What grows after water damage — and why speed matters

In plain English

Bacteria and fungi live in ordinary household dust and on building materials, and in a dry, routinely cleaned home they cause no trouble. Once water gets in or moisture condenses on surfaces, that ecology shifts: microbes start to multiply, which can harm indoor air, damage materials and create health risks. That's why the S500 wants mitigation to begin as soon as it is safe.

Most bacteria need very wet conditions to grow (water activity above about 0.95). Many of the molds that show up first after water damage need much less: some Penicillium and Aspergillus species can grow at a surface humidity of only about 66–70%, and some produce strong allergens and toxic substances. A musty, moldy or mildewy smell, caused by gases microbes release while they grow, is a warning sign even when nothing is visible.

Sewage is in a different league: it can carry dangerous bacteria such as Salmonella, Shigella and E. coli, more than 120 kinds of viruses, fungi, parasites and bacterial endotoxins. (The S500's definitions separately class sewage as Category 3 water.)

Key points

• Water changes the microbial ecology fast; start mitigation as soon as it's safe.
• Early molds can grow at ~66–70% surface humidity; most bacteria need ~95%.
• A musty odor is a warning sign of possible microbial growth.
• Sewage carries bacteria, viruses, parasites and endotoxins.

Official text

This topic is covered in our own words because the standard behind it is copyrighted; we cite it below rather than reproduce it.

Citations

• IICRC S500, ANSI/IICRC S500 Standard for Professional Water Damage Restoration — Sec. 2, Microbiology of Water Damage, pp. 24–25. Best practice

Related topics

• Category 1, 2 and 3 water: how dirty was the water? The water-damage industry sorts water by how contaminated it is, judged by where it came from and what it picked up on the way. The category decides how much can be... Best practice
• How mold affects health The most common effects are allergic: sneezing, runny nose, red eyes, rash. Mold can trigger asthma attacks in people who are sensitized to it, and it irritates the... Best practice
• Mycotoxins, “toxic black mold” and musty odors Some molds can produce toxins (mycotoxins) under certain conditions — more than 200 have been identified. But finding mold in a building doesn't mean toxins are... Best practice
• Water activity, humidity and the moisture levels mold needs R520 explains mold's moisture needs with water activity (aw): how much free water a material offers, which roughly matches the relative humidity the material would... Best practice