If your allergies feel worse than they used to, you’re not imagining it. About one in four U.S. adults now has a diagnosed seasonal allergy, and pollen seasons across North America have grown longer and more intense over the past three decades. The combination of shifting climate patterns, higher carbon dioxide levels, air pollution, and indoor triggers means more people are exposed to more allergens for more of the year.
Pollen Seasons Are Longer and Heavier
Between 1990 and 2018, pollen seasons across North America started roughly 20 days earlier and lasted about 8 days longer than they had at the start of that period. Total annual pollen counts rose by nearly 21% over those same years, with spring pollen (February through May) increasing by a similar margin. That’s not a subtle shift. It means weeks of additional exposure each year, and more pollen in the air on any given day during peak season.
Tree pollen typically leads the year, peaking in early to mid-spring depending on your region. Grass pollen follows in late spring and summer. Weed pollen, especially ragweed, dominates late summer through fall. In warmer climates like the southern U.S., overlapping seasons from species like oak, pine, rye grass, and ragweed can create near-continuous exposure from winter through autumn.
Why Rising CO2 Makes Pollen Worse
Plants use carbon dioxide to grow, and atmospheric CO2 levels have been climbing steadily. This directly affects how much pollen individual plants produce. In controlled greenhouse experiments, ragweed grown at double the normal CO2 concentration produced 61% more pollen per plant. That’s a dramatic increase from a single species, and ragweed is already one of the most potent allergy triggers in North America.
Higher CO2 doesn’t just increase pollen volume. Warmer temperatures push flowering seasons earlier and extend them later, compounding the effect. The result is more pollen, released over a longer window, from plants that are growing more vigorously than they did a generation ago.
City Air Makes Pollen More Potent
Living in a city doesn’t protect you from pollen, and it may actually make your reaction worse. Urban areas trap heat (a phenomenon called the urban heat island effect), which can extend local pollination seasons beyond what surrounding rural areas experience. But the bigger issue is air pollution.
Traffic-related pollutants like ozone and nitrogen dioxide don’t just irritate your airways on their own. They interact with pollen grains and appear to increase the allergenicity of certain species, including birch. Pollen that’s been exposed to these pollutants may carry more potent allergenic proteins on its surface. When you combine that chemically altered pollen with already-inflamed airways from breathing polluted air, the allergic response can be significantly amplified.
Thunderstorms Can Trigger Sudden Attacks
If you’ve noticed your allergies flare dramatically during or just before a thunderstorm, there’s a specific reason. Strong updrafts pull pollen grains high into the atmosphere, where humidity, lightning, and turbulent winds cause them to burst apart. A single pollen grain can release up to a thousand tiny allergenic fragments, called sub-pollen particles, that are small enough to bypass your nose and throat and settle deep into your lungs.
This can trigger what’s known as thunderstorm asthma, a sudden wave of severe respiratory symptoms that affects people with pollen allergies who may not normally have asthma. Large-scale thunderstorm asthma events have sent hundreds of people to emergency rooms in a single evening. If your symptoms seem to spike with storm systems, this mechanism is likely the explanation.
Indoor Allergens That Persist Year-Round
Pollen isn’t the only culprit. Dust mites, microscopic relatives of ticks and spiders, thrive in warm, humid indoor environments. They feed on shed skin cells and concentrate in bedding, upholstered furniture, and carpeting. Symptoms from dust mite allergy often feel worse at night or during cleaning, when their allergens get stirred into the air. Ongoing exposure causes repeated sneezing, a persistently stuffy nose, facial pressure, and can worsen eczema or trigger asthma attacks.
Mold spores are another year-round indoor trigger, especially in damp bathrooms, basements, and buildings with poor ventilation. If your symptoms never fully clear up between pollen seasons, indoor allergens are a likely contributor.
Why More People Are Allergic Than Before
The increase in allergies isn’t only about more pollen. The rate of allergic disease itself has risen in developed countries over the past several decades, and one leading explanation focuses on how modern environments shape immune development early in life.
A newborn’s immune system needs exposure to a range of bacteria and other microbes to learn how to calibrate its responses. Certain bacterial molecules act like training signals, flipping molecular switches on immune cells that help them distinguish genuine threats from harmless substances like pollen or pet dander. In very clean household environments, common in developed countries, children may not encounter enough of these microbial signals during the critical window when their immune systems are maturing. The result is an immune system more likely to overreact to harmless proteins, which is exactly what an allergy is.
Epidemiological studies support this pattern: allergic diseases and asthma are more common in homes with lower levels of bacterial exposure. Children raised on farms, around animals, or in larger families with more microbial diversity tend to develop fewer allergies. This doesn’t mean dirt prevents allergies in a simple way, but it does help explain why allergy rates keep climbing even as hygiene and healthcare improve.
The Compounding Effect
No single factor explains why your allergies are bad right now. What’s changed is that multiple forces are pushing in the same direction simultaneously. Pollen seasons start earlier and last longer. Individual plants produce more pollen because of rising CO2. Air pollution makes that pollen more irritating to your airways. Indoor allergens provide year-round baseline exposure. And a growing share of the population has immune systems primed to overreact in the first place. Each factor amplifies the others, which is why so many people feel like their allergies are getting progressively worse each year. They probably are.