Mold spore counts explained

A mold spore count is the number of mold spores in a cubic meter of air, and the AAAAI says the higher the count, the greater the chance that people with hay fever will have symptoms outdoors.¹ In the United States, the AAAAI’s National Allergy Bureau reports current pollen and mold spore levels from counting stations around the country.²

What airborne mold spores are

Molds are tiny fungi related to mushrooms but without stems, roots or leaves. They can be almost anywhere, including in soil, on plants and on rotting wood, and their spores float in the air much like pollen.³ There are roughly 1,000 species of mold in the United States, and many of them aren’t visible to the naked eye.⁹

Spores work like seeds: they travel through the air, and disturbing a mold source can send them airborne.¹⁰ Everyone breathes in mold spores, but some people have an allergic reaction or asthma symptoms if exposed to too much of the fungus.⁴ Not every mold causes allergy symptoms, and the most common ones that do include Alternaria, Aspergillus, Cladosporium and Penicillium.⁴ AAFA notes that many molds grow on rotting logs and fallen leaves, in compost piles, and on grasses and grains.¹⁰

How stations count mold spores

The AAAAI describes the counting instrument as usually sitting on a rooftop, where it collects spores for 24 hours before it is taken to a lab and the collected material is analyzed.² An AAAAI research summary says outdoor molds are captured mainly with Hirst-type samplers, which draw in 10 liters of air a minute and give daily results, as for pollen.⁷

A handout on running a sampling station, written by Estelle Levetin of the University of Tulsa for an AAAAI aeroallergen course, gives more detail. It calls a roof three to six stories, or 30 to 60 feet, above the ground ideal, not close to overhanging vegetation.⁵ It describes a Burkard spore trap that draws in air at 10 liters a minute onto a greased tape and samples 14.4 cubic meters of air in 24 hours.⁵ The Rotorod it describes spins rods with greased leading edges at 2,400 revolutions a minute and is generally run for 10% of the time.⁵ The handout notes that Burkard-type traps catch small particles efficiently, including small fungal spores such as those of Penicillium and Aspergillus.⁵

The handout gives 400 times magnification for reading pollen and 1,000 times, under oil immersion, for fungal spores, with the raw counts converted into a concentration per cubic meter of air.⁵ The AAAAI research summary notes that recognizing spores by their shape under the microscope takes significant expertise.⁷

Which spores are counted

Outdoor molds are numerous: between 25 and 50 different taxa, especially in spring and summer.⁷ The same research summary calls Cladosporium the most frequent mold in temperate countries.⁷ Here is how a 2021 AAAAI aeroallergen course handout on airborne fungal spores, also by Levetin, describes three of the spore types:

Spore	What the handout says
Cladosporium	Generally the most abundant airborne spore type in temperate areas; in many areas found year-round, with the highest levels from late spring through early fall
Alternaria	Frequently the second most abundant of the dry-air spores; commonly several hundred to several thousand spores per cubic meter; peak levels usually in late summer or fall
Aspergillus and Penicillium	Similar spores; the two can be told apart in a culture but not on spore trap slides

Everything in this table comes from that handout on airborne fungal spores.⁶ It also says hourly Cladosporium levels can exceed 100,000 spores per cubic meter in some areas.⁶ It groups two other spore types, ascospores and basidiospores, as spores of moist air.⁶

What raises outdoor counts

In the United States, outdoor mold spores begin to increase as temperatures rise in spring, peak in July in warmer states and October in colder ones, and can be found year-round in the South and on the West Coast.³ Unlike pollen, molds do not die with the first frost, but most outdoor molds become inactive during the winter.¹⁰ AAFA says allergy symptoms from fungus spores are most common from July to the first hard frost.¹⁰

AAFA also says some spores spread in dry, windy weather, and others spread with fog or dew when humidity is high.¹⁰ The fungal spore handout adds that dry-air spores, Cladosporium and Alternaria among them, usually peak in the afternoon, when humidity is low and wind speeds are highest.⁶ It says moist-air spores need moisture in the air, which generally limits their dispersal to periods during and after rain or times of high humidity, and that basidiospores are often most abundant in the humid hours before dawn.⁶ Raindrops striking a surface with fungi growing on it can also release dry-air spores, mainly with the first drops.⁶

Why our pollen pages show no mold level

AllergyNest’s pollen levels come from Google’s Pollen API. Google’s coverage list for the United States names three pollen types (grass, trees and weeds) and 11 plants, and no mold.¹¹ So our metro pages show no mold level, and how our index works explains what the pollen level does cover.

The AAAAI also separates counts from forecasts: it notes that pollen and mold counts measure the amount of allergens present in the air, while forecasts are often based on past pollen data and general weather forecasts.³

Indoor mold and outdoor counts

Mold is found both indoors and outdoors, and it can get into a home through open doors, windows, vents and heating and air conditioning systems, or ride in from outside on clothing, shoes, bags and pets.¹² Indoors it grows where there is moisture, such as around leaks in roofs, windows or pipes, or where there has been a flood.¹² The most common indoor molds are Cladosporium, Penicillium and Aspergillus.¹²

There is no official number to compare an indoor sample with: the EPA says no standards or threshold limit values have been set for airborne concentrations of mold or mold spores.¹⁴ The CDC does not recommend mold testing, noting that there are no set standards for an acceptable quantity of mold in a home.¹² Air sampling also gives information only for the moment the sample was taken.¹⁵ NIOSH adds that results from very short-term samples may not represent actual exposures, and that spore counts from them cannot be interpreted in relation to health risks.¹³

Where measured counts are published

The National Allergy Bureau is the nation’s only pollen and mold counting network certified by the AAAAI, and it compiles pollen and mold levels from certified stations across the country.³ An AAAAI guide to setting up an NAB account puts the network at approximately 84 counting stations in the United States, plus one in Canada and six in Argentina.⁸

The AAAAI says the relationship between pollen and mold levels and hay fever symptoms can be complex, and that symptoms may be affected by recent contact with other allergens, the amount of pollen exposure and a person’s sensitivity to pollen and mold.³ An allergist can diagnose whether mold is behind a person’s symptoms.⁴ For questions about mold allergy, talk to an allergist.