Cleaning & Restoration Association News

An Update on Microbial Growth

Q: 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 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."

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.

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.

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