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The short version
The CDC names Cladosporium, Penicillium and Aspergillus as the most common molds indoors, and a few others turn up wherever there has been water damage. You can't tell which mold you have, or how risky it is, from its color. The good news is that you rarely need to: the CDC says to remove mold whatever its type and fix the moisture that let it grow.
The names are still worth knowing, because the kind of mold, and especially how much water it needs, says a lot about the problem behind it. A little Penicillium on a humid closet wall and a patch of Stachybotrys on soaked drywall call for very different fixes. This guide walks through the common ones, where they grow in NYC homes and what to do about each situation.
What the mold says about the water
Every mold needs moisture, but not the same amount. Health agencies group indoor molds by how much water they need to grow, and that is the most useful way to think about them, because it points straight at the cause.
Damp is enough
Can grow on materials that are only damp, such as in a humid room or on dusty surfaces.
Surfaces that stay damp
Needs surfaces that stay damp, like window frames with condensation or a shower that never dries.
Soaked materials
Needs a lot of water for a long time, which usually means a leak, serious condensation or a flood.
Groups follow Table 1 of the World Health Organization's 2009 dampness and mold guidelines. Chaetomium, which that table doesn't list, is placed using INSPQ's fact sheet. Aureobasidium isn't in the table either: INSPQ says it needs a lot of moisture, but the figure INSPQ gives falls at the top of the WHO's middle band, so we show it there. The sources don't always agree, some kinds in each group need more or less water than others, and only a lab can say for sure which mold is which.
If you find one of the molds in the "Soaked materials" group, keep in mind what the WHO says: molds that need that much water generally mean serious condensation or real water damage, from leaks, flooding, or poor insulation combined with poor ventilation. Running a fan won't be enough on its own.
Where did you find it?
Most people meet mold in a particular spot, not by name. Pick yours to jump to what it usually means and what to do.
- 1Bathroom ceiling, walls and caulk
- 2Windows and the walls around them
- 3A stain on the ceiling or wall
- 4Under the sink or behind the fridge
- 5Around an air conditioner
- 6Basement walls, floors and carpet
- 7After a flood or a burst pipe
The common indoor molds
The NYC Health Department says hundreds of kinds of mold are common in the United States, and mold can be gray, black, green, yellow or orange, furry, slimy or powdery. Below are the kinds that come up most often in homes. Where U.S. agencies don't describe a mold in detail, we use the fact sheets of INSPQ, the public health institute of Quebec, Canada. The descriptions of what they look like are typical, not a way to identify them: the same mold can look different on different materials, and different molds can look alike.
Cladosporium
- Water:
- Surfaces that stay damp
- Often looks:
- In lab cultures, olive-green to brown, velvety or powdery
Cladosporium is one of the three molds the CDC names as most common indoors, and it is also everywhere outdoors, so some of what turns up inside simply blows in. Indoors, INSPQ says it often discolors paint, paper and fabrics that stay damp, and painted window frames are a classic spot. It sits in the middle of the moisture scale: it needs surfaces that stay damp, which in most homes means condensation.
INSPQ calls Cladosporium a common and important allergen, linked to hay fever and asthma. It notes that most kinds found in the environment don't cause infections, though people with severely weakened immune systems remain at risk of fungal infections, including from Cladosporium.
Often found on: Painted window frames,Damp paint and paper,Fabrics
Penicillium
- Water:
- Damp is enough
- Often looks:
- In lab cultures, usually green, sometimes white
Penicillium is another of the CDC's three most common indoor molds, and the WHO says it can be found in most houses. It needs relatively little water, so it can grow on materials that are only damp rather than wet. INSPQ lists drywall, wallpaper glue, damp particleboard and carpet among the places it grows.
INSPQ describes it as a very common allergen linked to hay fever and asthma, and says infections in people are rare, with the cases that do occur mostly in people with weakened immune systems. Because it doesn't need much moisture, check for high humidity and surfaces that stay slightly damp, not just for leaks.
Often found on: Drywall,Wallpaper glue,Carpet,Particleboard
Aspergillus
- Water:
- Damp is enough (most kinds)
- Often looks:
- Varies by kind; on building materials A. fumigatus is usually gray to grayish green
Aspergillus completes the CDC's list of the most common indoor molds. There are many kinds, and the WHO says Aspergillus, like Penicillium, can be found in most houses. Most kinds need relatively little water, though some, including Aspergillus fumigatus, need much more. INSPQ says A. fumigatus grows well on damp building materials, drywall and house dust, where it usually looks gray to grayish green.
Aspergillus can cause an infection called aspergillosis. The CDC says its spores are common indoors and outdoors and most people don't get sick from them. The people most at risk have weakened immune systems or lung disease, including people who recently had an organ or stem cell transplant. The infection doesn't spread from person to person.
Often found on: Damp building materials,House dust,Drywall
Alternaria
- Water:
- Soaked materials
- Often looks:
- In lab cultures, greenish black or olive brown
Alternaria is mainly an outdoor mold, and INSPQ says there is usually more of it outside than inside. Indoors it turns up in the dust on floors, carpets and mattresses, and on window frames that stay damp. The WHO puts Alternaria alternata among the molds that need the most water, though INSPQ gives a lower figure, close to Cladosporium's. Either way, growth on building materials points to condensation or a leak.
Its main health concern is allergy. INSPQ calls it a potent allergen and associates it with asthma attacks, and the WHO says an allergy to Alternaria has been linked to asthma starting, persisting and getting worse. INSPQ also says Alternaria infections are seldom reported but can no longer be called rare, and that people with weakened immune systems are most at risk.
Often found on: Damp window frames,Carpet and mattress dust
Stachybotrys chartarum (often called black mold)
- Water:
- Soaked materials
- Often looks:
- Greenish black; shiny and slimy when wet, powdery or sooty when dry
Stachybotrys chartarum is the mold most people mean by "black mold." The CDC describes it as greenish black and says it grows on materials high in cellulose, the plant fiber in paper and wood, like drywall, fiberboard and paper. It also needs constant moisture, which comes from water damage, leaks, condensation, water seeping in or flooding. INSPQ adds that it needs very wet conditions, and that it looks black, shiny and slimy while wet and powdery or sooty once it dries out.
The useful thing to know is how much water it needs. If it really is Stachybotrys, something has been wet for a long time, and the leak matters as much as the mold. As for health, the CDC says that at present no test exists that proves a link between Stachybotrys and particular symptoms, and that all molds should be treated the same for health risks and removal. More on that in the next section.
Often found on: Water-damaged drywall,Paper,Fiberboard
Chaetomium
- Water:
- Soaked materials
- Often looks:
- In lab cultures, cottony and white at first, then gray or olive
Chaetomium is a water-damage mold. INSPQ says that indoors it lives on cellulose materials like wood and drywall and needs a lot of moisture, so like Stachybotrys it usually means a leak or a flood. In lab cultures it grows cottony and white at first and turns tan-gray or gray-olive with age.
INSPQ reports that it can cause allergies and that confirmed infections in people are rare, though serious infections have occurred in people with very weakened immune systems. It also says Chaetomium is known to produce many potent mycotoxins, which are toxic substances some molds can make. As the next section explains, the WHO found the evidence that mycotoxins in indoor air cause health problems to be extremely weak. Either way, treat Chaetomium like any other mold: remove it and fix the water.
Often found on: Wet drywall,Wood
Aureobasidium
- Water:
- Surfaces that stay damp
- Often looks:
- In lab cultures, white, pink or cream at first, then black
Aureobasidium is a good reminder that black growth isn't necessarily "black mold." INSPQ says that in lab cultures it starts out white, pink, cream or yellow and turns black as it ages, and that indoors it grows on surfaces that stay damp in bathrooms and kitchens, such as tile grout, shower curtains and window sills.
INSPQ says it is a confirmed allergen but isn't known to produce mycotoxins, and that infections are rare and mostly seen in people with weakened immune systems. For a dark line along the grout or the bottom of a shower curtain, the EPA's advice for bathrooms applies: better ventilation and more frequent cleaning.
Often found on: Tile grout,Shower curtains,Window sills
What about black mold?
"Black mold" sounds like a diagnosis, but it isn't one. The EPA says "black mold" and "toxic mold" are not species or specific kinds of mold. The news media use the words for molds that may produce mycotoxins, or for Stachybotrys chartarum in particular. Molds come in many colors, and the EPA adds that nobody can tell by looking whether a mold is producing mycotoxins.
Much of the fear around black mold traces back to a cluster of infants with bleeding in the lungs in Cleveland in the 1990s. Early reports suggested a link with Stachybotrys. In 2000, though, the CDC reported that reviewers had found the evidence didn't adequately support those findings, and that the link should be considered not proven. In 2004 the Institute of Medicine, as quoted in the WHO's 2009 guidelines, found the evidence was inadequate or insufficient to say whether there is any link.
Mycotoxins, the toxic substances some molds can produce, do exist: the EPA says more than 200 have been identified from common molds, and some are known to affect people, though for many there is little health information. But the WHO found the evidence that mycotoxins play a role in health problems from indoor air "extremely weak," and said it wasn't clear whether the levels in damp buildings are high enough to cause harm.
None of that means mold is harmless. The CDC says mold can cause a stuffy nose, sore throat, coughing or wheezing, burning eyes or a skin rash in some people, while others have no reaction at all. People with asthma or a mold allergy can react severely, and people with weakened immune systems or chronic lung disease can get lung infections from mold. The WHO found enough evidence that people living in damp or moldy buildings are at higher risk of breathing symptoms, respiratory infections and worse asthma, and that fixing the dampness can reduce these problems. The EPA also points out that mold doesn't have to be alive to cause an allergic reaction, so killing it isn't enough. It has to be removed.
The practical answer is the same for every color: remove the mold, fix the water, and if you think mold is affecting your health, talk to a doctor. The EPA cautions against assuming mold is the cause of symptoms without checking.
Where mold grows, room by room
The New York State Department of Health names four places in a home that are always prone to mold: bathrooms, window moldings, the seal on the refrigerator door, and the surfaces on and around air conditioners. Leaks, basements and floods complete the list below.
1. Bathroom ceiling, walls and caulk
Steam from showers is one of the moisture sources the state Health Department lists, and shower stalls, tiles and curtains are where mold shows up first. The EPA's advice for a bathroom where mold keeps coming back is simple: run the exhaust fan or open a window when you shower, and clean more often. A dark film on grout is often a surface mold like Aureobasidium. Mold that spreads across the ceiling or comes back within weeks usually means the fan isn't moving enough air, or there's a leak.
If you rent in NYC, note that the city's housing code doesn't count mold on tile or grout as a violation, but mold on the ceiling or walls is the landlord's to fix.
2. Windows and the walls around them
When warm, humid indoor air meets a cold window or wall, water condenses on it. The EPA says to dry condensation quickly and treat it as a sign of high humidity. To cut it down, the EPA suggests lowering humidity, improving ventilation, insulating cold surfaces and raising the temperature. Cladosporium and Alternaria are both known for growing on damp window frames. Condensation can also form on a wall behind furniture pushed up against it, which is one of the places the EPA says mold hides.
3. A stain on the ceiling or wall
The CDC says mold grows wherever there's moisture, such as around leaks in roofs, windows or pipes. In apartment buildings a brown ring on the ceiling often comes from the apartment above, and in rowhouses from the roof. The mold you can see may be the smaller part of the problem: the EPA lists the back of drywall, the top of ceiling tiles, and the inside of walls around leaking pipes among the places mold hides. A musty smell with no visible mold is the usual clue, and the state Health Department says the smell is often the first sign of a problem.
Be careful about opening things up to look. The EPA warns that disturbing hidden mold, for example by pulling off wallpaper, can release a lot of spores, and suggests bringing in an experienced professional if you suspect hidden mold.
4. Under the sink or behind the fridge
Plumbing leaks are one of the most common moisture sources, and the cabinet under a sink is where a slow drip goes unnoticed for months. The state Health Department also lists the refrigerator door seal as a place mold always seems to find. Elsewhere in the kitchen, the EPA recommends venting stoves to the outside where you can, since cooking adds moisture to the air.
5. Around an air conditioner
Air conditioners pull water out of the air, and that water has to go somewhere. The EPA says to keep drip pans clean and drain lines flowing. Window units can also let rain in around the frame. If you suspect a central system has mold inside it, the EPA says not to run it, because it could spread mold through the building.
6. Basement walls, floors and carpet
Damp basements are on the state Health Department's list of moisture sources, along with carpet laid on concrete or below ground. The CDC advises against carpet in places that may get a lot of moisture, such as basements and bathrooms. A dehumidifier helps, but water coming through the walls needs to be fixed at the source.
7. After a flood or a burst pipe
Time is what matters most. The EPA says that if wet materials are dried within 24 to 48 hours after a leak or spill, in most cases mold won't grow. The CDC's advice after a flood is to dry everything within 24 to 48 hours if you can, move anything that can't be cleaned and dried in that time out of the house, and photograph it for insurance.
NYC Emergency Management adds two safety points. Don't turn on any lights or appliances until an electrician has checked the system, because flood water may be electrically charged. And pump out a flooded basement slowly, because pumping it out too quickly can cause serious damage to the building's walls. If the water was sewage or other contaminated water, the EPA says to call a professional who has experience with it.
What to do about it
Whatever the mold is called, the steps are the same, and the first one matters most.
- Find and fix the water. The EPA calls moisture control the key to mold control, and says that if you clean up mold without fixing the water problem, the mold will most likely come back.
- Clean small areas yourself. If the moldy area is less than about 10 square feet, roughly 3 feet by 3 feet, the EPA says you can usually handle it. Scrub hard surfaces with detergent and water and dry them completely. The EPA suggests an N-95 respirator (a close-fitting mask that filters fine particles), long gloves and goggles without vent holes, and the NYC Health Department says to keep children and pets away until the area is clean. If you have any health concerns, the EPA says to talk to a health professional before you start. That matters most if you have asthma, allergies, a lung condition or a weakened immune system.
- Throw out what can't be cleaned. Porous materials like ceiling tiles, carpet and drywall may have to go if they get moldy, because mold fills their pores and is hard or impossible to remove completely.
- Don't paint over it, and go easy on bleach. Paint and caulk over mold won't stop it growing. The EPA doesn't recommend bleach as a routine part of cleanup, and you should never mix bleach with ammonia or cleaners that contain it.
- Keep the air drier. The EPA recommends keeping indoor humidity below 60 percent, and ideally between 30 and 50 percent. The CDC says to keep it no higher than 50 percent. A humidity meter from a hardware store costs about $10 to $50, according to the EPA.
Bring in a professional when the area is bigger than about 10 square feet, when the water was sewage or flood water, when you suspect mold inside walls, or when someone in the home is very sensitive to mold. The state Health Department recommends a licensed contractor for mold problems larger than 10 square feet and for people who are highly sensitive. The EPA considers cleanup finished only when the moisture problem is fixed, the mold has been removed, and you can't see or smell any mold.
If you rent in NYC, report leaks and mold to your landlord rather than tackling a big job yourself. Owners of buildings with three or more apartments are legally responsible for fixing both, and our guide to NYC mold laws explains the deadlines and how to report it.
Do you need a mold test?
Often, no. The EPA says that in most cases, if you can see mold, sampling is unnecessary, and that no federal limits have been set for mold or mold spores, so there is nothing official to compare results against. The CDC doesn't recommend mold testing either, noting that there are no standards for an acceptable amount of mold in a home and that sampling can't tell you whether someone will get sick. The New York State Department of Health puts it plainly: knowing the type of mold doesn't change the advice.
Testing does earn its place in some situations. The EPA says sampling can help find the source when it isn't clear, separate mold from soot or dirt, and show whether a cleanup worked, since after a professional cleanup the mold in indoor air should be similar to the mold in the air outside. It can also help in a legal dispute. A test can't tell you whether mold is making someone sick, so for health concerns, talk to a medical professional. An air sample is only a snapshot of one moment, and the EPA says results are hard to interpret without experience, so sampling should follow a plan and be done by a professional.
That's how we use it. When mold is visible, we measure it and find the moisture. When the source is hidden, or when a job needs proof that remediation worked, we take samples for an independent lab and explain what the results mean.
Sources
The facts in this guide come from the government agencies, laws and public health sources listed below. We read each one again before publishing and update the guide when they change. Practical tips from our own work are never presented as an agency's advice.
- CDC: About mold
- CDC: Facts about Stachybotrys chartarum
- CDC MMWR (2000): Update on pulmonary hemorrhage among infants, Cleveland, Ohio
- CDC: About aspergillosis
- CDC: 8 tips to clean up mold
- US EPA: A brief guide to mold, moisture and your home
- US EPA: Mold course, chapter 1
- US EPA: Mold testing or sampling
- US EPA: Mold remediation in schools and commercial buildings, chapter 3
- NYC Health Department: Mold
- NYC Health Department: Prevent and get rid of mold (brochure)
- New York State Department of Health: Mold and your home (publication 7287)
- NYC Emergency Management: Flooding
- NYC HPD: Indoor Allergen Hazards (Mold, Mice, Roaches and Rats)
- Local Law 55 of 2018, full text (NYC), including the tile and grout exception
- World Health Organization (2009): Guidelines for indoor air quality, dampness and mould
- INSPQ (public health institute of Quebec, Canada): Cladosporium fact sheet
- INSPQ: Penicillium fact sheet
- INSPQ: Aspergillus fumigatus fact sheet
- INSPQ: Alternaria alternata fact sheet
- INSPQ: Stachybotrys chartarum fact sheet
- INSPQ: Chaetomium fact sheet
- INSPQ: Aureobasidium pullulans fact sheet
This guide is general information written in plain English, not legal or medical advice. Rules and guidance change, so for your own building go by your notice, if you have one, and the current rules from the agencies listed. For health questions, please talk to a medical professional. Last checked November 2026.