4 Common Types of Mold and How to Prevent Mold Growth

Zombie media is a staple of modern pop culture. From iconic films like “Night of the Living Dead” to shows like “The Walking Dead,” audiences are captivated by stories of society overturned by reanimated corpses. But “The Last of Us,” a hugely popular video game and TV series, offers a fresh twist. Its apocalypse is caused by an evolved fungus, Ophiocordyceps unilateralis, a real-life parasite that compels ants and spiders to act in strange ways.

While Cordyceps can’t infect humans in reality, it sparks curiosity about what could happen if fungi evolved to threaten us. In the meantime, plenty of other fungi and mold already lurk in our homes and buildings, feeding this fascination with the unknown.

In case you have an issue with mold growth and to give you a better idea what you might be dealing with, here’s a list of 4 Common Types of Mold and suggestions on how to prevent mold growth. But first, we’ll explain three classes of mold.

How is Mold Classified?

Molds are generally classified into three categories: allergenic, pathogenic, and toxigenic. Allergenic molds primarily cause allergic reactions, especially in individuals with respiratory conditions like asthma. Pathogenic molds are more harmful, capable of infecting humans, particularly those with compromised immune systems, leading to infections. Toxigenic molds are the most dangerous, as they produce mycotoxins—harmful chemicals that can cause serious health issues. Prolonged or high-level exposure to mycotoxins can be lethal, making this type of mold particularly hazardous.

Four Common Types of Mold

 

1. Stachybotrys

 

Mold Category: Toxigenic

Where it’s found: High moisture and humid areas, including HVAC systems

Stachybotrys is one of the world’s 10 most hazardous fungi. This toxic mold, including around a dozen species, is among the most dangerous you can encounter. One common species, Stachybotrys chartarum, known as Black Mold, often infects homes and buildings. Areas contaminated by black mold, referred to as “sick buildings,” can cause prolonged exposure symptoms like nausea, vomiting, and bleeding in the lungs and nose, as well as diarrhea. People with allergies or weakened immune systems, especially children, are at greater risk.

Stachybotrys chartarum has a green-black appearance and thrives in high-moisture areas on cellulose-rich materials like fiberboard, gypsum liner paper, straw, and hay. While it prefers humid conditions, it can grow across a range of temperatures. It commonly appears after water damage from leaks or infiltration and after natural disasters like flooding.

 

2. Aspergillus

Mold Category: Pathogenic and Allergenic

Where it’s Found: In the air and in HVAC systems

Aspergillus is another common type of mold. It can live both indoors and outdoors and, like other types of fungus, it spreads spores into the air. In humid environments, you may find this type of mold growing in the HVAC system of a home or building.

“A clogged air-conditioning drain line can create mold issues within an HVAC system,” says Bill Ivey, Ivey Engineering’s principal consultant. “Since mold requires moisture to grow and condensation collects in the drain line, this is the perfect breeding ground for mold.”

Aspergillus can cause a disease called Aspergillosis, which has several subtypes depending on the location of symptoms and the specific Aspergillus strain involved. Common symptoms include coughing, shortness of breath, and other cold-like signs.

Avoiding Aspergillus spores is challenging since they are naturally present in the environment. However, reducing exposure in high-risk areas can help. While most people can inhale Aspergillus spores daily without issue, those with weakened immune systems or lung conditions face higher health risks.

For those concerned about exposure, testing and antifungal treatments are available. Of the 180 known Aspergillus species, only about 40 impact human health.

 

3. Cladosporium

 

Mold Category: Pathogenic and Allergenic

Where it’s Found: Both indoors and outdoors in high-moisture and humid areas

Cladosporium is another common mold found in homes and buildings, with over 40 species. Like Stachybotrys chartarum, it thrives in areas with high moisture and humidity, typically in basements, ceilings, bathrooms, and around sinks and faucets.

Visually, it appears as clusters of brown and green spots and can quickly spread across large surfaces. However, it can be difficult to distinguish from Stachybotrys chartarum due to its similar appearance.

Most Cladosporium species have minimal impact on human health, but long-term exposure can worsen allergies and asthma. Regardless of whether it causes symptoms, it should be safely removed from any affected space.

4. Penicillium

 

Mold Category: Allergenic

Where it’s Found: On spoiled food and walls

Penicillium, commonly known as Green or Blue Mold, is the most frequently found mold indoors. As its name suggests, it appears bluish green with a fluffy texture. This allergenic mold can trigger allergy symptoms, asthma, heart inflammation, and pose serious risks to immunocompromised individuals.

Some Penicillium species are famously used to produce penicillin, a groundbreaking 20th-century antibiotic. Found worldwide, it thrives in cool, damp environments—even in your fridge. Common in temperate and subtropical regions, it grows on decaying organic material and is used in certain moldy cheeses. Indoors, it can appear between tiles, on wooden floors, or in damp basements, especially after water damage.

Mold Prevention and Cleanup

Mold typically becomes a problem once it’s visible or noticeable by smell. You might be able to treat small areas of non-toxic molds, like allergenic ones, with bleach or vinegar. However, the Environmental Protection Agency recommends hiring a contractor if the mold covers more than 10 square feet or if you’re unsure of the type.

“Make sure the contractor has experience with mold removal,” Ivey says. “A trained professional specializing in mold remediation will have the right equipment and expertise to safely remove it.”

Beyond health concerns, mold can also reduce property value by 20 to 37 percent, making professional removal a wise investment. Mold remediation costs range from $500 to $8,000, depending on the mold’s location. In severe cases, such as an entire house, treatment can reach $30,000.

Preventing mold is essential for maintaining a clean, healthy environment, even if you’re not sensitive to spores. Using dehumidifiers, fixing water leaks as soon as possible, and applying antifungal paint on walls are ways to keep mold at bay.

Ivey adds that it’s also important to maintain your home or building’s A/C system to prevent possible mold growth. Knowing other causes of mold can help in preventing it. Check out our list of four common causes of mold.

We hope you enjoyed reading our list of the most common types of mold found in homes and buildings. If you’re looking for a plumbing or HVAC expert witness to investigate potential causes of excessive moisture or water leaks, give Ivey Engineering a call.