2019s not an impossible problem to address.<\/p>\n Cities can make better decisions about planting trees, as Dr. Ziska noted. They can also adjust the brightness of streetlights and billboards. And Dr. Katz suggested that data on how lights change the allergy season could be used to improve pollen forecasts so that people can \u201creduce their exposures and take their allergy medications at appropriate times.\u201d<\/p>\n That way, moving to somewhere dark won\u2019t be the only option to breathe easier.<\/p>\n