Black, Slimy, Fuzzy: What Does Stachybotrys Mold Look Like?

Black mold usually means Stachybotrys chartarum, which the CDC describes as a greenish-black mold^[1]. So what does black mold look like? Typically dark, often slimy patches on materials that have stayed wet, such as the paper face of drywall, ceiling tiles and wallpaper^[1][2]. But many common molds are just as dark, so color alone cannot tell you what you have^[3].

This guide covers what Stachybotrys looks like and where it grows, the molds most often mistaken for it, why you cannot confirm it by eye, and what the EPA and CDC say to do whatever the species turns out to be.

What does Stachybotrys look like?

Stachybotrys chartarum shows up as greenish-black growth on cellulose-rich material that has been wet for a long time^[1]. Three features describe it best: color, texture and where it is growing.

Color

The CDC calls it greenish-black, not jet black^[1]. Its spores are dark brown to black when mature^[2].

Texture

Stachybotrys spores are held together in slimy masses^[2]. Colonies produce a polysaccharide "slime" that coats both the threads and the spores^[4]. So active growth tends to look wet rather than dusty. NIEHS describes mold in general as fuzzy and thick, while mildew is flat and powdery^[5].

Where it grows

Location is a stronger clue than color. Stachybotrys grows on high-cellulose materials such as fiberboard, gypsum board and paper, and it needs constant moisture from leaks, flooding, condensation or water infiltration^[1]. In more than 5,300 samples from water-damaged Danish buildings, Stachybotrys was linked mainly to gypsum board, wallpaper and glass fiber^[6].

It is a mold of very wet conditions. In lab tests, its spores germinated fastest and its threads grew best at the wettest water levels studied, between 25 and 30 °C^[7]. Mold growth in general can start at much drier levels, but significant amounts of mycotoxins are produced only once materials are very wet (a water activity of about 0.95)^[8].

In practice: dark, slimy growth on drywall or ceiling paper next to a leak or a past flood fits Stachybotrys. Dark spots on a tile shower wall or a dry window frame are more likely something else.

What is commonly mistaken for Stachybotrys?

The most common look-alikes are Cladosporium, Aspergillus niger and ordinary mildew. NIEHS lists Alternaria, Aspergillus, Cladosporium and Penicillium as common indoor molds^[5]. Some of them, such as Cladosporium and A. niger, form dark colonies.

Cladosporium

Cladosporium is the mold you are most likely to meet. In 12,026 air samples from 1,717 US buildings, it was among the most common airborne fungi indoors and outdoors, in every season and region^[13]. A worldwide survey of about 520 indoor isolates described 46 species, with colonies in shades of olive-grey and grey-green and a velvety to fluffy texture^[9].

In homes without reported mold problems, Cladosporium was abundant on sills beneath condensation-prone windows^[10]. On water-damaged wood, Cladosporium herbarum was one of the main species found^[6].

Aspergillus niger

Aspergillus niger belongs to a group known as the black aspergilli^[11]. It is best known for food spoilage, including "black mold" of stored onions^[12]. Indoors, it was associated with wet concrete and floor materials rather than drywall^[6]. It is also an opportunistic cause of ear infections (otomycosis) and other infections^[11].

Mildew and other dark molds

Mildew is flat and powdery, while mold tends to be fuzzy and thicker^[5]. Our mold vs. mildew guide covers that difference in detail.

In a Danish study of 72 moldy building materials, Penicillium, Aspergillus, Chaetomium and Ulocladium all turn up more often than Stachybotrys^[14]. Alternaria is another common indoor mold, and in the larger Danish dataset it was associated with damp wood^[5][6].

Can you tell Stachybotrys from regular mold by looking at it?

No. Nobody can reliably confirm Stachybotrys by eye, and nobody can see whether a mold is producing toxins.

When researchers sorted 122 Stachybotrys isolates from water-damaged buildings, they needed microscopy, colony growth and chemical profiles together. They found two different species and two chemical types within S. chartarum^[3].

The numbers also argue against assuming the worst. Stachybotrys made up only 3.9% of more than 5,300 samples taken from visible mold growth^[6]. It was found in the indoor air of 6% of 1,717 US buildings tested^[13]. Several species often grow together on the same material^[6], so one dark patch can hold more than one mold.

Is "toxic" mold a separate kind?

"Toxic black mold" is not a separate species. Stachybotrys chartarum itself comes in chemical types that differ in which toxins they make, and output also depends on how wet the material is. Significant amounts are produced only when materials are very wet^[8][3].

The CDC's position is that you do not need to know which mold you have, because all molds should be treated the same for health risk and removal^[1].

How to check a dark mold patch at home: 5 steps

You cannot name the species at home, but you can tell whether you have a mold problem and how big it is.

1. Look at what it is growing on. Dark growth on the paper of drywall, ceiling tiles or wallpaper fits Stachybotrys. Growth on wood, sills or concrete more often points to other molds^[1][6].
2. Find the water. Mold needs moisture. Check for roof or plumbing leaks, past flooding, and condensation on windows, walls or pipes^[15].
3. Use your nose. A musty smell with no visible source suggests hidden mold. Check the back of drywall or wallpaper, the top of ceiling tiles, the underside of carpet pads, and around leaking pipes^[15].
4. Estimate the area. The EPA uses about 10 square feet, a 3 by 3 foot patch, as the line between a do-it-yourself job and one that needs more help^[15].
5. Do not disturb hidden growth. Pulling wallpaper off moldy walls can release a large burst of spores^[15].

A musty odor is worth taking seriously. In a meta-analysis of 16 studies, mold odor at home was linked to a higher risk of developing asthma (summary estimate 1.73, 95% CI 1.19-2.50)^[16].

Do you need a mold test to confirm it?

Usually not. The EPA says sampling is unnecessary in most cases when you can see mold^[15]. No federal limits exist for mold or spores, so a result cannot show whether a home "passes"^[15]. The CDC adds that no test proves a link between Stachybotrys and particular symptoms^[1].

Air tests have a specific blind spot for this mold. Stachybotrys spores are often rare in the air of buildings with heavy growth, possibly because the spores are large or growth is hidden inside walls^[4]. Air sampling alone also missed much of the fungal variety found on surfaces in the Danish study^[6].

If you do need testing, for example after cleanup, the EPA recommends professionals who follow recognized lab methods^[15]. Our guide to mold testing at home compares the options.

Is Stachybotrys worse than other molds?

The evidence does not single it out as clearly as its reputation suggests. A review in the Journal of Allergy and Clinical Immunology linked S. chartarum to building-related symptoms but called a causal connection "far from being unequivocally proven"^[17]. The CDC says a suspected link with lung bleeding in infants has not been proven^[1].

What is better established is that damp, moldy homes in general affect breathing. Visible mold was linked to a higher risk of new asthma in pooled studies (summary estimate 1.29, 95% CI 1.04-1.60)^[16]. It was also linked to a higher risk of rhinitis (1.82, 95% CI 1.56-2.12)^[18], including allergic rhinitis. Mold allergy can cause sneezing, a runny nose, red eyes and skin rash^[15].

For the full health picture, see is black mold dangerous?, our guide to black mold poisoning symptoms and treatment, and the warning signs of mold toxicity. If you have persistent symptoms, the CDC advises seeing your physician^[1].

What should you do if you find mold that looks black?

Treat it like any other mold: remove it and fix the water source^[1][15]. The EPA's steps for areas under about 10 square feet are:

1. Fix the leak or moisture source first. Cleaning without fixing the water problem means the mold will most likely come back^[15].
2. Protect yourself. The EPA suggests an N-95 respirator, long gloves and goggles without vent holes^[15].
3. Scrub hard surfaces with detergent and water, then dry them completely^[15].
4. Replace porous materials such as ceiling tiles and carpet if they are moldy, because mold fills their crevices^[15].
5. Do not paint or caulk over mold. Paint applied over moldy surfaces is likely to peel^[15].
6. Skip routine bleach. The EPA does not recommend biocides such as chlorine bleach as a routine step, and bleach must never be mixed with ammonia cleaners^[15]. Dead mold can still cause allergic reactions, so it has to be removed, not just killed^[15].

To keep it from returning, dry water-damaged items within 24 to 48 hours^[15]. Keep indoor humidity below 60%, ideally between 30% and 50%^[15]. Running a bathroom fan or opening a window usually keeps recurring shower mold to a minimum^[15].

When should you call a professional?

Call in help when the job is bigger or riskier than a small, visible patch. The EPA points to professional guidance or a contractor when^[15]:

• mold covers more than about 10 square feet, or there has been a lot of water damage;
• the heating, ventilation or air conditioning system may be contaminated (do not run it in that case);
• the water damage came from sewage or other contaminated water;
• you suspect hidden mold inside walls, ceilings or ductwork.

If you have health concerns, the EPA says to consult a health professional before starting cleanup^[15]. When hiring a contractor, check references and ask them to follow EPA or industrial hygiene guidelines^[15].

This article is for general educational purposes and is not medical advice. Mold species cannot be identified from photos or descriptions, and this guide does not replace a professional inspection where one is needed. Talk to a qualified healthcare professional about symptoms you think may be related to mold, particularly if you are pregnant, immunocompromised, taking prescription medication, or managing a diagnosed condition such as asthma.