🧫 Petri Dishes: The Mood Rings of Mold Testing

Here’s an uncomfortable truth from someone who’s spent decades investigating indoor air quality issues:

You know those DIY Petri dish mold test kits you see everywhere? They’re about as scientifically valid as a mood ring. Worse yet, they aren’t just useless—they’re actively harmful.

These kits—also known as gravity plates or settling plates—are little more than glorified middle school science fair experiments. They occupy a special place in our industry, which we call “junk science.” That’s when a scientific tool or principle is misused in a way that misleads and confuses. Unfortunately, that’s true for nearly all at-home kits.

And that’s exactly why the GOT MOLD? test kit exists. We got sick and tired of seeing people waste their money—or worse, be taken for a ride, which happens all too often.

Scientific Nonsense

Let’s break down why these kits fail the basic requirements of scientific testing:

No Outside Sample = No Context

Imagine trying to assess your cardiac health after a stress test without knowing your resting heart rate and blood pressure. Without that baseline, you’d have no idea whether your numbers were normal or concerning. That’s exactly what DIY Petri dishes do with mold. They purport to measure mold spores inside your home without taking outdoor levels into consideration, completely ignoring the bigger picture.

Mold spores are normal in nearly every environment — indoors and out — but their types and quantities vary depending on weather, location, season, and time of day. Without an outdoor sample for context, these tests leave you with results that exist in a vacuum, making it impossible to determine what’s truly “normal” or “abnormal” in your home. In other words, presence does not equal a problem.

False Positives aka The Most Dangerous Kind of Wrong

Ready for another analogy? These tests are like a car alarm that goes off every time a leaf falls on it. No one is stealing your car, but your alarm might make you think the threat is constant. Petri dish tests are almost always “positive” because mold spores exist everywhere. This creates unnecessary fear and can lead to expensive and unnecessary interventions. For the average consumer, false positives are particularly dangerous because they often trigger expensive and disruptive actions that a professional would never recommend without more information.

Testing for “Toxic Mold”?

Here’s something the kit manufacturers won’t tell you: most molds will never grow on these plates. Research shows that nearly 80% of species go undetected using this method, even when sampling is done by professionals. The molds that do show up are usually the fast-growing, common and abundant background molds—not the ones you’re probably worried about.

If you’re concerned about “toxic mold” or “black mold” like Stachybotrys or Chaetomium — species often associated with water damage and heightened health risks — these kits are the worst way to test for them. Why? Because those molds don’t culture well on store-bought Petri dishes. You’ll miss the very ones you’re trying to find. Detecting these problem molds, known as “water damage indicators,” requires an entirely different approach. Using a off-the-shelf gravity plate for this is like fly fishing for sharks—you’re using the wrong tool for the wrong job.

The Psychology of Fear & Confirmation Bias

Petri dish mold tests prey on two common human vulnerabilities: the fear of the invisible and the need for validation. When people suspect they have a mold problem — especially if they’re dealing with unexplained symptoms that others dismiss — they often seek out information and solutions that confirm their concerns.

It’s a perfect setup for confirmation bias: “See? I knew something was wrong!” This can lead to a counterproductive and often costly rabbit hole of questionable solutions — electronic air cleaning devices, expensive “detox” protocols, unnecessary mycotoxin tests, eye-poppingly pricey remediations, and sometimes even repeated relocations — often without any improvement to health.

To be clear, we’re not saying you don’t have a mold problem or that your symptoms aren’t real. But when a test yields a positive result nearly 100% of the time, you have to question its value—and the motivations of those who recommend it. After all, they almost always have something much more expensive to sell. A broken clock is right twice a day, but that doesn’t make it an accurate or reliable way to tell time.

We see the same dynamic at play with a popular dust test called ERMI, which has similar shortcomings.

Building Science vs. Test-First Mentality

Here’s what really grinds my gears: Relying on these junk science kits completely ignores the most fundamental principle of mold investigation—finding the source of moisture.

Every mold problem begins and ends with moisture. Period.

Instead of wasting time and money on unreliable products, start with your senses and ask these critical questions:

•Do you see, smell, or feel something unusual?

•Is there a musty odor, occasionally or persistently?

•Is there a history of leaks, floods, or water intrusion?

•Are there visual signs of moisture, like stains, discoloration, condensation, or visible mold?

•Does anyone in the household feel better when they leave the building?

If you answered YES to any of these, chances are you have a mold issue.

Your next step isn’t testing—it’s locating the moisture source. Check for leaky pipes, excessive humidity, remnants of past floods, or any other signs of water intrusion, and fix them immediately.

Only after addressing the moisture should you assess the extent of the mold and decide what needs to be removed or remediated. This is where reliable testing methods come into play, not before.

The Professional Perspective

Want to hear something telling? Not one reputable indoor air quality professional I know uses Petri dish sampling anymore. They are so 1999, especially in this age of DNA-based analysis. Yet this outdated method, and others like it, persist in the consumer market. Why? Because they’re cheap to produce and great at generating false alarms that drive further sales. 

The Industry’s Self-Contradicting Logic

Here’s where it gets really rich: Some companies selling these kits cite in their marketing materials government agencies like OSHA and the EPA, and professional organizations like the American Conference of Governmental Industrial Hygienists (ACGIH) as proof of efficacy. But these organizations don’t promote Petri dish usage. In fact, they explicitly recommend against them! It’s like name-dropping Weight Watchers to sell a doughnut diet.

What’s more, many of these same companies often include disclaimers stating that they can’t assess health risks due to a lack of regulatory standards, but then provide arbitrary “health effect” tables to bolster their claims. Talk about having your cake and eating it, too! This recent excerpt from a Immunolytics report is a perfect example:

A Better Approach

Don’t rely on unreliable tests to start your mold investigation. Instead, begin with a systematic self-inspection of your space. To support this practical, common-sense approach, we’ve created a free eBook, How To Find Mold, which includes professional guidance, inspection checklists, and tips for identifying problem areas.

Focus on finding actual evidence of a moisture issue — past or present — since moisture is the root of every mold issue. Document what you find, and trust your senses. If you decide to test, choose methods that provide actionable, reliable data from reputable sources, not fear-driven false positives from questionable products.

Good science isn’t about confirming your suspicions—it’s about uncovering the truth. Don’t let the allure of quick answers lead you down a fraught, expensive, and ultimately ineffective path. With the right approach, you can find real, sustainable solutions.

The Bottom Line

Petri dishes are the snake oil of the mold industry. They promote anxiety and confusion without delivering actual answers. They’re scientifically invalid, misleading, and can delay proper investigation while commonly triggering hasty, unnecessary, and expensive interventions. It’s a slippery slope of misinterpretation and misinformation, often resulting in a destructive cycle of throwing good money after bad. 

In a time when truth has become a loosely defined term, it’s hard to know who to trust, but just because something gets repeated enough, doesn’t make it true. 

Be your own advocate. Ask the hard questions. Do the research. And learn to trust your gut.

In the world of mold investigation, there are no shortcuts and no silver bullets.

Start with the basics:  Tune into your senses. If you see something, smell something or feel something, do something. Find the source(s) of moisture and fix it. And when testing your home, don’t fall for junk science.

Your health and your wallet will thank you.

P.S. Our next piece in this theme is on the mycotoxin urine panels which are so popular these days. Stay tuned!