Is it allergy season in Illinois?

Illinois's flat topography and vast agricultural landscape create near-ideal conditions for pollen dispersal — nothing to slow the wind, and vast acreage of grass and weed crops. Chicago sits at the convergence of Great Lakes moisture and Midwest prairie, ranking consistently in the AAFA's top 10 hardest allergy cities. The state's allergy profile is dominated by a brief but intense tree season, followed by severe grass and ragweed stretches.

Illinois's four main regions — Chicago Metro, Central Prairie, Northern IL, and Southern IL — each have distinct allergy profiles shaped by the Great Lakes, agriculture, and latitude.

Chicago Metro

Top-10 allergy city — lakefront concentration

Lake Michigan creates a natural pollen trap: wind off the lake pushes airborne pollen toward the shore and then back inland on lake breezes. The urban heat island advances tree pollen 1–2 weeks ahead of rural Illinois. Grass and ragweed seasons are severe in the suburbs, where prairie-style landscaping and agricultural land begin immediately west of the city.

Trees · Apr–May Grass · Jun–Jul Ragweed · Aug–Sep

Central Prairie

Springfield and Peoria — peak grass country

The heart of the Illinois prairie belt. Flat terrain and intensive corn and soybean farming create exceptional grass pollen dispersal. Springfield and Peoria both experience the state's highest grass pollen counts. Tree season is relatively brief — the severe months are June and July for grass, then late August–September for ragweed.

Grass · Jun–Jul (intense) Ragweed · Aug–Sep Prairie amplification

Northern Illinois

Rockford — early ragweed, compressed season

The Rockford area bridges the Great Lakes region and the open prairie. Birch pollen from surrounding woodlands is noticeable in April. The season is compressed slightly by cooler northern temperatures, but ragweed is intense — the region sits directly in the high-ragweed belt that extends into southern Wisconsin.

Birch · Apr Grass · May–Jul Ragweed · Aug–Sep

Southern Illinois

Earlier spring, longer overall season

Southern Illinois has a markedly earlier spring than Chicago — tree pollen can begin in late February near the Ohio River. The longer warm season extends both grass and ragweed duration. This region has mixed deciduous forest plus agricultural land, so tree diversity is higher and the season more layered than the central prairie.

Trees · Mar–May (earlier) Grass · May–Jul Ragweed · Sep–Oct

Each row shows a full year of pollen for one region — trees in blue, grasses in green, weeds in amber. Look for where all three rows go quiet at the same time — that's your window.

Oral Allergy Syndrome (also called pollen-food allergy syndrome) causes tingling or mild itching in the mouth when you eat certain raw foods during the relevant pollen season. The proteins in the food are similar enough to the pollen protein that your immune system cross-reacts. Cooking usually deactivates the offending protein, so the same food cooked may cause no reaction.

Oak & Birch pollen

Stone fruits & apples

Apples, peaches, cherries, plums, pears, almonds, hazelnuts, carrots, celery

Peak risk April–May in IL. Oak and birch both significant in the Chicago area and northern counties. Peeling fruit helps — most of the cross-reactive protein is in the skin.

Grass pollen

Tomatoes, potatoes & melons

Tomatoes, potatoes, kiwi, watermelon, cantaloupe, oranges (in some cases)

Peak risk May–July — especially intense in the Central Prairie belt. Illinois's extensive timothy and bluegrass agricultural land makes this one of the highest-risk states for grass OAS.

Ragweed

Melons & bananas

Watermelon, cantaloupe, honeydew, banana, zucchini, cucumber, sunflower seeds

Peak risk August–September. Illinois produces more ragweed per acre than almost any other state — Chicago and Springfield are both high-burden cities for ragweed OAS.

Not medical advice. If you suspect OAS, speak with an allergist — it can sometimes progress, and symptoms that extend beyond the mouth should be evaluated.