Seasonal allergies are caused by your immune system overreacting to airborne pollen from trees, grasses, and weeds, as well as outdoor mold spores. About one in four U.S. adults (25.2%) has a diagnosed seasonal allergy, making it one of the most common chronic conditions in the country. The trigger isn’t the pollen itself but your body’s misguided immune response to it, treating harmless plant proteins as dangerous invaders and flooding your system with histamine and other inflammatory chemicals.
How Your Immune System Creates the Problem
Pollen grains are tiny, lightweight particles that plants release to reproduce. They’re biologically harmless. But in people with seasonal allergies, the immune system tags specific pollen proteins as threats. The first time you’re exposed, your body produces antibodies against those proteins. Every subsequent exposure triggers those antibodies to activate specialized cells that release histamine into your nasal passages, eyes, and airways.
Histamine is what causes the familiar symptoms: sneezing, itchy and watery eyes, nasal congestion, and a runny nose. It dilates blood vessels and increases mucus production as part of an inflammatory response designed to flush out pathogens. The problem is there’s no actual pathogen to flush out, just pollen.
The Three Waves of Pollen Season
Seasonal allergies follow a predictable calendar tied to which plants are pollinating. Most people are allergic to one or two types of pollen rather than all of them, which is why your symptoms may flare at specific times of year.
Spring (trees): Tree pollen is the first wave, starting as early as February in warmer climates and running through May. Birch, oak, and cedar are among the most allergenic species. Cedar can even pollinate during winter months, causing what some regions call “cedar fever.”
Late spring and summer (grasses): Grass pollen takes over from late spring through summer. The worst offenders include Bermuda, Kentucky bluegrass, fescue, Johnson grass, and Bahia grass. Grass pollen counts peak on warm, breezy days.
Fall (weeds): Ragweed dominates the fall allergy season and is the single most common trigger for autumn symptoms. A single ragweed plant can release up to a billion pollen grains in one season. Other allergenic weeds include cocklebur, lamb’s quarters, and burning bush.
Outdoor mold spores add a fourth layer. They begin increasing in spring as temperatures rise, peak in July in warmer states and October in colder ones, and persist year-round in the South and along the West Coast.
Why Some People Get Allergies and Others Don’t
Genetics plays the biggest role in determining whether you’ll develop seasonal allergies. If both of your parents have hay fever and pollen allergies, your risk rises substantially. One large study found that 37.5% of people in the highest-risk group (both parents with hay fever and pollen allergy) had allergic rhinitis. If neither parent has allergies, your odds drop significantly, though they don’t reach zero.
But genetics alone doesn’t explain the rapid rise in allergy rates over the past several decades. That’s where the environment comes in.
The Role of Early Childhood Exposure
In 1989, a British researcher named David Strachan noticed something curious: children from larger families were less likely to develop hay fever. He proposed that early exposure to infections, passed along by older siblings through “unhygienic contact,” was somehow protective. This became known as the hygiene hypothesis.
The idea has evolved considerably since then. Research on children raised on farms found roughly a sixfold reduction in hay fever prevalence compared to non-farm children, suggesting that broad microbial exposure in early life helps calibrate the immune system. Children in families practicing stricter hygiene habits (more frequent hand washing, antiseptic use) showed slightly higher rates of allergic conditions. In one striking example, infants whose pacifiers were washed with antiseptic had double the risk of food allergy compared to those whose pacifiers were cleaned less aggressively.
The underlying biology is still debated. The prevailing thinking is that early microbial exposure trains the immune system to distinguish genuine threats from harmless substances like pollen. Without that training, the immune system is more likely to overreact.
Why Allergy Seasons Are Getting Worse
If your allergies feel worse than they did a decade ago, you’re probably right. Rising temperatures and higher carbon dioxide levels are making pollen seasons longer and more intense. Warmer springs cause trees to flower earlier, extending the overall pollen window. Higher CO2 concentrations act as a fertilizer for plants, boosting pollen production.
The scale of this effect is notable. In controlled experiments, ragweed grown under CO2 levels projected for later this century produced 230% to 272% more of its primary allergenic protein compared to plants grown at current CO2 levels. That means not just more pollen in the air but more potent pollen.
Warmer temperatures are also shifting the geographic range of allergenic plants. Species that once couldn’t survive in cooler northern climates are expanding their territory, introducing new allergens to populations that haven’t encountered them before.
Cities Can Make It Worse
Urban environments tend to intensify seasonal allergies through what’s known as the heat island effect. Cities run warmer than surrounding rural areas because of pavement, buildings, and reduced tree canopy. That extra warmth causes trees to flower earlier in cities than in nearby rural zones. Research in Detroit found that oak flowering dates were almost perfectly predicted by February nighttime temperatures, and warmer urban pockets saw trees pollinating days ahead of cooler areas.
This creates an extended, staggered pollen season across a single metro area. Trees in the warmest parts of a city release pollen first, followed by trees in cooler neighborhoods, stretching out total exposure time. Air pollution common in cities can also irritate airways independently, making allergic reactions feel more severe.
How Pollen Counts Are Measured
Local pollen counts, reported in grains per cubic meter of air, can help you plan your day. The thresholds differ by pollen type because some are more potent than others:
- Tree pollen: Low is 1 to 14, moderate is 15 to 89, high is 90 to 1,499, and very high is above 1,500.
- Grass pollen: Low is 1 to 4, moderate is 5 to 19, high is 20 to 199, and very high is above 200.
- Weed pollen: Low is 1 to 9, moderate is 10 to 49, high is 50 to 499, and very high is above 500.
- Mold spores: Low is under 6,500, moderate is 6,500 to 12,999, high is 13,000 to 49,999, and very high is above 50,000.
Most weather apps and websites report daily pollen counts by category. On high or very high days, limiting outdoor time during peak hours (typically mid-morning through early afternoon) and keeping windows closed can meaningfully reduce your exposure. Showering and changing clothes after spending time outdoors helps remove pollen you’ve carried inside.
What Triggers a Late-Onset Allergy
Seasonal allergies can develop at any age, not just in childhood. Moving to a new region exposes you to plant species your immune system hasn’t encountered before. After a year or two of repeated exposure, your body can begin producing antibodies against local pollen, and symptoms appear seemingly out of nowhere. This is why people who never had allergies as kids sometimes develop them in their 20s, 30s, or later after relocating.
Hormonal changes, particularly during pregnancy or menopause, can also shift immune function enough to trigger new allergic responses or worsen existing ones. And because pollen seasons are getting longer and more intense, cumulative lifetime exposure is increasing for everyone, potentially pushing people past the threshold where their immune system begins to react.