by Ryan Taylor

Settle (gravity) plate sampling, sometimes called “petri dish sampling” is a common, low-cost method marketed to homeowners and practitioners as a way to “test the air for mold.”

In practice, settle plates can be useful for demonstrating that biological material is depositing in a space and for flagging grossly abnormal indoor conditions. However, the method has fundamental physical and microbiological limitations that prevent it from functioning as a defensible exposure assessment tool, a reliable indicator of hidden water damage, or a basis for remediation recommendations or clearance decisions.

This paper explains what gravity plates actually measure, why they behave the way they do, and how those realities lead to important conclusions about their value and limitations that are supported by physics, microbiology, research and experiential application.

The goal is not to discredit the sampling method outright, but to define its appropriate role within a professional or DIY indoor air quality (IAQ) assessment.

1. Introduction

Gravity plates, also called settle plates, are Petri dishes containing agar growth media (usually a malt extract agar or MEA) that are opened to the indoor environment for a defined period (often 30–60 minutes), then allowing the sample to incubate, usually for 3-7 days. During exposure, airborne particles settle onto the agar surface by gravity and airflow dynamics.

After the plate is closed and incubated, viable microorganisms that are both capable of growing on that media and growing under those incubation conditions will form visible colonies (World Health Organization [WHO], 2009). Settle plates are appealing because they are inexpensive, readily available, simple in concept, and visually persuasive. A plate covered in colonies feels like direct proof of “mold in the air,” while a plate with little or no growth feels reassuring.

The challenge is that the method of collection does not measure the same thing that occupants experience (inhalation exposure), and it selectively amplifies some organisms while missing others. If those constraints are not understood, the test results are easily over-interpreted or mis-interpreted.

A scientifically defensible interpretation begins with a clear statement of what the method measures:

• A gravity plate is a deposition-and-growth test. It primarily reflects (1) what particles deposit onto the plate surface during the exposure window and (2) what organisms successfully grow under the chosen media and incubation conditions.
• Gravity plates are different from contact plates, which seem similar but are used to sample a surface directly instead of allowing airborne propagules to fall passively onto the sample media.

That is not the same as measuring the concentration of respirable bioaerosols in the breathing zone, nor is it the

same as diagnosing the presence or absence of an indoor mold source.

2. What Gravity Plates Measure (And Why That Matters)

2.1 Deposition is not the same as exposure

Most indoor mold exposure discussions center on what is inhaled. And while more involved conversations, especially those in the functional medicine space, also address contact exposure, mycotoxins, mVOCs, Beta Glucans, and co-contaminants like endotoxins and actinomycetes, the conversation as it relates to the expressed uses of culturable gravity plates also focus on inhalable mold particulates.

Inhalation depends on what remains suspended in air at breathing height, the particle size distribution, ventilation systems, movement and activity patterns, and the occupant’s time spent in the space. Gravity plates, in contrast, largely capture what settles to the surface during the exposure period. Settling is strongly influenced by particle size and aerodynamic behavior. Larger and heavier particles settle more readily, while smaller, buoyant particles can remain suspended for long periods and may not deposit efficiently onto a plate.

This is one of the key reasons settle plates often overrepresent certain outdoor-associated and larger-spored genera (e.g., many dematiaceous molds) and underrepresent small-spored or fragment-dominated exposures. The practical consequence is that gravity plates can show substantial growth in spaces where deposition is high without necessarily indicating high inhalation exposure. They can also show minimal growth in spaces where inhalation-relevant particles are present but do not deposit well or do not grow well on the chosen media.

2.2 Growth on Agar is a selective filter with inherent bias

A colony on an agar plate requires multiple conditions to be met:

1. A particle containing a viable propagule (i.e. spore or hyphae) must land on the plate.
2. That propagule must germinate under the incubation temperature and humidity.
3. The organism must be physiologically suited to the nutrients and inhibitors (if any) in the media.
4. The organism must compete successfully against other organisms present.

These are powerful filters, which have both advantages and disadvantages, but they also create inherent sampling bias. Many organisms that matter in the built environment are present as fragments, nonviable spores, or stressed propagules, none of which will readily propagate and form colonies. Even viable spores can fail to grow if the specific media, temperature, or competition is unfavorable (Sharma & Pandey, 2010).

The result is that settle plates do not provide a comprehensive picture of what is in the breathable air on their own. They provide only a partial picture of what successfully settled in that exact space and could grow.

3. Strengths and Appropriate Uses

3.1 Accessibility and Rapid Engagement

Gravity plates lower the barrier to entry for testing. For a homeowner who is uncertain whether an odor or visible staining warrants attention, a settle plate can be a first step that prompts curiosity and motivates action. In professional practice, that can have value because indoor environmental problems often persist or expand due to delay, denial, or uncertainty. Having easily accessible testing methods available at a moment’s notice, while imperfect, is often better than nothing.

The reasoning here is practical: a tool that people will actually use, and which does not require professional training or expertise, can sometimes move a situation from inaction to investigation. That does not make the tool analytically strong, but it can make it operationally useful in the earliest stage of awareness.

3.2 Flagging Grossly Abnormal Conditions

In some environments, microbial deposition can be so elevated that a settle plate becomes heavily colonized quickly. When a plate is rapidly overgrown, especially when overgrown with numerous colony forming units and many distinct fungal genera, it is reasonable to conclude that the environment is experiencing unusually heavy deposition of viable propagules and/or the sampling was performed during a high-disturbance event.

This is most supportive when the result is extreme and consistent across multiple plates and exposures, and when there is a plausible physical explanation (e.g., active disturbance of contaminated materials, obvious visible growth, active or recent water damage, very poor filtration, soiled ventilation systems, or poor housekeeping).

The logic is simple: while the method is biased, it is still responsive to large changes. In other words, gravity plates are poor at precision, but sometimes adequate at detecting an indoor air quality signal when the signal is very strong.

3.3 Education and Communication Value

Settle plates can help occupants understand that indoor microbial ecology is real and dynamic. The visible emergence of colonies on the sample media provides an intuitive demonstration that air and dust contain biological material that we cannot otherwise see with the naked eye.

Professionally, this can be useful in several contexts, for example, to explain why dust control matters, to show how cleaning methods influence resuspension, or to demonstrate how filtration and ventilation affect what deposits in a space.

The key is to frame the plate as an educational demonstration, and not a definitive diagnosis.

3.4 Limited Qualitative Trend Observation Under Controlled Conditions

In tightly controlled settings, like pharmaceutical settings, settle plates can be used for qualitative trending. Trend analyses require the same location, same height, same exposure time, similar occupancy/activity conditions, similar season, and consistent handling/incubation. This can provide insight into fluctuations in the environment over time, or can even be used to create a control when studying alterations like the effects of air filtration devices, the effects of opening and closing windows, the impact of chemicals on the indoor microbial environment, and more (PharmaState Academy, 2017; Booth, 2021; Parenteral Drug Association, 2014).

Even then, the trend is not purely “mold level.” It is a trend in deposition-and-growth under those conditions. In DIY applications, these controls are rarely if ever maintained and almost never documented, which is why trend claims in the DIY space are generally unreliable.

4. Limitations and Their Underlying Reasoning

4.1 Lack of a Known Sampled Air Volume

A core requirement of exposure-oriented air sampling is knowing the volume of air sampled so that results can be expressed as a defined concentration (e.g., spores per cubic meter, CFU per cubic meter). Settle plates, unlike defined calibration methods such as Anderson Cassette sampling, do not sample a defined air volume. They sample only what settles onto a defined surface area over time. Many DIY and professional users alike believe these samples are an air sampling method. And while they are advertised as such, they are possibly more closely categorized as a surface sample with relation to airborne particulates, as they only show the relationship between the air and surface where the sample media is set.

Because of this, gravity plates cannot support meaningful quantitative statements such as “the indoor air has X concentration” or “the air is within/above a guideline range.” Without defined volume, there is no denominator to connect what grew on the plate to what was in the air or what the inhalation exposure risk is.

This also undermines indoor/outdoor comparison. Outdoor reference sampling is often used in IAQ investigations because outdoor air is a major contributor to indoor fungal profiles. But an outdoor settle plate is not a reliable baseline for an indoor settle plate, because the deposition dynamics (wind, turbulence, sunlight, temperature, and particle behavior) differ dramatically. This same dilemma applies to comparing one sample in the middle of a room to another sample positioned in close proximity to a forced air return or supply register.

4.2 Strong Sensitivity to Airflow, Disturbance, and Replacement

Settle plate results are heavily influenced by micro-environmental factors:

• HVAC cycles can change turbulence and deposition.
• Human, pet, and mechanical activity can resuspend settled dust, and while agitation can be used to simulate this during testing, there is no established standard for whether this is a more valid exposure assessment (WHO, 2009).
• Doors and windows change pressure and airflow patterns.
• Plate height and proximity to supply/return registers can alter deposition and the velocity of air and particulate movement.

This matters because two plates placed a few feet apart can yield very different outcomes, but do not necessarily represent closer proximity to a mold growth source or area of higher indoor amplification. The plate is not only measuring what is in the room, but also what the airflow delivered to that specific surface during that specific window of time.

From an industrial hygiene perspective, this is a reproducibility problem. In scientific processes, this is what is referred to as replication crisis. As such a method that cannot be reliably reproduced under typical field conditions has limited diagnostic value, and should be considered with caution.

4.3 Competitive Overgrowth and Misleading Visual Dominance

A common observation, especially in DIY settings, is that a single colony with one apparent epicenter expands rapidly and overtakes the plate, while other colonies remain small, stall, or are incapable of propagating altogether. Sometimes this leads the user to falsely conclude that the air in the sampled space contains an excessive amount of that specific mold genera.

This happens because fungi differ substantially in:

• Germination speed
• Hyphal extension rate
• Nutrient utilization
• Production of inhibitory metabolites, including anti-fungal mycotoxins

A fast-growing organism can dominate the available nutrients and physically overgrow slower organisms. The visual impression (“this mold is everywhere”) is often a laboratory artifact that is the result of growth dynamics, not a reflection of airborne abundance or health significance. Likewise, some filamentous fungi produce toxins used for competing, like patulin which is produced by some genera of penicillium and aspergillus. In these scenarios, the “chemical warfare” of one species might prevent the ability of other species to grow despite being present (Reverberi et al., 2010).

The takeaway from this knowledge is important: plate dominance ≠ environmental dominance. A single viable spore of an aggressive grower can produce a plate-covering colony.

4.4 Media Bias and Under-Detection of Building Relevant Organisms

Most consumer plates use general-purpose media, which is designed to support broad fungal growth. That seems advantageous, but it also introduces bias:

• Some building-relevant and water-damage genera of fungi are slow-growing and may be missed within typical incubation windows.
• Some genera are outcompeted by faster saprophytes.
• Some species may require different media or conditions.
• Most readily available plates use malt extract agar (MEA), but in a study of five important aspergillus species, the most reliable cultures used sabouraud dextrose agar (SDA) or potato dextrose agar (PDA). (Ali et al., 2016).

The absence of a “feared” genus like stachybotrys, fusarium, or chaetomium on a plate does not, therefore, support a conclusion that it is absent from the building. Likewise, finding a common outdoor mold genera on a plate does not, by itself, support a conclusion of indoor amplification.

4.5 Poor Alignment with Health Questions

Many DIY users use gravity plates in an effort to answer health-relevant questions like “Is this environment making me sick?” A gravity (settle) plate culture cannot reliably answer that question because:

• It does not measure inhalation-relevant concentration.
• It does not measure fragments, which can be a major component of inhalation exposure.
• It does not measure mycotoxins, VOCs, endotoxins, actinomycetes, or other co-contaminants.
• It does not account for individual susceptibility.

From a professional standpoint, settle plates are a brut environmental observation tool at best. Translating settle plate growth directly into health risk is not scientifically defensible.

5. Value Statement: What Settle Plates Can Contribute

A balanced view recognizes that settle plates can contribute something, just not what many users assume.

Their most defensible value is as an early-stage indicator that conditions might warrant further investigation, particularly when results are both extreme and consistent across multiple samples in the same space. They can also support client education by making invisible viable particulates visible.

Their value declines sharply, however, when the question shifts from “Should we look closer?” to “What is the exposure?” or “Is remediation needed?” or “Did remediation work?” Those questions require methods that measure concentrations, evaluate building ecology, and integrate moisture history, visual and direct microscopic materials assessment, air exchange, and occupant patterns.

6. Common Misuses and Misconceptions (And Why They Persist)

6.1 Misuse: Treating Colony Count or Plate Coverage as a Numeric Environmental Mold Level

It is common to see DIY interpretations like “I had 40 colonies, therefore my mold level is high.” This fallacy persists because counting feels objective. But without a sampled air volume and without controlling deposition variables, colony count is not the same as concentration or environmental exposure.

Moreover, colony count is influenced by what can grow and what may be suppressed. Counting colonies often gives a false sense of precision. For this reason, culture-based methods, and especially passive settle plate cultures, should be considered primarily for initial qualitative data. These should be approached with skepticism on quantitative questions (Eduard, Heederik, 1998; Macher, 1999).

6.2 Misuse: Using Plates to Declare a Space "Safe" or "Unsafe"

Some users treat settle plates like a pass/fail safety test. Because people want certainty, and plates produce a tangible, visible artifact, it is easy to accept this reasoning.

Professionally, a pass/fail conclusion on this sampling method alone is simply not supportable, and more data is needed. A “clean” plate can occur in a contaminated building if conditions are unfavorable for deposition or growth, sporulation is limited or inhibited, air filtration is preventing airborne communication of viable propagules, or if contamination is hidden and not aerosolizing during the exposure window. Conversely, a “dirty” plate can occur in a normal environment during high outdoor spore seasons, during cleaning disturbance, inside a ventilation system with high velocity air flow, or near a doorway.

6.3 Misuse: Using Plates as Post-remediation Verification (PRV)

PRV is intended to assess whether remediation goals were met, whether the fungal ecology has been restored to a Condition 1/Normal Fungal Ecology, whether the structural threat to the building has been eliminated, and to provide documentation to materially interested parties that the space is ready for reconstruction and safe re- occupancy. This requires analysis methods with defined sampling parameters, defensible comparability to a baseline, and interpretation in context.

Gravity plates are unsuitable for a PRV because:

• They are non-quantitative.
• They have disproportionate sampling bias when compared to other generally accepted methods.
• They are highly disturbance- and airflow-dependent.
• They are vulnerable to overgrowth artifacts (one viable spore may overgrow the entire plate).
• They cannot be reliably compared to benchmarks.

6.4 Misconception: "My Plate Was Completely Covered, So My Air Must Be Horrible"

As discussed earlier, a single viable spore can produce a dominant colony that overtakes a plate. This misconception is rampant because the visual metaphor is strong: people see the growth rapidly spreading outward and assume the environment was heavily contaminated.

The correct interpretation is far narrower: one viable spore or hyphae landed, germinated quickly, and grew aggressively on that medium. It is possible that other spores were present and were competed out. It is also possible that no other spores were present.

6.5 Misconception: No "Scary Mold" on the Plate Means No Problem

This fallacy persists because people assume the plate is a comprehensive detector. It is not. Media bias, competition, incubation conditions, and nonviable fragments all contribute to this false reassurance.

A professional assessment prioritizes moisture history, material inspection, and a variety of both air and surface sampling methods when sampling is warranted.

7. Professional Conclusion

DIY settle (gravity) plate culture sampling is best understood as a deposition-and-growth demonstration rather than an exposure measurement. The method can be helpful in limited circumstances – particularly as a low-cost way to illustrate that viable organisms can settle to the surfaces in an environment and to flag grossly abnormal conditions that warrant further investigation.

At the same time, the physical mechanics of deposition and the biological selectivity of agar growth impose unavoidable limitations and biases. These limitations prevent settle plates from serving as a reliable diagnostic tool, a defensible clearance method, or a direct proxy for health risk.

A scientifically responsible conclusion is therefore three-part:

1. Gravity plates can have limited practical value when used as a preliminary indicator or educational tool, especially when interpreted conservatively.
2. Gravity plates should not be used as a standalone decision-making tool for remediation scope, clearance, or health attribution.
3. Environmental assessment sampling should be conducted by a trained professional who understands the limitations of the methods used and has relationships with accredited laboratories for analysis.

In short, settle plates can tell you that viable organisms deposited and grew under those specific sampling conditions, but they cannot reliably tell you what occupants inhaled, whether the building has an indoor amplification source, or what actions are warranted or unwarranted without broader context and more appropriate assessment methods.

8. References

Ali, S. R. M., Fradi, A. J., & Al-Aaraji, A. M. (2016). Comparison between different cultural medium on the growth of five Aspergillus species. World Journal of Pharmaceutical Research, 5(8), 9–16.

Booth, C. M. (2021, April 5). An introduction to trending in environmental monitoring programs. Outsourced Pharma. https://www.outsourcedpharma.com/doc/an-introduction-to-trending-in-environmental- monitoring-programs-0001

Eduard W, Heederik D (1998). Methods for quantitative assessment of airborne levels of non-infectious microorganisms in highly contaminated work environments. American Industrial Hygiene Association Journal, 59:113–127.

Macher J, ed. (1999). Bioaerosols: assessment and control. Cincinnati, American Conference of Governmental Industrial Hygienists.

Parenteral Drug Association. (2014). Technical Report No. 13-2: Fundamentals of an environmental monitoring program—Annex 1: Environmental monitoring of facilities manufacturing low bioburden products (PDA Technical Report No. 13-2).

PharmaState Academy. (2017, November 13). Environment monitoring by settle plates in aseptic facility. https://pharmastate.academy/environment-monitoring-by-settle-plates-in-aseptic-facility/

Reverberi, M., Ricelli, A., Zjalic, S., Fabbri, A. A., & Fanelli, C. (2010). Natural functions of mycotoxins and control of their biosynthesis in fungi. Applied Microbiology and Biotechnology, 87(3), 899–911.

Sharma, G., & Pandey, R. R. (2010). Influence of culture media on growth, colony character and sporulation of fungi isolated from decaying vegetable wastes. Journal of Yeast and Fungal Research, 1(8), 157–164.

World Health Organization. (2009). WHO guidelines for indoor air quality: Dampness and mould. WHO Regional Office for Europe.