Bathroom mold (C) Daniel FriedmanHow to Find & Test for, Remove & Prevent Mold in Bathrooms

  • BATHROOM MOLD - home CONTENTS: how to recognize, clean off & prevent bathroom mold growths. Is bathroom mold dangerous? When does bathroom mold growth or similar black or brown mold growths in buildings suggest hidden leaks and water damage? How do we address mold growth behind or under bathroom or kitchen cabinets or vanities?
  • POST a QUESTION or READ FAQs about the diagnosis, cleanup & prevention of mold growth in bathrooms

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Bathroom mold causes, cures, prevention: this article gives advice on how to find, test for, remove & prevent mold in bathrooms, including mold found on bath tiles, moldy tile grout, moldy caulk, and hidden bathroom mold. Brown mold, and sometimes darker black mold commonly found on bath tile grout may be harmless, a cosmetic clean-up job handled with bathroom cleaners.

On occasion mold in bathrooms may indicate a more serious hidden leak behind a cabinet, vanity, wall, or floor. In such cases there may be hidden rot, insect damage, or a larger mold reservoir that needs attention. Here we explain how to decide what to do about bathroom mold, how to remove it, how to prevent new bath mold growth, and when to dig deeper into building cavities.

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Is Bathroom Mold a Health Risk or Indoor Air Quality Problem?

Black mold on bath tile grout (C) Daniel Friedman

This bathroom mold diagnostic procedure helps identify the presence of or locate the probable sources of mold reservoirs in bathrooms, and helps decide if there is a need more invasive, exhaustive inspection and testing such as cutting openings to inspect wall, ceiling or floor cavities.

Obviously such destructive steps should be avoided if at all possible, so first let's look at which kinds of bathroom mold indicate cosmetic versus more serious trouble.

Allergenic bathroom mold: Our photo at left shows the most common place to find mold in a bathroom, on the tile grout or caulk.

Do you need to test this brown stuff to identify it as mold or to identify the mold genera/species? Normally not. The cleanup procedures are the same regardless of the mold identification. However on occasion, there is a reason to test and identify mold growth in buildings.

See MOLD TEST REASONS for details.

Bath tile mold closeup (C) Daniel FriedmanUsually this mold is a member of the Cladosporium sp. group, often C. sphaerospermum - among the most common molds found, and possibly allergenic.

The total area of mold in this case is trivial and unlikely to be harmful to anyone. It is primarily a cosmetic issue. A close up of this bathroom mold on tile grout is shown at below (left).

There is no need to perform a mold test to identify small areas of mold such as those shown here. Just remove the mold and correct the conditions that contribute to mold growth.

Whenever cleaning mold, especially when opening a wall or ceiling cavity where you have found leak damage, remain alert for the discovery of a large moldy area (more than 30 sq.ft.). If a large area of mold is discovered, stop work, seal off the work area, and consult a professional.

Our photos above show close ups of moldy bath tile grout. Mold may also be found in bathrooms on bath caulks (page to photo) and on the surface of ceramic tiles and even toilets and sinks where those surfaces have become coated with soap scum or organic dust and debris. These molds may appear to be "black mold" but on closer look they are usually brown members of the Cladosporium family.

Hidden Mold in Bathrooms - How to Evaluate the Risk of Harmful and Hidden Bathroom Mold

A bit of looking around may disclose larger and more problematic mold contamination in bathrooms. Here are some places to look:

  • Look for hidden bathroom mold on the back side of wallpaper, especially above and around or near showers. Mold genera/species growing on paper surfaces such as on the back side of vinyl wallpaper or on drywall, painted or bare, are more likely to be harmful, possibly airborne species such as Aspergillus or Penicillium as well as the less easily airborne black bath mold Stachybotrys chartarum.
  • Look for hidden bath mold behind floor trim and in wall cavities where the floor has been wet, and in the lower portion of wall cavities where floors have been wet.
  • Hidden bathroom mold may be present in wall cavities around bath tub or shower controls that have leaked into the wall cavity. If your bathroom provides an access panel to the tub or shower controls from the other side of the wall, remove the panel and look for water stains there.
  • Bathroom mold may be hidden in the ceiling, on the ceiling side of drywall around a leaky vent fan or light fixture opening, especially over or near a shower; Bathroom moisture related mold may also be present in other ceiling or attic spaces if the bath vent fan duct has not been properly sloped and routed outdoors.
Bath vanity floor mold (C) Daniel Friedman
  • Bathroom vanity mold is often found growing on the back side or under-side of bath vanity cabinets - those un-coated surfaces pick up moisture and invite mold growth, particularly if the vanity is made of chipboard or pressboard.
  • See our Photo Guide to Mold Under Vanity Cabinets below. Our photo (left) shows mold found under a built-in bathroom storage chest, visible only after the bottom drawer was removed to expose the wood subflooring.
  • Insulation mold over bathrooms: on occasion, especially where a bathroom is not well ventilated or where the occupants do not use the vent fan system (maybe it's too noisy?) we find non-visible mold in ceiling or wall insulation, especially around ceiling penetrations for lights or fans.
  • Marble wall mold in baths? Mold may even be found on hidden drywall surfaces behind solid marble bath walls, as we show in our second case study
  • of PHOTO GUIDE to Mold Behind Wall Marble below.
  • Mold below bathroom floor tiles? Mold may be found growing on wood subfloors under bath floor tiles or sheet vinyl flooring if the bathroom floor has been wet or flooded.

    If a ceramic bath tile floor has become loose we suggest exploring for damaged subfloors - otherwise simply repairing the tiles may not be a durable fix. If the bathroom is not on a basement or slab, inspect the ceiling areas below the bathroom for leak stains. Stained areas of drywall ceiling below a leaky bathroom or bath fixture usually merit further investigation.

Photo Guide to Finding Mold Under or Behind Bathroom Vanity Cabinets

Photograph of inspecting below a bath vanity for hidden mold. Photograph of inspecting below wallpaper for hidden mold.

If there has been protracted leakage or spillage under built-in cabinets such as bath vanities, there may be a mold cleanup job under or behind these components.

We removed this bath vanity after receiving complaints of recurrent moldy odor in this bathroom. No amount of cleaning of other bath surfaces had reduced the mold smell. A slight slope in the bathroom floor had been sending tub spillage behind this bath vanity for a decade or more. Water spillage was inconsistent - it depended on who used the shower and how much water they splashed onto the floor.

What makes a lot of sense sense is to study the building carefully to decide on the building points at most risk of having been wet from leaks due to construction details or other site observations. That's where one would make a test cut. Like behind the vanity cabinet near the most moldy corner in our photo, above right.

More mold-contaminated cabinets can be seen


Also see BROWN HAIRY BATHROOM MOLD for a Q&A on the harmfulness of "hairy brown mold" found in a bathroom.

Identifying & Removing Dense Brown Hairy Mold Found On Bathroom Floor & Under Carpeting

Question: what is this hairy brown mold found in my bathroom? Is brown hairy mold dangerous? How do I get rid of it?

I live in Melbourne Australia and I have a major issue with my bathroom floor. Please see my brown hairy mold photographs (below). The images are of brown mold growths on the bathroom floor, & afterwards when I had scraped them off the floor.

I do understand that no competent expert would pretend to ID mold [or any other microscopic particle] from just a field photo.

But I have no idea what I'm dealing with and of course am concerned for my health and my family's. Can you tell me please - is brown hairy mold dangerous? When I scraped the protrusions off the bathroom floor, it turned into a powder & hair like structures stood on end. I would be most grateful if you could please tell me what this could be and if it is dangerous.

 I'm wanting to send a specimen away for analysis & hope that I can locate a government organisation in Australia, such as the Agricultural Department, hopefully they can do this at no cost. - G. P., Melbourne, Australia

Brown hairy bathroom mold under carpet (C) GP DF Brown hairy mold on bathroom floor (C) GP DF

Reply: Examination of fungal material in an aerobiology lab is needed for sure identification of mold

Your photos are blurry and no competent expert would pretend to identify a mold genera or species just from email photos.

But with that caveat stated, the pattern and character of the mold in your photos resembles like a fungus Stemonitis sp.

- see BROWN MOLD PHOTOS where we include photographs of Stemonitis sp. found on a bathroom floor and growing on oriented strand board (OSB) or "waferboard" in a basement.

Enlarging your photos [click any image to see an enlarged version] we see the mold was thick under carpeting (above left) meaning that in at least this area of the building, mold growth, including in hidden cavities such as walls, floors, ceilings, is likely to be extensive.

Watch out: And one would certainly expect other leak damage; that "wrinkled" looking floor baseboard trim board - if that's what it is in photo #1 at above left - means there have been leaks and probably rot.

If the mold test lab or aerobiology lab technician who examines a mold test sample you provide is familiar with myxomycetes, s/he should easily recognize the fungus from an actual sample. - you can use the clear adhesive tape sampling procedure

at TEST KITS for DUST, MOLD, PARTICLE TESTS and send the sample to a qualified local lab of your choice [not to us].

Brown hairy bathroom mold under carpet (C) GP DF Brown hairy mold on bathroom floor (C) GP DF

Some Brown Hairy Molds are Probably Harmless

Stemonitis sp. is a Myxomycete ( a class of fungus) that is not listed in our


nor in the authoritative Atlas of Clinical Fungi, deHoog et als. as a known toxic mold but as we warned you earlier, having so much fungal growth and leakage as we saw in that lifted-carpeting photo, you should expect that other genera/species are likely to be present even if you don't see them yourself from a superficial inspection.

Watch out for Other Leaks & Hidden Mold

Watch out: At a minimum you'd be smart to assume there are allergenic molds present and if there is more than about one square meter of moldy material, to use appropriate protection measures during cleanup as well as, of course, finding and correcting the leaks that led to these conditions. In most circumstances it is not necessary to test mold to identify the genera species.

That information will not change how mold should be removed or prevented in buildings. But on occasion, such as for medical reasons or for control of a larger, costly mold remediation project, there are reasons (MOLD TEST REASONS) to identify mold on surfaces or in building air or dust.

This article is part of our series: MOLD in BUILDINGS which describes how to find mold and test for mold in buildings, including how and where to collect mold samples using adhesive tape - an easy, inexpensive, low-tech but very effective mold testing method.

Reader Question: is this stuff in our bathroom black mold?

Bathroom mold suspect (D) D Friedman and KLI was wondering if you could tell by the pictures if this is black mold? They are pictures of my shower.

The trim by the shower, behind the toilet, is gross too. And on the other side of the wall is another bathroom.

The last picture is of the tile in that bathroom and it has cracked and caved in.

If you could take a look, your opinion would be greatly appreciated. - K.L.

Reply: visible black stuff or mold-suspect materials + evidence of leaks and water spillage can be a clue that says look more carefully for leak damage and hidden mold in buildings

Bathroom mold suspect (D) D Friedman and KLA competent onsite inspection by an expert usually finds additional clues that help accurately diagnose a problem with leaks, mold, and hidden mold. That said, here are some things to consider:

  • We cannot reliably identify mold, harmful in particular, from just your photos. Sometimes a close-up, more sharply focused, of mold-suspect material on a surface can unequivocally be identified as "mold" without of course permitting identification of the mold genera/species.

    But nevertheless, your photos show wet deteriorated conditions in a bathroom, leaks around a shower door, onto the wall and floor and floor trim.
  • Those conditions make some mold growth likely; such as on grout, possibly on tile, more likely too on wood, and on drywall, and possibly in wall cavities that have been wet - an area hidden from view.
  • In sum, it's likely that there is some mold in your bathroom; But I did not see in your photos any large mold area.
  • Take a look at MOLD APPEARANCE - WHAT MOLD LOOKS LIKE for help in recognizing mold growth in buildings.

    Also see MOLD / ENVIRONMENTAL EXPERT, HIRE ? for help in figuring out if you need to bring in a mold professional - not something that appears to be the case from your photos.

Watch out: don't assume that all "black mold" is harmful nor that it is the most important mold in your home. While some dark colored molds are indeed potentially harmful, others can be insignificant or even simply of cosmetic concern. However the conditions that produced the mold growth that you see can indeed have produced other, less easy to spot, molds, including hidden mold in building cavities or light colored but harmful molds that move throughout building air (such as Aspergillus sp. or Penicillium sp.) and that could be of more concern.

If you have a small area of mold or even mold-suspect material (less than 30 sq ft or less than 10 ft by some EPA sources with which I disagree) then spending on testing or professional clean-up are not normally appropriate, with the warning that if in the course of ordinary cleaning and renovations you discover a larger reservoir, a professional should be consulted.



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