What Are Common Allergy Triggers?

Outdoor Allergy Triggers: Pollen and Mold

Pollen from wind-pollinated trees, grasses, and weeds is the most prevalent outdoor allergy trigger. Tree pollen (birch, oak, cedar, maple) peaks in spring. Grass pollens (Timothy, Bermuda, Kentucky bluegrass) peak in late spring and early summer. Weed pollens — particularly ragweed, which produces up to 1 billion pollen grains per plant per season — peak in late summer and fall. A single ragweed pollen grain can travel 400 miles on wind currents.

Outdoor mold spores from Alternaria and Cladosporium species peak in late summer and fall, particularly after rainfall. Mold allergy frequently co-occurs with pollen allergy and can significantly extend the outdoor allergy season beyond pollen season.

Indoor Allergy Triggers: Dust Mites, Pets, and Mold

House dust mites are the leading cause of perennial (year-round) allergic rhinitis and asthma worldwide. These microscopic arthropods live in bedding, upholstered furniture, and carpets, feeding on shed human skin cells. Their fecal protein particles (Der p 1, Der f 1) are potent allergens that become airborne during movement and vacuuming.

Pet dander allergens — Fel d 1 from cats and Can f 1 from dogs — are lightweight proteins that remain airborne for hours and adhere to clothing, walls, and furniture. Cockroach allergens (Bla g 1, Bla g 2), particularly prevalent in urban housing, are significant asthma triggers. Indoor mold from Aspergillus and Penicillium species grows in bathrooms, basements, and areas with water damage.

Food Allergy Triggers: The Nine Major Allergens

The FDA designates nine major food allergens responsible for the vast majority of food allergic reactions: peanuts, tree nuts, milk, eggs, shellfish, finfish, wheat, soy, and sesame. These foods must be declared on US food labels. Peanuts, tree nuts, shellfish, and finfish cause the highest rates of severe anaphylaxis and fatality.

Beyond the major nine, oral allergy syndrome (pollen-food allergy syndrome) causes localized mouth itching from raw fruits and vegetables that cross-react with pollen proteins. People with birch pollen allergy often react to apples, peaches, and hazelnuts. These reactions are usually mild and symptoms are eliminated by cooking or peeling the food.

• Peanuts: legume, one of the most common causes of fatal anaphylaxis
• Tree nuts: cashews, almonds, walnuts, pecans, pistachios — each is a distinct allergen
• Shellfish: shrimp, crab, lobster, clams (tropomyosin is the major allergen)
• Milk: casein and whey are the major cow's milk allergen proteins
• Eggs: ovomucoid (heat-stable) and ovalbumin (heat-labile) are key allergens
• Wheat: omega-5 gliadin (exercise-induced), gliadins and glutenins
• Sesame: FDA-designated major allergen since January 1, 2023

Medication, Insect, and Occupational Triggers

Penicillin is the most commonly reported drug allergy, though over 90% of patients with reported penicillin allergy test negative on formal evaluation. Other common drug triggers include aspirin and NSAIDs (via COX-1 inhibition), radiocontrast media, sulfonamides, and general anesthetics. Drug allergy evaluation by an allergist can safely de-label most patients who do not have true allergy.

Insect venom allergy from bee, yellow jacket, wasp, hornet, and fire ant stings causes approximately 40–100 deaths annually in the US. Venom immunotherapy reduces anaphylaxis risk by 95% over a 3–5 year course and is strongly recommended for anyone who has had a systemic reaction to insect sting. Latex allergy is an important occupational trigger for healthcare workers.