After the flood, the menace of mold

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NYC, Jersey soaked. What’s the story with mold?

As the Hurricane Sandy disaster fades from the headlines, residents are still trying to figure out what to do. With mold growing on their walls, possessions and furnishings, are their homes safe to enter? Do they need expert help to clean up and start over? Can they even salvage their properties?


Many people gathered outside severely damaged home with debris cluttering yard

Homeowners and neighbors clean up houses damaged from flooding from Hurricane Sandy in New York on Nov. 21, 2012.

The situation is reminiscent of Hurricane Katrina in 2005, but the science is much more distinct. After years of debate, it’s accepted that mold — the common name for a range of multi-cellular fungi — can be extremely unhealthy.

Mold can engage in a kind of biological warfare with the human occupants by

triggering allergies and asthma

releasing reproductive spores that irritate the respiratory system

making poisonous chemicals called mycotoxins that can damage genes, suppress the immune system or kill cells

One sick building

Indoor mold is a common villain in “sick building syndrome,” which can also be due to a multitude of other toxic compounds. In the United States, the syndrome dates to the energy crisis of the 1970s, says David Straus, professor of microbiology and immunology, who has been studying indoor mold for almost 20 years. “Architects began building tight and you could not open a window, to conserve energy costs.”


Large black dots of mold overtaking light yellow paint on indoor wall with square painting in center.

After Hurricane Katrina, New Orleans, melanie innis
After a flood, flamboyant mold growths warn of invisible toxins and spores.

In addition, misguided efforts to reduce air flow with impermeable materials caught water inside the dwelling, setting the stage for mold. “When they began to get water damage, organisms, toxins and spores would get concentrated inside,” Straus says, “and people inside, every time they took a breath, would inhale these toxins.”

Scientists have recently found these toxins in the cells and fluids of people who lived in fungus-infected buildings.

As we saw in New Orleans in 2005, and in New York and New Jersey this year, mold problems reach their zenith after a flood. Among the many types of mold, Stachybotrys chartarum earns the medal for producing the worst mycotoxins. These so-called trichothecenes inhibit protein synthesis and kill cells, causing mucosal bleeding, cognitive dysfunction, extreme rashes, vomiting and nausea. “Stachybotrys trichothecenes are considered biological warfare weapons by the U.S. Army,” Straus says.

Although trichothecenes can kill, “I don’t want to scare people,” Straus adds. “You are not going to die if you enter a building with Stachybotrys. The dose makes the toxin, and most of the time, the amount of trichothecene is sufficient to make people sick, but not to kill them.”


Air-masked person removing drywall in flood-damaged kitchen

Coney Island, N.Y., Luna Park NYC
Clean-up after a flood can be a nasty, dangerous job.

The idea that environmental mold could cause such a range of ailments took time to reach acceptance, but a 2002 report by the Centers for Disease Control (CDC) largely settled the question, Straus says.

Are people over-reacting to the sight of mold? “I think people are correctly paranoid,” Straus says.

A matter of the lung

Molds and the material and compounds they release can cause respiratory injury through mechanisms of allergy, irritation, inflammation and toxicity. They can even ingest toxic chemicals like lead from paint, and release them into the air.


Microscope cross-section, long strip with bulbous top sprouting small spheres

Phase contrast micrograph of spore formation and release in Aspergillus, a fungus implicated in indoor mold injury. Chains of asexual spores bud off from the hyphae called conidiophores. Most Aspergillus species are saprophytes, feeding on decaying organic matter.

The indoor molds that grow after a flood primarily affect the respiratory system, says Jay Portnoy, director of allergy, asthma and immunology at Children’s Mercy Hospital and Clinic, Kansas City, Missouri. “When mold grows on a surface that’s damp, it produces a lot of things that get into the air. Spores contain allergens that can become airborne, causing asthma and nasal allergy, so people have trouble breathing. They can also produce volatile organic compounds which produce that moldy, musty smell. It’s very irritating, and even if you are not allergic, can cause respiratory problems. And the sheer number of particles they produce as they make spores can be irritating. There is a lot of harm that can occur in a moldy environment.”

We’ve heard that neurological symptoms can develop in a water-damaged house1, and while mycotoxins can affect the nervous system when eaten, “whether they can produce enough mycotoxin to harm you through inhalation” is open to question, Portnoy says. “Some fanatics believe all their problems are caused by mold. Other people say there is no way mold can release enough mycotoxin to cause systemic problems. We don’t know, this is speculation. Opinion tends to dominate when facts are missing.”

One thing is certain: when you know how to look for mycotoxins, you find them. In a survey of water-damaged buildings in Sweden, Erica Bloom and colleagues looked for seven common mold-related toxins. They found at least one mycotoxin on 66 percent of building materials, 11 percent of the settled dust samples, and 51 percent of dust samples that they cultured (allowed to grow). The researchers concluded that “(a) molds growing on a range of different materials indoors in water-damaged buildings generally produce mycotoxins, and (b) mycotoxin containing particles in mold-contaminated environments may settle on surfaces above floor level.”2


Aerial view of neighborhood with houses submerged almost up to their gutters in water.

Photo: Matt Ewalt
Bacteria have plenty of time to grow in these water-logged conditions, but in general, mold is more active in damp conditions found after a flood recedes. Notice roof damage due to Hurricane Katrina’s wind…

Is mold the whole story?

Scanning electron micrograph of Penicillium chrysogenum

Scanning electron microscope image of mold, chain of spherical spores standing vertically

In this common fungus, asexual reproductive spores, called conidia, are maturing as they approach release. Penicillium, which grows on bread, is the source of penicillin, an early antibiotic. But when inhaled, P. chrysogenum can release an “allergen that can cause a significant allergic inflammatory reaction in the lungs of mice”3

Bystanders in biological warfare?

Competition drives many molds to make toxic chemicals, says allergist Jay Portnoy. “Molds are microorganisms that are trying to eke out a living in a niche with lots of competition from parasites, bacteria, and other molds, and they try to clear out an ecological niche for their kin by producing mycotoxins that are toxic to other molds and bacteria.”

This can be a good thing, since mycotoxins that obliterate bacteria are called antibiotics, and Streptomyces, a large group of bacteria that can be found in wet buildings, is the biggest source of antibiotics. And cyclosporine, a fungal chemical that deters competing fungi, is used to prevent immune rejection after organ transplants.

The struggle between microbes can help us or hurt us, Portnoy notes. “This is bio-warfare, little bombs of mycotoxins. But the mold does not care about us; we are innocent bystanders.”

While Straus sees mold “as the primary problem” in water-damaged buildings, the microbial morass in a flooded building can also contains bacteria, and that concerns Jack Dwayne Thrasher, a California toxicologist. He notes that many of these types of bacteria can either infect a person, produce disease-causing toxins, or both.

For example, Thrasher says streptomyces bacteria produce the toxin valinomycin, which “is more toxic than many of the mycotoxins produced by molds, and the streptomyces are human pathogens in their own right.”

Both major categories of bacteria (gram-negative and gram-positive) can produce toxins, Thrasher adds. “We have been trying to look at the entire indoor environment, to see just exactly what is going on, which agents are present? Toxic bacterial metabolites need to be considered as being part of very complex and diverse microbial exposures in ‘moldy’ buildings.”

Still, bacteria have gotten a lot less attention than mold, Thrasher says. “The CDC and EPA [Environmental Protection Agency] are overlooking everything except for mold. It’s a horrible oversight. There has been publication after publication, by a variety of scientists around the world, on mold and bacteria in these environments.”

In a 2011 study of homes with severe moisture damage4, all 69 dust and building-material samples contained at least one of the 186 toxins or suspected toxins under study. “For the first time, the presence of toxic bacterial metabolites and their co-occurrence with mycotoxins were shown for indoor samples,” the authors wrote.


Air-masked person with crowbar in room showing studs with drywall completely removed

After Katrina, a room is stripped bare of drywall, to deprive mold of a hospitable, cellulose-rich surface.

What to do?

And so the primary advice is something that people are already doing: hauling stuff to the curb. After Katrina, says Straus, “lay people learned a lot about what to do. The first thing is what you see, ripping out all the cellulose-containing building material, Sheetrock, carpet. If you leave them in, they are going to grow mold, and you will have all the associated health problems, so you need to get all the waterlogged material out.”

Drywall, the most popular interior wall material, is coated with paper, and mold spores sprout on it within minutes, says Straus, and become visible within a day or two.

Contaminated, porous materials like drywall and carpet need to be dumped, not bleached, says Straus. “We usually don’t recommend putting bleach on things. It may kill mold in the immediate time frame, but as long as things stay wet, mold will grow.”

And although bleach will kill mold at the surface, the slender hyphae of fungus penetrate deep into wood, which bleach cannot reach. “Wood, obviously, is much more difficult to remove, and wood framing is almost impossible,” says Straus. “If framing is dried quickly, mold is not going to grow on it, nothing like the way it grows on Sheetrock. The best thing to do is get all water-damaged material out immediately, and the things you can’t get out, try make sure they are dried as quickly as possible.”

After Katrina, houses that were flooded “really could not be salvaged in any intact form,” Portnoy says. Mold spores “start to germinate the minute they become wet, and there is literally no way to stop that. Many houses had to be pretty much gutted.”

Then, once everything is dried, rebuilding can begin.

Wood panel wall completely covered in multi-colored mold

Photo: Rose Vines
Mold grows on materials containing cellulose — its food. This fungal zoo decorated a pantry cupboard after Hurricane Katrina.

Extreme Home Resuscitation: A do-it-yourself project?

Cash-strapped homeowners usually don’t need expert assistance, at least at first, says Straus, since problematic mold will be visible or at least stinky. “If you don’t see it or smell it, I don’t think you are going need to bring anybody in for testing.”


Large yellow tractor claw raised over smashed house after flattening it

After Katrina, this New Orleans home could not be saved.

But at a certain point, expert advice makes sense, Portnoy says. “There are experts who specialize in this area; the typical home inspector may not have all the qualifications you need.”

Labs can test samples of dust and building material, Thrasher says. A test for 36 common mold species would cost around $250. Testing for specific mycotoxins, however, is much more expensive: around $600 per sample.

Many people wear a paper mask while removing moldy crud, and these masks may filter out spores and particles, but not mycotoxins. Thrasher, however, advocates more protection. “Treat it like a highly dangerous situation. Don’t do any cleaning without respiratory protection, up to and including a complete hazmat suit.”

The ultimate fate of a drenched building “is going to vary from building to building,” says Straus. “Many houses in New Orleans, the ‘shotgun shacks,’ had to be torn down. People left and the houses stayed wet for days, got filled with Stachybotrys; and you can’t save a house that is completely inundated with Stachybotrys.”

— David J. Tenenbaum


Terry Devitt, editor; Emily Eggleston, project assistant; S.V. Medaris, designer/illustrator; David J. Tenenbaum, feature writer; Amy Toburen, content development executive


  1. A water-damaged home and health of occupants: a case study, Thrasher JD et al, J Environ Public Health.2012;2012:312836
  2. Molds and Mycotoxins in Indoor Environments — A Survey in Water-Damaged Buildings, Erica Bloom et al, Journal of Occupational and Environmental Hygiene, 6: 671–678, 2009
  3. Molds, mycotoxins, and sick building syndrome, David C Straus, Toxicology and Industrial Health, 25(9-10) 617–635, 2009
  4. Co-occurrence of toxic bacterial and fungal secondary metabolites in moisture-damaged indoor environments, M. Taubel et al, Indoor Air 2011
  5. The biocontaminants and complexity of damp indoor spaces: more than what meets the eyes, Jack D Thrasher and Sandra Crawley, Toxicology and Industrial Health, 25(9-10) 583–615.
  6. Moldy house photos
  7. Mold and health:CDC’s advice
  8. What is mycology?
  9. Tons of mold and fungi teaching tools
  10. How dangerous is mold growing on food?
  11. Mold and moisture in a home: What to do
  12. When it is–and isn’t–okay to eat moldy cheese
  13. One month after Hurricane Sandy, there’s still work to do