What Is a Mold Count? The Other Number in the Air
Pollen gets the headlines, but it is not the only thing being counted in your air. Alongside every pollen reading sits a second figure that explains a different set of bad days. So what is a mold count? In one sentence: it is a measurement of how many fungal spores are suspended in a cubic metre of outdoor air — the mold counterpart to a pollen count, produced by the same stations, reported on the same pages, and driven by a completely different biology.
Key takeaways
- The AAAAI defines a mold count as the number of mold spores in a cubic metre of air; the higher the count, the greater the chance hay-fever sufferers will experience symptoms outdoors.
- The counts come from the National Allergy Bureau's certified counting-station network — the same stations that produce pollen counts, running rooftop samplers and identifying spores under a microscope.
- Spores run 2 to 100 microns across, small enough to float great distances on gentle breezes, so outdoor counts reach the thousands per cubic metre where pollen counts run into the hundreds.
- Two genera dominate outdoor air: Cladosporium, often the most abundant and strong through the warm months, and Alternaria, which builds through late summer into the autumn leaf-fall.
- There is no official threshold behind the number: the EPA does not regulate mold or mold spores in the air, so bands like Low, Moderate, and High do the interpretive work.
So, What Is a Mold Count, Technically?
The American Academy of Allergy, Asthma & Immunology's glossary defines it directly: a mold count "is based on the measurement of the number of mold spores in a cubic meter of the air," and the higher the count, the greater the chance that hay-fever sufferers will experience symptoms outdoors. The AAAAI points readers to the National Allergy Bureau (NAB) as the trusted resource for these counts — the same certified counting-station network that produces pollen counts, staffed largely by trained volunteers who run rooftop samplers and identify what lands in them under a microscope.
The units deserve a moment, because they explain the scale. Molds are fungi, and they reproduce by spores that the U.S. EPA's mold course sizes at 2 to 100 microns across — small enough, as the EPA puts it, to float through the air and be carried great distances by even the gentlest breezes. Outdoor spore counts therefore run into the thousands per cubic metre on ordinary days, where pollen counts run into the hundreds. The two regulars in outdoor counts are Cladosporium and Alternaria, genera that feed on decaying vegetation and thrive wherever plant matter rots in warm, damp air. When you see a mold row on a Pollen Atlas city page — the New Orleans pollen count page, say, in a humid Gulf spell — it is summarising this spore traffic, in band terms, the same way the pollen index summarises grains.
Why the Number Moves: Humidity, Storms, and Seasons
The EPA's course makes a second point that governs everything about mold counts: the number of spores in the air "fluctuates from season to season, day to day and even hour to hour," and spores will not grow if moisture is not present. Mold counts are weather instruments in a way pollen counts only partly are:
- Humidity feeds them. Damp stretches let outdoor fungi grow and sporulate; the count follows the moisture.
- Storms scramble them. Some spore types are released in dry, windy conditions and peak on breezy afternoons; others are discharged during and after rain. A thunderstorm can either scrub the count down or spike it, depending on which fungi dominate locally.
- Autumn piles them up. Fallen leaves and crop residue give outdoor molds an enormous food supply, which is a big part of the story in our companion post on mold allergies in fall.
Local sources feed the regional number, too. A city ringed by farmland breathes the spores off its fields at harvest; a city of old trees breathes its own leaf litter every autumn; gardens, compost, and mulch beds add their share at neighbourhood scale. None of that changes what the count is — a regional, per-cubic-metre measure taken at the station — but it explains why two neighbourhoods under the same count can feel different, and why the mold band tends to climb in the weeks when a city's vegetation is turning over fastest. Morning dew is another quiet driver: spores that settled overnight can re-aerosolise as surfaces dry and breezes pick up, giving many days a late-morning lift in the count that has nothing to do with anything you did.
This is also why mold-heavy cities cluster where the moisture is. Compare the mold readings behind the Houston pollen count and Jacksonville pollen count pages across a wet week with a desert city's, and the count behaves like a humidity map — a pattern you can watch develop on the pollen heatmap.
The Usual Suspects: The Spores Behind the Number
A mold count is a total, but the total is built from characters worth knowing, because they explain the count's seasonal personality. Two genera dominate outdoor air in most of North America:
- Cladosporium is the workhorse — often the most abundant spore type in outdoor counts, feeding on decaying leaves and vegetation, and characteristically strong through the warm months. When a summer mold reading sits stubbornly elevated for weeks, Cladosporium is usually the bulk of it.
- Alternaria is the heavyweight — its spores are much larger, it thrives on grasses, grains, and rotting plant matter, and it tends to build through late summer into the autumn leaf-fall. Alternaria is also the genus most often named in studies of spore-driven symptom days, which is why counters report it separately where they can.
Around those two orbits a supporting cast: Aspergillus and Penicillium types (familiar from indoor contexts but present outdoors too), plus the great seasonal flushes of basidiospores — mushroom and bracket-fungus spores — that rise on damp nights in the fall. The mix matters because the genera keep different hours. The dry-air spores, Cladosporium and Alternaria among them, are typically released and carried best in dry, breezy conditions and peak on those afternoons; the wet-weather spores discharge during rain and high humidity. So two consecutive days with the same mold total can be built from opposite casts — one a dry-wind day, one a rain day — and feel different to a mold-sensitive reader. This is also why a single thunderstorm can send the count either way: it washes the dry-air spores out of the sky while triggering the wet-weather ones, and which effect wins depends on the local fungi. A mold count, read properly, is a tiny weather forecast folded inside a biology report.
| Spore group | Release conditions | When they peak |
|---|---|---|
| Dry-air spores — Cladosporium and Alternaria among them | Released and carried best in dry, breezy conditions | Breezy afternoons |
| Wet-weather spores — basidiospores among them | Discharged during rain and high humidity | Damp nights, with a great seasonal flush in the fall |
What Is a Mold Count Compared With a Pollen Count — and What It Is Not
The mechanics are twins: same samplers, same stations, same grains-or-spores-per-cubic-metre mathematics as the counts explained in what is a pollen count. The meaning differs in two ways. First, pollen is seasonal by plant, while outdoor mold is present in some quantity nearly all year in mild climates — it has peaks, not a true off-season. Second, there is no official threshold behind the number. The EPA states plainly that it does not regulate mold or mold spores in the air, so a mold count is a level, not a standard: it tells you how today compares with typical days, and bands like Low, Moderate, and High do the interpretive work.
And one boundary worth drawing sharply, because the keyword splits here: an outdoor mold count says nothing about the air inside your home. A high outdoor count does not mean your walls have a problem, and a low one does not clear a musty basement. Indoor mold is a moisture story specific to a building — the EPA's guidance centres on finding and fixing the water source — whereas the count on a city page is about the weather-like behaviour of spores across a region. Two different questions; don't let one number answer both.
Frequently Asked Questions
What is a mold count measured in?
Mold spores per cubic metre of air. The AAAAI's glossary defines the count as "the measurement of the number of mold spores in a cubic meter of the air," produced by certified counting stations coordinated by the National Allergy Bureau.
Why does the mold count swing so much from day to day?
Because it is a weather instrument. The EPA notes the number of spores in the air fluctuates from season to season, day to day, and even hour to hour — humidity lets outdoor fungi grow and sporulate, and a thunderstorm can either scrub the count down or spike it, depending on which fungi dominate locally.
Is there an official safe level for a mold count?
No. The EPA states plainly that it does not regulate mold or mold spores in the air, so a mold count is a level, not a standard: it tells you how today compares with typical days, and bands like Low, Moderate, and High do the interpretive work.
Does a high outdoor mold count mean there is mold in my home?
No — the two are different questions. An outdoor count describes the weather-like behaviour of spores across a region, while indoor mold is a moisture story specific to a building, and the EPA's guidance centres on finding and fixing the water source.
If outdoor mold seems to track your symptoms — worse on humid, leaf-littered weeks, quieter in dry cold snaps — bring that pattern to a specialist: you can find an allergist through the Pollen Atlas directory. This article is general information only and is not medical advice. And to see today's mold reading alongside the pollen index for your city, with free 5-day forecasts worldwide, head to PollenAtlas.com — where what is a mold count stops being a mystery line and becomes a number you can plan around.