Your Mold Spore Count Came Back High: Now What?

Air sample mold reports tend to look alarming. They're usually full of unfamiliar Latin names, color-coded charts, and counts that look big to anyone who's never seen one before. But interpreting them well requires comparing the right numbers to the right baseline — which most people don't do.

Step 1: Always compare indoor to outdoor

A reputable mold air sample collects an outdoor control on the same day, in the same conditions. The fundamental question isn't 'how high is my indoor count' — it's 'how does my indoor count compare to outdoors.' Indoors should be lower than outdoors and have a similar species mix. If indoors is higher, or if certain species are present indoors but not outdoors, that's the actual signal.

Step 2: Look at species, not just totals

• Cladosporium and Alternaria — common outdoor species, almost always present, usually fine

• Aspergillus and Penicillium — frequent indoor problem species, especially in damp areas

• Stachybotrys and Chaetomium — rare in outdoor air; if elevated indoors, you have an active wet building material

• Basidiospores and Ascospores — outdoor wood-rot fungi, normal in country settings

Step 3: Read the lab's interpretation, not random Internet charts

Mold air sample counts have no universal 'safe' threshold the way water has MCLs. The IICRC, AIHA, and ACGIH have all declined to publish enforceable indoor limits because the variation is too high. The lab's own interpretation, written by a professional, is more useful than any list of numbers you'll find online.

Step 4: Decide what to do

1. If indoors looks similar to outdoors and there's no Stachybotrys or Chaetomium, you're likely fine — focus on humidity control

2. If indoor totals are 2x outdoor or higher, look for hidden moisture sources (HVAC, behind walls, under floors)

3. If problem species (Stachybotrys, Chaetomium) are present indoors but not outdoors, hire a remediator

4. If you're symptomatic regardless of counts, treat the symptoms as data and investigate further

What not to do

Don't fog the house with antimicrobial spray, don't run an ozone generator, and don't replace your HVAC because of one air sample. Mold problems are moisture problems. Find the moisture, dry it out, fix the leak, and re-test in 30 days. The answer is almost always boring and structural — not chemical.