Why does the IICRC S520 Mold Remediation Standard forbid the use of biocides?
Note: At the time of writing this response, the IICRC has not yet released the S520 Standard and Reference Guide for Professional Mold Remediation (Second Edition) and therefore, my responses are based upon the S520 (First Edition).
The S520 (First Edition) does not prohibit the use of biocides on mold-remediation projects. Rather, it discusses the limitations of what the application of such products can accomplish. Because of these limitations, it was decided that the physical removal of molds is a more effective approach. Section 4.4 of the Standard states:
(Principles: Contamination Removal)
"Physically removing mold contamination is the primary means of remediation. It is highly recommended that mold contamination be physically removed from the structure, systems and contents to return them to Condition 1 status. Attempts to kill or encapsulate mold generally are not adequate to solve the contamination problem."
The principles of biocide use endorsed by the S520 are drawn directly from "ACGIH Bioaerosols: Assessment and Control."
(Chapter 10 of the Reference Guide, page 119)
"15.2 Remediators must carefully consider the necessity or advisability of applying biocides... The goal of remediation programs should be removal of all microbial growth. This generally can be accomplished by physical removal of materials supporting active growth and thorough cleaning of non-porous materials. Therefore, application of a biocide would serve no purpose that could not be accomplished with a detergent or cleaning agent."
"16.2 Biocide use should not be considered if careful and controlled removal of contaminated materials is sufficient to address a problem."
"16.2.4 Biocide application is not recommended in the restoration of water-damaged indoor environments except where they have suffered extensive sewage backup. Widespread pollution from raw sewage represents a significant health risk from a variety of infectious agents, and biocides may help to control and contain these agents during the restoration process..."
In addition to the above comments that were quoted in the S520, Bioaerosols: Assessment and Control goes on to state that:
15.4 Remediators must carefully consider the necessity and advisability of applying biocides when cleaning microbially contaminated surfaces [see 16.2.3]. The goal of remediation programs should be removal of all microbial growth. This generally can be accomplished by physical removal of materials supporting active growth and thorough cleaning of non-porous materials. Therefore, application of a biocide would serve no purpose that could not be accomplished with a detergent or cleaning agent.
Consistent with these principles, the S520 discourages the use of biocides. Chapter 7 of the Reference Guide, page 82, states, "Biocide application is discouraged and is not considered effective for mold remediation." However, the S520 also recognizes that, "there may be specific instances where professional judgment dictates that biocides be applied." The Reference Guide continues on to discuss situations where deviation from removal processes might be appropriate, stating:
"The Principles of Mold Remediation (Sections 4.3 and 4.4 of this Standard) state that mold must be controlled at its source. Further, it is highly recommended that mold be physically removed during remediation, and that attempts to kill or encapsulate it are inadequate remediation measures. At the same time, these principles recognize that unique circumstances may arise and that biocides and encapsulants may be considered in specific situations."
Nowhere in the S520 does it state that biocide use is never appropriate. Rather, in agreement with "ACGIH Bioaerosols: Assessment and Control," it recognizes the fact that biocides are not effective at solving mold contamination problems and requires physical removal of contaminants. It leaves the door open for biocide use in unusual situations, on a case-by-case basis.
So when would biocide use be appropriate on a mold remediation project? Killing mold only eliminates its ability to grow in or on a host. There remains the possibility of allergic and toxigenic reactions to the exposure to dead molds. In order for molds to colonize a human host, it generally requires that the individual’s immune system not be functioning competently. While the application of a biocide to remediated surfaces may not be effective, there might be a desire to attempt to kill those molds that might be potentially pathogenic. Most molds found in contaminated buildings are not pathogenic.
In a recent article entitled Effect of plasterboard composition on Stachybotrys chartarum growth and biological activity on spores (Murtoneimi, T et al Applied Environmental Microbiology), it was noted that "Spores collected from all wallboards exhibited cytotoxicity to macrophages. Biocide application did not reduce fungal growth; however, spores collected from biocide treated board exhibited the highest degree of cytotoxicity." The implication is that biocide treated drywall with Stachybotrys chartarum growth was more toxic that if it had not been treated with a biocide. In addition, the mold spores were still viable. Additional research is needed to determine whether the application of a biocide to mold growth triggers mycotoxin production as a defense mechanism.
In addition, there is some belief that biocide application on Category 1 water projects might be appropriate as a preventative against other microbial growth. The primary purpose of such an application is to control bacterial growth when drying is delayed, with the possibility of limiting mold germination and growth. Since the purpose is not to deal with existing mold contamination but rather to prevent it, such use in no way contradicts the S520. In a case where a property has been subjected to a Category 3 water intrusion and there is a subsequent mold problem, it would be appropriate to apply biocides for the purpose of remediating the concurrent bacterial problem.
The viewpoints presented in this article are my personal opinions and not an official position of the IICRC, the IICRC S520 Standard Committee or the S520 Edit Committee.
— Jim Holland
I’ve noticed that the current version of the S500 now includes wind driven rain as part of Category 3. Does that mean that wind driven rain is always considered contaminated?
Your observation that the current edition of the IICRC S500 Standard and Reference Guide for Professional Water Damage Restoration (3rd Edition) includes a reference to wind-driven rain, as a part of Category 3, is correct. The 2nd edition of S500 reads as follows:
3 Definitions
category 3 water: grossly unsanitary water arising from sewage or other contaminated water sources and having the likelihood of causing discomfort or sickness if consumed by or exposed to humans. This category includes all forms of seawater, ground surface water and rising water from rivers or streams. Category 3 water referred to as "black water."
5.3 Category 3 — Black Water:
Category 3 water is referred to as "Black Water." Black water contains pathogenic agents and is grossly unsanitary. Black water includes sewage and other contaminated water sources entering or affecting the indoor environment. Gray water that is not removed promptly from the structure and its contents may be reclassified as black water. Toilet backflows that originate from beyond the toilet trap is considered black water contamination, regardless of visible content or color.
Category 3 water includes all forms of flooding from seawater, ground surface water and rising water from rivers or streams. Such water sources carry silt and organic matter into structures and create black water conditions.
The water is considered to be category 3 water in situations where structural materials and/or contents have been contaminated with such contaminants as pesticides, heavy metals, or toxic organic substances.
The S500 3rd Edition was changed to:
Category 3 — Category 3 water is grossly contaminated and can contain pathogenic, toxigenic or other harmful agents. Examples of Category 3 water can include, but are not limited to: sewage; toilet backflows that originate from beyond the toilet trap regardless of visible content or color; all forms of flooding from seawater; ground surface water and rising water from rivers or streams, and other contaminated water entering or affecting the indoor environment, such as wind-driven rain from hurricanes, tropical storms, or other weather-related events. Such water sources may carry silt, organic matter, pesticides, heavy metals, regulated materials, or toxic organic substances.
The phrase in question is "and other contaminated water entering or affecting the indoor environment, such as wind-driven rain from hurricanes, tropical storms, or other weather-related events." This phrase can be read in different ways.
First, the reader may assume the phrase means all "wind-driven rain from hurricanes, tropical storms, or other weather related events" is contaminated; another interpretation might be that the phrase lists possible other sources of contaminated water.
When reading any document, it is important for the reader to consider the context in which statements are made. In this case, the second sentence of the Category 3 definition in the 3rd Edition says that "Examples can include." According to the American Heritage dictionary the word "can" is defined as there being a "possibility" under the specified circumstances. Therefore, it is not an absolute.
The S500 elaborates on what is included in determining the Category of the water in the following section:
12.1.8 Determining the Category of water
The Categories of water, as defined by this document, refer to the range of contamination in water, considering both its originating source and its quality after it contacts materials present on the job site. Time and temperature can also affect the quality of water, thereby changing its Category. Restorers should consider potential contamination...
Notice, the phrase in question states, "contaminated water" is "entering or affecting the indoor environment." There are two key factors to consider. First, is that the water is "contaminated." If it is contaminated then "YES" it is a Category 3. However, what if the wind-driven rain that enters is not contaminated? Then, in my personal opinion it is not a Category 3. Secondly, what if the water that enters is not initially a Category 3 and is allowed to remain for a period of time without any mitigation or restoration? In my opinion it can become a Category 3 if the levels of bacteria are allowed amplify to a point where there is gross contamination. As stated in my response to a similar question published in this magazine in the October 2007 issue, "it is possible for a Category 1 [or 2] to deteriorate to a Category 2 or 3 and possibly develop into a Condition 3 [mold growth]."
The problem is that restorers need to make a preliminary determination at the commencement of work as to whether to employ procedures necessary for Category 3 or Condition 3 (mold) remediation. The Standard provides examples for restorers to use as initial guidance in making that "preliminary determination." Therefore, it would be reasonable for a restorer to assume wind-driven rain could be a Category 3 and proceed accordingly. However, if there is a question as whether it is actually a Category 3, then the standard provides for a mechanism to answer that question. The only way to make such a determination is to perform an assessment (e.g., sampling and laboratory analysis).
9.8 Developing a Preliminary Determination
In most cases pre-remediation assessment by an IEP on a water damage restoration project is not necessary. However, if the preliminary determination shows that one or more of the following elevated risk situations are present and the damage is Category 3, then restorers should consider using an IEP to make a pre-remediation assessment. Considerations may include:
- occupants are high risk individuals; or
- a public health issue exists; or
- there is a risk of adverse effects on worker or occupant health; or
- occupants express a need to determine the identity of a suspected contaminant; or
- contaminants are believed to have been aerosolized; or
- there is need to determine, rather than assuming, that the water actually contains microbial contamination.
Notice the last bullet point partially addresses this issue.
In conclusion, the S500 does provide examples in order to assist restorers in making a preliminary determination. The examples are not definitive or absolute. An assumption can be made based upon the examples provided. However, the only definitive way to make such a determination is to have an assessment of the water performed by an IEP.
As a final clarification, the viewpoints presented in this article are my personal opinions and not an official position of the IICRC, the IICRC S500 Standard Committee or the S500 Edit Committee.
— Jim Holland
The S500 says that "Restorers can install one or more air filtration devices ... AFDs provide additional airflow while simultaneously removing soils or contaminants from the air ...” So, if we decide to install an AFD to remove contaminants, should we...
Q.) The S500 says that "Restorers can install one or more air filtration devices... AFDs provide additional airflow while simultaneously removing soils or contaminants from the air..." So, if we decide to install an AFD to remove contaminants, should we also have our workers wear N95 respirators while working in the area?
If I might be allowed to read between the lines, I would assume that you are saying that since an AFD is on the water loss that there must be contamination present. Therefore, if there is contamination present then shouldn't workers wear N95 filtering facepiece respirators?
A.) There are two issues to address: first, whether or not there is contamination; and secondly, whether N95s are appropriate personal protection equipment to be worn by workers while in the area.
To begin let's look at the S500 and see what it says. The S500 Standard states at 12.1.20:
Airmoving devices inherently tend to aerosolize soils and contaminants present in the environment. As water evaporates from surfaces and materials, such as carpet, more particles often become aerosolized, creating possible health, safety, comfort and cleanliness issues. To minimize or control aerosolization of particles, restorers should consider implementing the following:
Restorers can install one or more air filtration devices or AFDs (scrubbers), depending on the AFD’s size and obstructions within the structure. AFDs provide additional airflow, while simultaneously removing aerosolized soils or contaminants from the air within a room. Restorers should consider repositioning AFDs on each monitoring trip.
The "standard of care" is that the restorer "should consider implementing" what is stated. In order for a restorer to implement these recommendations, a consideration of the available facts would be necessary. Therefore, the use of an AFD in this instance is not part of the "standard of care," but is based upon the professional judgment of the restorer. That being the case, what information would a restorer consider? The most obvious consideration is that there is a substantial amount of dust and debris that has accumulated in and under the carpeting. Without knowing or suspecting the make up of the dust, the installation of an AFD as an air scrubber would be a reasonable precaution. So, the answer to the first question is "possibly."
Let's consider a different scenario. If you know that there is contamination, then it would be the "standard of care" to install an AFD, probably as a negative air machine. At 12.5.2, S500 states:
The most effective way to ensure that gaseous and aerosolized contaminants do not spread is to isolate work areas by establishing critical barriers or by erecting containment (plastic sheeting) and maintaining adequate negative air pressure within contained work areas relative to adjacent areas.
If you have performed an inspection and developed a preliminary determination that contamination does not exist, the use of respiratory protection is not generally necessary. If, however, you have made a preliminary determination that contamination is actually or possibly present and that workers will likely be exposed to aerosolized contaminants, then the use of respiratory protection would likely be a part of the "standard of care."
Now the question is whether or not an N95 filtering facepiece respirator is adequate to protect workers. A recent article published in the Journal of Occupational and Environmental Hygiene, entitled Respiratory Protection Provided by N95 Filtering Facepiece Respirators Against Airborne Dust and Microorganisms in Agricultural Farms, concluded that the assigned protection factor for N95 filtering facepiece respirators against microorganisms (mean aerodynamic size < 5 µm) seems to be inadequate. While this information was based upon work protection factors (WPF) and was preliminary, it demonstrated that microorganisms were able to penetrate the filter media or circumvent the facepiece easier than non-organic dust. WPF is a measurement of the protection provided in the workplace by a properly functioning respirator when correctly worn and used. The article further stated that "For fungal spores in the mean aerodynamic size of 3.7 to 18.9 µm, the protection factors before correction resulted in the overestimation ranging from 41% to 75%.”" The "overestimating" refers to the overestimating of the WPF.
While it is true that we do not know what the levels of contaminants may be while using an AFD as described above, there is a concern about what level of protection is provide while wearing an N95 filtering facepiece respirator. In the case of very little contaminant, an N95 may be adequate. If the levels are considerable, in my opinion, an N95 is probably inadequate. The decision to wear respiratory protection is left to the employer. The level of protection is complicated by the fact that there are not established exposure limits for many microorganisms.
— Jim Holland
Recently our company has received a number of new assignments from insurers and attorneys asking us to review restorer’s invoices, scope of work and overall compliance of the restoration/remediation project with the S500 and S520 Standards.
The Effects of Time — From Category 1 to Category 3
Q.) Recently our company has received a number of new assignments from insurers and attorneys asking us to review restorer's invoices, scope of work and overall compliance of the restoration/remediation project with the S500 and S520 Standards.
As a result we have noticed that there is confusion about the effects of time on Category 1 water damage restoration projects. The S500 describes Category 1 water as water that originates from a sanitary water source and does not pose substantial risk from dermal, ingestion, or inhalation exposure. It is the only Category of water that makes reference to source. S500 later addresses the possible consequences of what can happen if the water intrusion is left unattended for a period of time. However, the S500 does not say to what extent the change will actually occur. The question arises whether it is correct to assume that a Category 1 water loss that is not attended to for 4 to 7 days (or more) has changed from Category 1 to a Category 3.
A.) The simple answer to that question is, "it depends..."
I recently spoke to a restorer who told me that "the S500 says that in 7 days a Category 1 water damage changes to a Category 3." I asked him where in the Standard it says that. He made reference to Figure 1 on page 88 of the 3rd Edition. He failed to read the text at the bottom of the page which reads:
It is true that some molds can colonize in 72 hours and bacteria can start to multiply in just a few hours. These time frames are based upon ideal growing conditions and not necessarily the conditions that you find in the indoor environment. In some circumstances these microorganisms may take considerably longer to grow due to less favorable conditions. It was not intended for the time frames mentioned here or reflected in this graphic to be absolutes, but a starting point at which you might want to be concerned.
The S500 does not say that Category 1 water turns to Category 3 in any specific period of time. In fact, a Category 1 water intrusion into an uncontaminated indoor environment may never turn into a Category 3. On the other hand, a Category 1 water intrusion into an already grossly contaminated indoor environment may instantly become a Category 3. Whether it does or not is dependent upon whether the water, in conjunction with the environment, has degraded into what is considered "grossly contaminated." It may only be "significantly contaminated" and therefore a Category 2. Examples are given to help clarify what is meant by significantly or grossly contaminated. The definitions are intentionally vague and not absolute. While that may sound like a criticism, it is not. The Standard cannot cover every conceivable microbiological condition that might exist in the indoor environment.
Let's consider what the S500 does say about the deterioration of Category 1 Water over time.
9.6 Determining the Category of water
It is important to remember that the Category of water initially determined can change during the course of the project (reference Standard Figure 1, To Prevent Amplification of Microorganisms, Prompt Response is Necessary for all Categories of Water Intrusion).
12.1.3 Rapid Response
If sufficient time elapses, Category 1 water can deteriorate to Category 2 or 3 water, requiring that more elaborate procedures...be followed.
12.1.8 Determining the Category of water
The Categories of water, as defined by this document, refer to the range of contamination in water, considering both its originating source and its quality after it contacts materials present on the job site. Time and temperature can also affect the quality of water, thereby changing its Category.
The definition of "can" in the S500 is:
can: when the term can is used in this document, it signifies an ability or possibility open to a user of the document, and it means that a referenced practice or procedure is possible or capable of application, but is not a component of the accepted "standard of care" to be followed.
While the S500 and S520 definition of "can" refers to a practice or procedure the common definition of "can" applies when used in other contexts within the document. Category 1 water that remains in a building overtime has the ability, possibility or capability to change to a more contaminated Category. But it is not an absolute as to how contaminated it will become over any period of time. Interestingly at 12.5.5.3 it explains that:
The degree of contamination represented by a Category 2 water loss can vary from minor (just slightly more contaminated than Category 1 water) to extreme (contaminated just short of Category 3 water).
So, the questions arises: Is it appropriate to have a Category 1 water loss that is left unattended for 4 to 7 days (or more) and then assume that the Category has changed to a Category 3 since there is mold growth? Or has it changed from a Category 1 to a Category 2 and possibly a Condition 3 with the potential of a Condition 2? (Condition 1,2 and 3 are terms defined by the IICRC S520 Standard and Reference Guide for Professional Mold Remediation).
On any Category 1 water intrusion in an uncontaminated building there will simultaneously exist a Condition 1. In other words, in an uncontaminated building that is subjected to a Category 1 water intrusion there concurrently exists a Condition 1. As water is allowed to remain, both "Category" and "Condition" are subject to change.
The answer to the first question (Is it appropriate to have a Category 1 water loss that is left unattended to for 4 to 7 days or more and then assume that the Category has changed to a Category 3 since there is mold growth?) is "no." It is not appropriate to assume that a Category 1 water loss that is left unattended for 4 to 7 days or more has changed to a Category 3 since there is mold growth present in the building.
The answer to the second question (Or has it changed from a Category 1 to a Category 2 and possibly a Condition 3 with the potential of a Condition 2?) is "yes" it is possible for a Category 1 to deteriorate to a Category 2 or 3 and possibly develop into a Condition 3 with the potential of Condition 2. However, you should use the appropriate Standard to make decision about the remediation procedures. In my opinion, it would not be appropriate to say that your Category 1 water intrusion that sat for 5 days is a Category 3 because mold might have grown.
Consistently throughout the S500, the reader is directed to the S520 if mold is suspected or discovered. For example, the Foreword states:
This standard also does not address mold remediation; please reference the IICRC S520 Standard and Reference Guide for Professional Mold Remediation for information directly related to mold remediation.
Similar statements can be found in the following S500 Standard sections:
3 Definitions
7.5 Mold
9.6 Determining the Category of Water
9.13.3 Crawlspace Inspection
9.13.4 Basement Inspections
11.2 Indoor Environmental Professional (IEP)
12.5 Drying Contaminated Structures: Category 2 and 3 Water
14.3 Time of Exposure (Contents)
As a final comment about the definition of Category 2 and 3; the reference to contamination has to do with what is in the water and how that water can then impact the structure and contents that it comes into contact with. That is not to imply that secondary damage is not possible, it certainly is possible. If you are working on a Category 2 or 3 water intrusion or an otherwise contaminated building, you need to be concerned about the safety of workers and occupants. But, it is important that you clearly delineate between Categories and Conditions as the basis for your decision to remediate.
As a final clarification, the viewpoints presented in this article are my personal opinions and not an official position of the IICRC, the IICRC S500 Standard Committee or the S500 Edit Committee.
— Jim Holland
When the S500 refers to 72 hours for microbial growth is it referring to mold or bacteria and does the timeframe change if we are in a hot, humid climate?
Actually the S500 Standard and Reference Guide for Professional Water Damage Restoration (Second Edition) does not refer to 72 hours in any instance. There is one reference to 48 hours in the Standard section 5.2 that says, "Time and temperature aggravate category 2 water contamination levels significantly. Gray water in flooded structures that remains untreated for longer than 48 hours may change to category 3 – black water..." I would like to point out that it refers to gray water and not to clean water. In addition, the standard says "may change." The newly released Third Edition of S500 makes the following statement in the Standard section at 9.6, "The cleanliness of Category 2 water can deteriorate for many reasons including but not limited to: contact with building materials, systems, and contents; mixing with soils and other contaminants. Factors that influence the potential organic and inorganic load in a structure include the age and history of the structure, previous water losses, general housekeeping, the type of use of the structure, and elapsed time or elevated temperature." There is also a reference to 72 hours at 9.10.8.2 that states, "The need for cushion replacement should be determined. Cushion (e.g., pad, underlay) should be replaced under the following conditions:
- cushion has remained saturated for approximately 72 hours or more, depending on temperature;
- cushion is saturated with either Category 2 or 3 water;"
The graphic at the end of the S500 Third Edition has been changed and the following clarifying language has been added.
In this document the use of the term microorganisms primarily refers to bacteria and molds. It is true that some molds can colonize in 72 hours and bacteria can start to multiply in just a few hours. These time frames are based upon ideal growing conditions and not necessarily the conditions that you find in the indoor environment. In some circumstances these microorganisms may take considerably longer to grow due to less favorable conditions. It was not intended for the time frames mentioned here or reflected in this graphic to be absolutes, but a starting point at which you might want to be concerned. The terms Clean, Gray and Black are used in this illustration to aid the reader in applying the concepts presented. This document has defined and uses the terms Category 1, 2, and 3 to describe the range of contamination in water.
It is true that some molds can colonize in 72 hours. Bacteria can start to multiply in just a few hours. However, there is more to the story. Most of the references that deal with the time frame for molds to germinate or to colonize are based upon ideal growing conditions and not necessarily the conditions that you find in the indoor environment.
The intent of the S500 and S520 is to provide the restorer or remediator with a general time frame that allows for an evaluation for the indoor environment. If the time frame is greater than 48 hours, then there may be some reason for concern. Then again, there may not be a reason for concern.
Mold spores take up water at varying rates. When mold spores germinate in an environment that is conducive to growth, their vegetative hyphae will grow into the substrate at which time it can be said that the mold has colonized the substrate. At this point, all that has happened is that there has been a microscopic colony established that will not change the appearance of the substrate surface. Could it be said that the mold has colonized? Yes! In some cases the term “colonized” means that the mold spore has not only germinated and become established, but it has also developed additional spores that have germinated, become established and the amount of growth is such that the colony is visible without the aid of magnification. This process can take many more days. You may not see visual evidence of mold growth for 6 to 14 days. In some cases, the growth environment may not be favorable and you will not see any evidence of mold growth for weeks, if ever.
Microbial organisms (e.g., molds and bacteria) go through the following 4 phases of growth.
- Lag Phase
Once the growing conditions become favorable, mold spores can germinate. This growth starts slowly and accelerates gradually. This phase is referred to as the lag phase.
- Exponential or the Log Phase
Exponential growth occurs for a brief period as the hyphae starts to develop. The new hypha extends into the substrate. The mass of hyphae is referred to as mycelium. As long as the growing conditions are favorable and there are nutrients available, the growth remains constant. This phase is referred to as the exponential phase.
- Stationary Phase
When the nutrients are depleted or the growing environment is not longer favorable, growth slows down or is completely stopped. During the stationary phase, hyphal growth stops. In some cases, nutrients are transferred to developing spores as a propagation mechanism. The spores are dispersed by air movement. This phase is referred to as the stationary phase.
- The Death Phase
During the death phase, the mycelium eventually dies off.
The lag and exponential phases vary in length of time depending upon what molds are present. Generally those molds that are xerophilic (capable of growth in relatively dry conditions) germinate and grow first. They are also referred to as primary colonizers. Xerophilics can germinate in conditions where the water activity (Aw) is below 0.80 and perhaps as low as 0.61. Penicillium, Aspergillus and Eurotium can germinate and colonize in these conditions. In ideal growing conditions, micro-colonies can develop in 48 to 72 hours. Secondary colonizers (mesophilic) can germinate and grow where the Aw is between 0.8 and 0.9. Secondary colonizers include species of Cladosporium, Ulocladium, Alternaria and some species of Aspergillus. Tertiary colonizers (hydrophilic) can germinate in conditions where the Aw is above 0.9. The presence of tertiary colonizers in a building is an indication that there has been a very wet condition for extended periods of time. Stachybotrys, Chaetomium, Trichoderma, Aureobasidium, actinomycetes and other bacteria require very wet conditions. Tertiary colonizers (molds) can take 7 to 14 days to develop micro-colonies. In all cases, it will take more time for the colonies to grow sufficiently to become visible.
Keep in mind that in any environment, bacteria are also present. When you have a very wet surface, bacteria can multiply very rapidly. In some cases, the odors that you might detect in wet buildings are actually bacteria. Bacteria go through the same phase of development and death as molds. As mentioned above, generally bacteria require a very high Aw in order to multiply. The lag phase for bacteria may only be 1 to 3 hours before it starts multiplying exponentially. The cells divide at a constant rate, doubling the population. In ideal growing conditions, E. coli bacteria will double in population every 17 to 20 minutes. At this growth rate, one E. coli bacterium could become a billion (1,000,000,000) bacteria in just 10 hours. In the case of Clostridium tetani which can result in Tetanus, the generation rate is even faster, perhaps doubling as quickly as every 6 minutes.
The S500 deals with bacteria primarily and molds as a secondary consideration. When reading the S500, keep in mind that the time frames mentioned are for microbial organisms that include more than just mold. Also keep in mind that these time frames are for ideal growing conditions. It was not intended for the time frames to absolutes, but a starting point at which we might want to be concerned. If you are living in a hot-humid climate, certain microorganisms will grow faster than others. There are molds and bacteria that prefer warmer or hot environments. Others will not grow as quickly.
— Jim Holland