Why are Aspergillus and Penicillium grouped together?

Their small, round spores look nearly identical under a standard microscope, so air-sample reports usually count them together as Aspergillus/Penicillium-type.

Are these molds found indoors?

Yes. They are among the most common indoor molds, growing on damp building materials, food and dust, and they are also common outdoors around decaying plants.

What symptoms do they cause?

Congestion, sneezing, cough and asthma symptoms in sensitized people. People with weakened immune systems should talk with their doctor about Aspergillus exposure.

How can I tell if my mold problem is indoors?

Compare an indoor air sample to an outdoor sample from the same day. Indoor counts much higher than outdoor counts suggest an indoor source.

Aspergillus & Penicillium Mold Guide | Spore Counts & Symptoms

Aspergillus & Penicillium Mold Guide

Aspergillus and Penicillium spores are common indoors and outdoors. Learn how they're counted, when levels rise, allergy symptoms, and how to find local mold data.

Quick answer

Aspergillus/Penicillium-type spores are tiny round spores found both indoors and outdoors. They are a key signal when comparing indoor and outdoor mold levels.

What they look like

Very small (roughly 2–5 µm), round, smooth or lightly roughened spores, often seen singly or in short chains. Because they are so similar, labs typically report them together.

Season and conditions

Outdoors they rise with leaf decay and compost in fall. Indoors they follow moisture — leaks, high humidity and water damage — and can be present year-round.

Reducing exposure

Fix leaks and dry wet materials within 24–48 hours.
Keep indoor humidity below 50% and use bathroom and kitchen exhaust fans.
Use a HEPA filter and avoid handling compost or mulch without a mask.

Frequently asked questions

Why are Aspergillus and Penicillium grouped together?: Their small, round spores look nearly identical under a standard microscope, so air-sample reports usually count them together as Aspergillus/Penicillium-type.
Are these molds found indoors?: Yes. They are among the most common indoor molds, growing on damp building materials, food and dust, and they are also common outdoors around decaying plants.
What symptoms do they cause?: Congestion, sneezing, cough and asthma symptoms in sensitized people. People with weakened immune systems should talk with their doctor about Aspergillus exposure.
How can I tell if my mold problem is indoors?: Compare an indoor air sample to an outdoor sample from the same day. Indoor counts much higher than outdoor counts suggest an indoor source.