Chicago Mold Count: How a Weekday Number Became a Local Institution
Few cities talk about mold the way Chicago talks about the Chicago mold count. For decades, a weekday count produced by a Chicago-area hospital's allergy department has been carried in local weather coverage, quoted on morning broadcasts, and treated by residents as part of the forecast — as routine as the lake breeze or the chance of storms. The Chicago mold count endures because outdoor mold in the Midwest is not a minor footnote to pollen season; on humid summer and fall days it can be the dominant airborne allergen in the region.
Understanding the method helps you read any mold number — Chicago's included — with the right expectations.
Key takeaways
- The Chicago mold count is a measured count, not a model: trained counters identify spores collected from the air, following the station methods of the AAAAI National Allergy Bureau.
- Pollen Atlas data shows Chicago's mold pressure typically builds through summer and peaks in late summer and fall, when warm, humid air and decaying plant material coincide.
- Mold is reported in spores per cubic metre on the NAB scale, a different unit and a different scale from pollen grains — the AAAAI National Allergy Bureau publishes both.
- Unlike pollen, outdoor mold in the Midwest does not end at first frost as abruptly; the CDC's allergens overview notes that some airborne allergens circulate across much of the year depending on region and weather.
What the Chicago mold count actually measures
The count measures fungal spores in outdoor air. Where pollen comes from flowering plants, spores are the reproductive particles of fungi — released by molds growing on soil, leaf litter, compost, crops, and decaying wood. A single summer day's air over Chicago can contain spores from many genera at once, and counters group them into categories that matter for allergy, including Cladosporium, Alternaria, and the spore types released after rain. Genera differ in season and behaviour: some thrive on dry, windy afternoons, others surge during and after wet weather.
That mix is why a mold count adds information a pollen count cannot. In Chicago, Illinois, tree pollen dominates spring and ragweed dominates early fall, but mold runs underneath both and often outlasts them, remaining a factor on mild days well into autumn. One genus in particular, Alternaria, often drives the late-summer peaks.
Spores are not pollen, and the numbers are not interchangeable
Spore counts run far higher numerically than pollen counts, because spores are produced in enormous quantities. A mold reading in the tens of thousands can be a routine humid day, while a pollen reading a fraction of that size would be extreme. Comparing the two numbers directly — or comparing a mold count from one service with a pollen index from another — misleads. Each has its own scale, its own thresholds, and its own season, which is why Pollen Atlas reports mold-influenced days within its city framework rather than folding everything into one figure.
How a Chicago mold count is produced
The traditional method is physical sampling plus microscopy. A sampling device on a rooftop draws in air or exposes a collection surface over a set period, trapping particles from a known volume of air. A trained counter then examines the sample under a microscope, identifies spores by shape and structure, tallies them by category, and converts the tally into a concentration — spores per cubic metre of air. The explainer on how pollen counts are measured walks through the same family of methods used for pollen grains.
Chicago's long-running count has historically followed that weekday rhythm: samples collected and read on weekdays, with the result published for the morning audience. Weekend weather still happens, of course, but the published tradition reflects the working pattern of a hospital-based counting station staffed by people, not sensors. It is a craft process — accuracy depends on the counter's training and consistency, which is precisely why the NAB certifies stations and standardises technique.
From rooftop sampler to morning report
Two consequences follow from the method. First, a measured count describes the previous sampling period, not a prediction: it tells you what was in the air, in remarkable taxonomic detail. Second, it is hyperlocal in origin — one sampler, one rooftop — yet spores travel so freely that a city count usefully represents a broad area on most days. Nearby Midwest cities such as Minneapolis, Minnesota and Columbus, Ohio share much of Chicago's mold calendar, even though their daily numbers differ.
What Chicago mold count levels mean
The National Allergy Bureau expresses mold on a standard scale, and Chicago-area reporting has long used those bands. The thresholds refer to total outdoor spores per cubic metre:
| NAB mold level | Spores per cubic metre | How a Chicago day tends to feel |
|---|---|---|
| Low | Below 6,500 | Typical of cool, dry, or winter days |
| Moderate | 6,500 to 12,999 | Common through much of summer |
| High | 13,000 to 49,999 | Humid stretches, harvest and leaf-fall periods |
| Very high | 50,000 and above | Peak late-summer and fall surges |
Thresholds, seasons, and the Midwest pattern
Season shapes the odds of landing in each band. Winter cold suppresses most outdoor fungal growth around Chicago, and counts sit low for months. Spring thaw and warming soil restart the cycle; summer humidity pushes counts into the moderate and high bands for long stretches; and fall adds fuel, as crops are harvested and leaves decay. The guide to mold allergies in fall explains why that season catches so many people off guard.
Rain complicates the picture in both directions. A downpour can briefly clear the air, yet the wet days that follow encourage spore release, and some spore types are actively discharged during rain itself. A single day's band is a snapshot; the run of days around it tells the truer story.
A counted slide versus modelled data
Most digital services now model mold rather than count it, estimating levels from weather, season, and land cover. Models offer clear advantages: daily coverage everywhere, weekends included, and a forecast for tomorrow rather than a report on yesterday. What they give up is the counted slide's specificity — the genus-by-genus detail a microscopist produces. Pollen Atlas takes the modelled route for scale, publishing city pages for the whole country and folding mold into each city's daily picture alongside tree, grass, and weed indexes.
The two approaches agree more often than they disagree on the question residents actually ask — is today a high-mold day? Comparing Chicago with the national city rankings adds context a single-station count cannot: whether the whole Great Lakes region is spiking together or Chicago is having a local event. The Asthma and Allergy Foundation of America folds pollen data into its annual city research, a reminder that measured counts still anchor the field even as models spread.
Using both traditions well
Treat the historic count as the region's reference instrument and the daily city page as your planning tool. If fall symptoms recur every year and you want to know whether mold is your trigger, the Pollen Atlas Find an Allergist directory lists specialists by location. This article is general information about a counting tradition and a data pattern, not medical advice.
Frequently Asked Questions
What is a high Chicago mold count?
On the National Allergy Bureau scale, high means 13,000 to 49,999 spores per cubic metre, and very high means 50,000 or more — levels Chicago typically reaches in humid late-summer and fall stretches.
Why is the Chicago mold count reported on weekdays?
It is reported on weekdays because the long-running count is produced by people: a hospital-based counting team collects and reads samples on working days, following a tradition local weather coverage has carried for decades.
What time of year is mold worst in Chicago?
Mold is worst in Chicago in late summer and fall, when heat, humidity, ripening crops, and decaying leaves combine; winter cold brings the lowest outdoor counts.
Is a mold count the same as a pollen count?
No. A mold count tallies fungal spores and uses its own scale and thresholds, while a pollen count tallies plant pollen grains — the two measure different particles and peak in different patterns.
Sources
AAAAI National Allergy Bureau counting standards and mold scale (accessed 2026); Centers for Disease Control and Prevention, Allergens and Pollen (accessed 2026); Asthma and Allergy Foundation of America, Allergy Capitals reports (2025–2026).
Conclusion
The Chicago mold count has earned its place in the city's weather culture: a hand-made, weekday measurement, produced the same careful way for decades, in a region where mold genuinely shapes the allergy year. Knowing how it is counted — sampler, slide, microscope, spores per cubic metre — makes its levels easy to interpret and easy to compare with the modelled city data now available everywhere. Either way, the Chicago mold count tells the same story: in the Midwest, the air's fungal load deserves the same attention as its pollen.
Pollen checks you daily. Check it back — free counts and 5-day forecasts, worldwide: PollenAtlas.com