The air carries a surprisingly wide range of allergens, from plant pollen and mold spores outdoors to pet dander, dust mite proteins, and cockroach particles indoors. What’s actually floating around you depends on the season, your location, and whether you’re inside or outside. Understanding the specific culprits can help you figure out what’s triggering your symptoms and how to reduce your exposure.
Pollen: The Biggest Outdoor Allergen
Pollen is the most common airborne allergen, and it comes in three major waves throughout the year: tree pollen in spring, grass pollen in summer, and weed pollen in late summer through fall. Trees can start releasing pollen as early as January in southern states and as late as May or June in the north. The trees most likely to cause problems include oak, elm, hickory, pecan, sycamore, walnut, and catalpa.
Grass pollen picks up in summer, with six species responsible for most allergic reactions: Bermuda grass, Johnson grass, Kentucky bluegrass, orchard grass, sweet vernal grass, and Timothy grass. Then from August through November, weeds take over. Ragweed is by far the worst offender, with pollen levels typically peaking in mid-September across much of the country. Other prolific weed pollen producers include sagebrush, pigweed, lamb’s quarters, plantain, and sheep sorrel.
These seasons are also getting longer. A 2021 study found that human-caused warming extended North American pollen seasons by an average of 20 days between 1990 and 2018. The freeze-free growing season has lengthened in 87% of major U.S. cities since 1970, giving plants more time to grow and release pollen earlier in spring and later into fall.
Mold Spores
Mold releases microscopic spores into the air year-round, though outdoor counts spike in warm, humid months. Common outdoor mold genera that trigger allergies include Alternaria and Cladosporium, which thrive on decaying leaves, compost piles, and soil. Indoors, mold grows in bathrooms, basements, kitchens, and anywhere moisture accumulates. Unlike pollen, mold doesn’t follow a neat seasonal schedule. A damp house can produce airborne spores in the middle of winter.
Indoor Allergens That Float in Your Home
Outdoor allergens get the most attention, but several potent allergens are generated inside your home and stay suspended in the air, especially when disturbed by foot traffic, vacuuming, or air circulation.
Dust mites are one of the most significant indoor triggers. They live in mattresses, pillows, carpets, and upholstered furniture, feeding on shed skin cells. The allergens they produce are digestive enzyme proteins found in their droppings. These particles are light enough to become airborne when bedding is shaken or carpet is disturbed, then inhaled before they settle.
Cat allergens come primarily from proteins in a cat’s skin oil glands and saliva, not the fur itself. These proteins are extremely small, sticky, and persistent. They cling to walls, clothing, and furniture, and have been detected in buildings where cats have never lived, carried in on people’s clothes. Dog allergens work similarly, originating from skin cells and saliva, though they tend to cause slightly less widespread sensitization than cat allergens.
Cockroach allergens are a major trigger in urban housing. Proteins from cockroach saliva, droppings, and decomposing body parts become airborne and are a well-documented cause of asthma, particularly in children. Mouse and rat allergens, primarily proteins found in urine and skin, have been detected in air samples from urban homes as well.
How Weather Makes Airborne Allergens Worse
Weather doesn’t just carry allergens around. It can actually break them into smaller, more dangerous particles. During thunderstorms, moisture causes pollen grains to absorb water and rupture. A single grass pollen grain can release roughly 700 sub-pollen particles when it bursts. These fragments are small enough to penetrate deep into the lungs, which intact pollen grains are too large to do. This is the mechanism behind “thunderstorm asthma,” where emergency room visits for breathing problems spike during or just after storms in pollen season.
Strong winds also play a role. When gusts blow pollen against hard surfaces like buildings or pavement, the mechanical friction breaks grains apart, producing higher concentrations of these tiny respirable fragments. Dry, windy days are generally the worst for pollen exposure, while rain washes pollen out of the air temporarily, though the moisture can trigger the rupturing effect described above.
When Airborne Pollen Triggers Food Reactions
If you’re allergic to airborne pollen, you may also react to certain raw fruits and vegetables. This happens because proteins in some foods are structurally similar to pollen proteins, and your immune system can’t tell them apart. The result is typically itching or tingling in the mouth and throat.
- Birch pollen allergy: reactions to pitted fruits (cherries, peaches, plums), carrots, peanuts, almonds, and hazelnuts
- Grass pollen allergy: reactions to peaches, celery, tomatoes, melons, and oranges
- Ragweed allergy: reactions to bananas, cucumbers, melons, and zucchini
Cooking these foods typically eliminates the reaction, because heat breaks down the proteins your immune system is confusing with pollen.
How to Check What’s in Your Air Right Now
Pollen and mold counts are tracked daily by monitoring stations across the country and reported on sites like pollen.com and local weather forecasts. These counts are measured in grains per cubic meter of air and rated on a low-to-high scale. Checking counts in the morning can help you decide whether to keep windows closed or limit time outdoors.
For indoor allergens, there’s no simple daily count to check. The most practical approach is reducing the conditions that produce them: using allergen-proof mattress and pillow covers for dust mites, running a dehumidifier to keep indoor humidity below 50% to discourage mold, and using HEPA filters in vacuums and air purifiers. Pet allergens are particularly difficult to eliminate because of how small and sticky the particles are. Even with regular cleaning, homes with cats or dogs will have measurable levels of airborne pet allergens.
If you’re unsure which airborne allergen is causing your symptoms, skin prick testing or blood tests can identify specific sensitivities, which makes avoidance strategies far more targeted than guessing based on the calendar alone.