Spring allergies are caused by your immune system overreacting to tree and grass pollen that becomes airborne as plants begin their reproductive cycle in warmer weather. The core problem isn’t the pollen itself, which is harmless, but a case of mistaken identity: your body treats these tiny grains as dangerous invaders and launches an inflammatory response that produces sneezing, congestion, itchy eyes, and a runny nose.
How Your Immune System Creates Allergy Symptoms
When you inhale pollen, it lands on the moist lining of your nasal passages and dissolves. If you’re allergic, your immune system flags those pollen proteins as threats. White blood cells called helper T lymphocytes activate and signal other immune cells to produce a specific type of antibody designed to recognize that exact pollen protein. These antibodies then attach to the surface of mast cells, which are packed throughout your nasal tissue, eyes, and airways, essentially arming them like tiny land mines.
The next time you breathe in the same pollen, the proteins latch onto those waiting antibodies and trigger the mast cells to burst open, releasing histamine and other inflammatory chemicals. Histamine is what causes the classic symptoms: it dilates blood vessels in your nasal lining (congestion), triggers mucus production (runny nose), and irritates nerve endings (sneezing, itching). This whole reaction can happen within minutes of exposure, which is why stepping outside on a high-pollen day can bring symptoms on fast.
Which Pollens Peak in Spring
Spring allergy season is a relay race between different types of pollen. Trees go first, followed by grasses, and the specific species depend on where you live.
In the continental United States and southern Canada, oak and cypress family trees produce the most abundant allergenic pollen. Regional patterns vary: Washington state is dominated by cypress and alder pollen, California by elm and olive, Texas by oak and cypress. In the Northeast and Midwest, tree pollen can start as early as February. In central and northern Europe, hazel and alder pollinate from December through April, followed by birch, which is one of the most potent allergenic trees on the continent. In Mediterranean countries, olive and cypress pollen are the primary triggers. Japan’s spring allergy season is driven almost entirely by Japanese cedar, which pollinates from February onward, followed by Japanese cypress through May.
Grass pollen overlaps with the tail end of tree season, typically ramping up in May and June across most of the U.S. In the Northwest, grass pollen peaks from May through July. This overlap means many people experience a continuous stretch of symptoms from early spring well into summer, sometimes without realizing the trigger has shifted from trees to grasses.
Why Some People Get Allergies and Others Don’t
Genetics plays a major role. Early research found that 48% of people sensitized to common environmental allergens had a family history of the same, compared to just 15% of non-sensitized people. Heritability estimates for allergic rhinitis (the clinical name for hay fever) run as high as 91%, meaning your genes account for most of the variation in who develops it. If both your parents have allergies, your odds are substantially higher than the general population’s.
That said, genes set the stage but don’t guarantee the show. Environmental factors, particularly how much pollen you’re exposed to during early life and what infections you encounter as a child, influence whether your genetic predisposition actually turns into active allergies. This is why some people develop spring allergies for the first time in adulthood after moving to a new region with different pollen types.
How Weather Shapes Your Worst Days
Not every spring day is equally miserable. Weather conditions dramatically affect how much pollen reaches your nose on any given afternoon.
Wind is the biggest amplifier. It carries pollen over long distances and stirs up grains that have settled on surfaces. Warm, dry, breezy days are peak misery for tree and grass pollen. Rain provides temporary relief by washing pollen out of the air, but it also promotes plant growth that increases pollen production in the following days. A rainstorm followed by sunshine and wind is a particularly bad combination.
Humidity adds a twist. High humidity can cause pollen grains to absorb moisture, swell, and burst open, releasing smaller fragments that penetrate deeper into your lungs and airways. Low humidity dries pollen out, making grains lighter and easier for wind to carry. Either extreme can worsen symptoms, though through different mechanisms. Grass pollen concentrations tend to be highest when conditions are warm and dry, while weed pollen (more of a late-summer problem) thrives in cool, moist conditions.
Pollen Counts and What the Numbers Mean
Pollen counts measure how many grains are present in a cubic meter of air. The thresholds differ by pollen type because some are more potent than others. For tree pollen, a count under 15 is considered low risk, 15 to 89 is moderate, 90 to 1,499 is high, and anything above 1,500 is very high. Grass pollen triggers symptoms at much lower concentrations: anything above 20 grains per cubic meter is considered high, and above 200 is very high.
These numbers explain why tree pollen season can feel relentless. Trees release enormous volumes of pollen, and counts in the hundreds or even thousands are common during peak weeks. Grass produces less total pollen but is highly allergenic at lower concentrations, so a “moderate” grass pollen day can feel just as bad as a “high” tree pollen day.
Pollen Seasons Are Getting Longer
If your allergies feel worse than they used to, you’re probably not imagining it. Analysis of roughly 30 years of historical data shows that pollen season now starts about 20 days earlier and lasts about 8 days longer than it did a generation ago. Warmer winters cause trees to begin pollinating sooner, and longer growing seasons extend the window for grass pollen as well. Higher concentrations of carbon dioxide in the atmosphere also stimulate plants to produce more pollen per season, so it’s not just about timing. The total pollen load is increasing too.
When Pollen Makes Certain Foods a Problem
If raw apples, cherries, or carrots make your mouth tingle or itch during spring, it’s not a coincidence. The proteins in certain fruits and vegetables are structurally similar to pollen proteins, so your immune system reacts to them too. This cross-reactivity, called pollen food allergy syndrome, affects a significant number of people with spring pollen allergies.
Birch pollen cross-reacts with apples, almonds, carrots, celery, cherries, hazelnuts, kiwi, peaches, pears, and plums. Grass pollen cross-reacts with celery, melons, oranges, peaches, and tomatoes. The reaction is usually limited to itching or tingling in the mouth and throat, and cooking the food typically eliminates the problem because heat breaks down the proteins responsible. Symptoms tend to be worst during pollen season, when your immune system is already on high alert.