Indoor Air Quality and Mold: What You Need to Know

You spend roughly 90% of your time indoors. That's about 21 hours a day breathing air that circulates through your home, your office, your car. Most people assume that air is clean -- or at least cleaner than the air outside.

The EPA measured the opposite. Indoor air can be 2 to 5 times more polluted than outdoor air -- and occasionally more than 100 times worse. One of the biggest drivers? Mold from water damage, present in roughly half of U.S. homes. And the standard tests most inspectors use miss the worst of it entirely.

What Is Indoor Air Quality and How Does Mold Affect It

Indoor air quality (IAQ) covers the full spectrum of pollutants inside a building: chemicals from cleaning products, furniture off-gassing, cooking particles, and biological contaminants like dust mites, bacteria, and mold. While all of these can be harmful, mold from water-damaged buildings is the one worth worrying about first.

When moisture lingers for more than 24 to 48 hours, mold can start growing. And once it's established, the problem is bigger than what you can see.

The invisible threat standard tests miss

Most people picture mold as black spots on a bathroom wall. That's the visible part. The real concern is what's invisible.

When mold grows in a water-damaged building, it doesn't just release intact spores. It sheds microscopic fragments -- tiny pieces of mold cell walls, bacterial byproducts, and chemical compounds called mycotoxins. Research published in ASM Journals found that these fungal fragments can be present at concentrations up to ~300 times higher than intact spores.

In other words: for every mold spore floating through your house, there could be hundreds of invisible fragments. They can carry allergenic and inflammatory material from the original mold.

Standard air testing -- often called spore trap testing -- captures what's airborne at one moment in time and relies on microscopy to count and identify spores. It can capture some larger fragments, but it tends to undercount the smallest fragments and it doesn't directly measure mycotoxins. So a "clean" air test, by itself, doesn't guarantee the space is safe -- it may just mean the snapshot didn't catch what you're actually being exposed to.

What testing actually works

That's where dust-based testing comes in. HERTSMI-2 (Health Effects Roster of Type-Specific Formers of Mycotoxins and Inflammagens) uses DNA analysis to measure 5 specific mold species linked to water-damaged buildings and associated with harmful effects on human health. Because it tests settled dust rather than a snapshot of air, it captures what's been circulating and accumulating since the last deep cleaning of that space -- which means you get a picture of actual exposure over time, not a single moment's sample.

HERTSMI-2 gives you a score you can act on:

• Below 11: Generally safe for sensitive individuals
• 11 to 15: Borderline -- further investigation advised
• Above 15: Significant contamination detected

For a broader view, the ERMI (Environmental Relative Moldiness Index) measures DNA from 36 mold species. The EPA developed ERMI as a standardized tool for comparing contamination levels across homes using data from a national survey of 1,096 U.S. residences. You can also calculate a HERTSMI-2 score from an ERMI test. Published research has linked these HERTSMI-2 scores to real health outcomes -- the thresholds aren't arbitrary.

Why Indoor Air Quality and Mold Matter for Your Health

Poor IAQ from mold isn't just uncomfortable. It makes people sick -- and the numbers are staggering.

The health impact is larger than most people realize

According to Lawrence Berkeley National Laboratory, 21% of current asthma cases in the U.S. are attributable to dampness and mold exposure -- approximately 4.6 million people whose breathing problems trace back to the air inside their buildings.

Dampness and mold are associated with a 30 to 50% increase in respiratory illness rates. A Mayo Clinic study found that fungus is likely the cause of most chronic sinusitis cases -- affecting an estimated 37 million Americans. And research across three U.S. cities found that homes of asthmatic children consistently had higher mold contamination than homes of non-asthmatic children.

The total annual cost? An estimated $22.4 billion in healthcare costs, lost productivity, and premature mortality.

Why some people get sicker than others

About 24% of the population carries a genetic variation (HLA-DR) that prevents their immune system from recognizing or effectively clearing mold toxins. For these individuals, mold exposure triggers an inflammatory response that doesn't shut off on its own.

Most people's bodies can recognize and remove mold toxins. But roughly 1 in 4 people have an immune system that can't "tag" these toxins for disposal. The toxins stay in the body, recirculate, and keep triggering inflammation -- even after the person has left the moldy environment.

This is why one person in a household can feel fine while another develops brain fog, crushing fatigue, and chronic sinus congestion. It's also why "just moving" doesn't always resolve symptoms. Their bodies need help breaking the inflammatory cycle.

This is testable and treatable

Mold-related IAQ problems are identifiable. You can test your environment. You can measure your body's inflammatory response. And with the right approach, you can get a clear path forward. MoldCo connects you with clinician-supported testing and treatment built on decades of published research.

Common Signs of Mold-Related Air Quality Problems

How do you know if mold is degrading your air? Look at two things: your home and your body.

What your home is telling you

• A musty or earthy smell. This is the single most reliable indicator of active microbial growth. The smell comes from microbial volatile organic compounds (mVOCs) -- chemicals released by actively growing mold and bacteria. If you smell it, something is growing.
• Visible water stains. Discoloration, bubbling paint, warped baseboards, or staining on ceilings, windowsills, and under sinks can point to past or current water intrusion.
• Condensation on windows. Persistent condensation, especially in winter, signals excess indoor humidity that fuels mold growth.
• A history of flooding or leaks. Even a "small" leak under a kitchen sink can create mold growth inside wall cavities within 24 to 48 hours.

What your body might be telling you

Poor indoor air quality from mold can produce symptoms that look like a dozen other conditions. The pattern matters more than any single symptom:

• Brain fog and difficulty concentrating
• Chronic fatigue that doesn't improve with rest
• Recurring sinus congestion or post-nasal drip
• Unexplained joint or muscle aches
• Symptoms that noticeably improve when you travel or spend time away from home

People often start forming their own hypotheses about what's causing it:

"My next two hypotheses are 1. mold (my house was flooded in a hurricane and remodeled with mold remediation) or 2. hormonal imbalance... Bloodwork is within normal levels... my brain fog had been getting extremely bad; not being able to focus on a conversation, being unmotivated to start or finish projects, feeling spacey and out of it." -- Reddit user, r/BrainFog

That last point -- feeling better away from home -- is a key signal. If your symptoms lift on vacation and return when you walk through your front door, your environment deserves a closer look.

Wondering if your home's air quality could be affecting your health? MoldCo's HERTSMI-2 dust test measures the DNA of 5 specific mold species linked to water-damaged buildings -- giving you a standardized score and context that standard air tests can miss. Explore testing options.

FAQ

How accurate are DIY mold test kits from hardware stores?

DIY kits can tell you whether mold is present, but they can't tell you how much, which species, or whether the levels are meaningful. Most hardware-store DIY kits are settle plates (Petri dishes you leave open) -- they mainly capture what lands on the plate and are heavily influenced by airflow, time, and placement. Because mold exists everywhere in small amounts, a "positive" result from a settle plate is nearly meaningless without context. For anyone with health concerns, ERMI or HERTSMI-2 testing provides a standardized score you can make decisions with -- DNA-based methods that identify specific species at specific quantities.

Can poor indoor air quality cause brain fog and fatigue?

Yes. When mold degrades your indoor air quality, microscopic fragments and toxins you inhale can trigger chronic inflammation. For the 24% of people with genetic susceptibility (HLA-DR variation), this inflammation can compromise the blood-brain barrier -- the protective layer that keeps harmful substances out of your brain. Once that barrier weakens, inflammatory compounds reach brain tissue, disrupting neurotransmitter function and blood flow. The result: brain fog, difficulty concentrating, memory issues, and debilitating fatigue. If these symptoms improve when you're away from a specific building, your environment is worth investigating.

My home smells musty but I can't find mold -- how do I locate hidden mold?

Trust your nose. A musty smell is the most reliable indicator of active microbial growth, even when nothing is visible. Check common hotspots: under kitchen and bathroom sinks, behind refrigerators, in crawl spaces and basements, around window condensation areas, and inside HVAC systems. Mold often grows inside wall cavities after leaks -- completely invisible from the outside. If you can't find the source visually, a HERTSMI-2 dust test can detect hidden contamination by measuring DNA from settled dust.

If you're trying to narrow down where the contamination may be coming from, consider running multiple HERTSMI-2 tests in different areas of the home (for example, a bedroom vs. a living area) to see which zones score worse.

What is the difference between spore trap testing and HERTSMI-2?

Spore traps capture a snapshot of airborne spores at one moment. They can identify certain spores, but they tend to undercount fragments (in one study, fragments were found at up to ~300x the concentration of intact spores) and don't directly measure mycotoxins. Results are also influenced by air movement, HVAC operation, time of day, and even sampler placement and height. HERTSMI-2 tests settled dust using DNA analysis of 5 specific water-damaged mold species. Because dust accumulates over time, it provides a more reliable picture of actual contamination. It also gives you a numeric score with established thresholds (below 11 is safe, above 15 is dangerous), which means you can make a clear decision instead of guessing.

How long after leaving a moldy environment should symptoms improve?

For most people, symptoms begin improving after they leave the exposure. But for the approximately 24% with genetic susceptibility (HLA-DR variation), biotoxins remain trapped in the body. The immune system can't tag them for removal, so they keep recirculating and driving inflammation -- potentially for months or years. These individuals typically need active treatment to bind and remove trapped toxins before they'll see consistent improvement. If you've moved and still feel sick, that's not unusual -- your body may need help clearing what's already accumulated.

If indoor air quality is driving your symptoms, the worst thing you can do is wait. Every month of delay is a month of inflammation your body doesn't need.

MoldCo gives you a clear path from uncertainty to answers: environment testing, health testing, and a clinician-supported plan. Most patients pay $150 to $300 per month, all-in.

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Any health-related claims made on this site have not been evaluated by the Food and Drug Administration (FDA). The information provided on this site is not intended as a substitute for professional medical advice, diagnosis, or treatment. Always seek the advice of your physician or other qualified health provider with any questions you may have regarding a medical condition. MoldCo assumes no responsibility or liability for any errors or omissions in the content of the references, nor for any actions taken in reliance thereon.