How Aspergillus Mould in Buildings Can Pose a Serious Health Risk

Recent news reports have raised concerns after a cluster of fungal infections linked to Aspergillus was investigated in a Sydney hospital. In that case, several transplant patients developed infections after being exposed to spores from this common mould, highlighting how environmental fungi can become a serious risk in certain settings.

Events like the recent hospital cases also highlight a broader issue: mould and airborne fungal spores are closely linked to indoor air quality and building hygiene, particularly in environments where vulnerable people are present.

Let’s take a look into what Aspergillus is, where it grows, why it can sometimes become dangerous, and what it means for indoor environments such as homes, workplaces and healthcare facilities.

What is Aspergillus?

Aspergillus is a type of mould (fungus) that occurs naturally in the environment. It belongs to a large group of filamentous fungi that grow in long thread-like structures and reproduce by releasing microscopic spores into the air.

These spores are extremely common and can be found in many places, including:

• Soil and compost
  
• Decaying leaves and plants
  
• Dust and outdoor air
  
• Damp indoor environments

Because the spores are so widespread, people inhale small amounts of Aspergillus every day without even realising it. In most healthy individuals, the immune system clears these spores before they can cause any harm.

Scientists have identified more than 20 species of Aspergillus capable of causing disease in humans, with Aspergillus fumigatus being the most commonly associated with infections. Other species such as A. flavus and A. terreus can also cause illness in certain situations.

Read related article: Is That Mould Dangerous? How to Identify Common Types at Home

When infections do occur, they are known as aspergillosis. Rather than being a single disease, aspergillosis is considered a spectrum of conditions that can range from mild allergic reactions to severe infections.

For example, medical literature describes several forms of aspergillosis, including:

• Allergic aspergillosis, where the body reacts to the mould spores
  
• Chronic pulmonary aspergillosis, which affects people with existing lung disease
  
• Invasive aspergillosis, a serious infection that can occur in people with weakened immune systems

Most people exposed to Aspergillus will never develop these conditions. But when large numbers of spores are inhaled, or when a person’s immune system is compromised, the mould can sometimes grow inside the lungs and lead to infection.

Read related article: Is Toxic Mould Making You Sick? Understanding Chronic Inflammatory Response Syndrome (CIRS)

How does Aspergillus affect the body?

Aspergillus affects the body primarily through inhalation of airborne spores. These microscopic spores are released into the air as the mould grows and reproduces. Because they are extremely small, they can easily be breathed deep into the lungs.

In healthy individuals, the immune system usually prevents infection. Once spores enter the airways, immune cells such as macrophages and neutrophils quickly recognise and destroy them. This is why most people are exposed to Aspergillus regularly without becoming sick.

However, problems can occur when large numbers of spores are inhaled or when a person’s immune system is weakened or compromised.

Spore entry and lung colonisation

When inhaled, Aspergillus spores can travel into the lungs and reach the alveoli, the tiny air sacs responsible for oxygen exchange.

If the immune system cannot eliminate them effectively, the spores may begin to germinate and develop into thread-like fungal filaments called hyphae. These hyphae allow the fungus to grow within lung tissue and extract nutrients from surrounding cells.

As the fungus expands, it releases enzymes and toxic metabolites that damage surrounding tissue and trigger inflammation.

This process can lead to symptoms such as:

• Persistent cough
  
• Fever
  
• Chest pain
  
• Shortness of breath
  
• Coughing up blood in severe cases

Allergic reactions

In some individuals, Aspergillus does not invade tissue but instead causes an allergic immune response.

This condition, known as allergic bronchopulmonary aspergillosis (ABPA), is more common in people with asthma or cystic fibrosis. In these cases, the immune system reacts strongly to fungal spores, causing airway inflammation, mucus buildup and worsening respiratory symptoms.

Invasive Aspergillosis

In people with severely weakened immune systems, the fungus can cause invasive aspergillosis, the most serious form of infection.

In this condition, the fungal hyphae can penetrate lung tissue and invade nearby blood vessels. Once the fungus enters the bloodstream, it may spread to other organs such as:

• The brain
  
• Kidneys
  
• Heart
  
• Skin
  
• Sinuses

A clinical case reported in medical literature described a patient developing cerebral aspergillosis, where the infection spread to the brain and formed abscess-like lesions visible on brain imaging scans. These infections can lead to neurological complications including headaches, seizures, confusion and other neurological symptoms.

Because the infection can damage blood vessels and surrounding tissue, invasive aspergillosis is considered a life-threatening medical condition and requires urgent antifungal treatment.

Read more about how mould toxicity affects the nervous system & hormones.

Who is most at risk of Aspergillosis?

For most healthy people, exposure to Aspergillus spores does not lead to illness. Our immune systems are generally very effective at recognising and removing these fungal particles before they can grow or cause infection.

However, some individuals have a much higher risk of developing aspergillosis, particularly when their immune defences are weakened or when their lungs are already compromised.

People with weakened immune systems

The highest risk group includes people whose immune systems are suppressed. This can occur due to medical treatments or certain health conditions that reduce the body’s ability to fight infections.

Higher-risk groups include:

• Organ transplant recipients, who take immunosuppressive drugs to prevent organ rejection
  
• Patients undergoing chemotherapy for cancer
  
• Stem cell or bone marrow transplant patients
  
• People taking long-term corticosteroids or other immune-suppressing medications

People with existing lung conditions

Certain lung conditions can also increase vulnerability to Aspergillus. Damaged or inflamed airways may provide an environment where spores can settle and grow.

People with higher risk include those with:

• Asthma
  
• Chronic obstructive pulmonary disease (COPD)
  
• Cystic fibrosis
  
• Previous tuberculosis or lung scarring

In these individuals, Aspergillus may trigger allergic reactions or chronic lung infections.

Hospital and healthcare patients

Patients in hospitals—especially those in transplant units, intensive care or oncology wards—are among the most vulnerable to invasive infections. This is why hospitals often place strong emphasis on air filtration, ventilation management and microbial control to reduce exposure to airborne spores.

Construction work near hospitals can sometimes increase risk by disturbing soil and releasing fungal spores into the surrounding air, which is why infection control measures are carefully monitored during building works.

How the environment influences exposure

Even though Aspergillus spores are present almost everywhere in the environment, the concentration of spores can vary significantly depending on environmental conditions. Certain environments can allow mould to grow and release larger numbers of spores into the air, increasing the likelihood of exposure.

Damp or water-damaged buildings are one of the most common sources of indoor mould growth. Moisture from leaks, condensation, flooding or high humidity can create ideal conditions for fungi to thrive on materials such as plasterboard, insulation, ceiling tiles and wood.

Ventilation systems can also play an important role. If HVAC systems are not properly maintained, dust and organic debris can accumulate inside components such as coils, drain pans and ducts. These areas may trap moisture, creating an environment where mould spores can develop and circulate through the building.

How to reduce mould growth indoors

Because mould spores such as Aspergillus are naturally present in the air, completely eliminating them is almost impossible. The goal of mould prevention is instead to control the conditions that allow mould to grow, particularly moisture and organic debris.

By managing these factors, you can significantly reduce the chances of mould developing indoors.

Control moisture and humidity

Moisture is the most important factor in mould growth. Damp environments allow fungal spores to settle, grow and reproduce.

To reduce this risk:

• Fix plumbing leaks or roof leaks as soon as possible
  
• Use exhaust fans in bathrooms and kitchens
  
• Keep indoor humidity levels ideally below 50–60%
  
• Dry water-damaged materials quickly after flooding or spills

Even small sources of moisture, such as condensation on windows or pipes, can create conditions that encourage mould growth over time.

Improve ventilation

Good ventilation helps remove excess moisture and prevents stale, humid air from accumulating indoors.

Practical steps include:

• Opening windows where possible
  
• Ensuring ventilation systems are functioning correctly
  
• Using exhaust fans in high-moisture areas
  
• Avoiding blocked air vents and return grilles

Buildings with poor airflow are more likely to experience persistent dampness and mould problems.

Address water damage quickly

Water-damaged materials such as plasterboard, carpets, ceiling tiles and insulation can become breeding grounds for mould if they remain wet for too long.

If water damage occurs:

• Dry affected materials as soon as possible
  
• Remove severely damaged materials if necessary
  
• Inspect surrounding areas to ensure mould has not spread

Early intervention is one of the most effective ways to prevent larger mould infestations.

Maintain HVAC and ventilation systems

Heating, ventilation and air-conditioning (HVAC) systems can influence indoor air quality throughout a building. Dust, moisture and organic debris may accumulate in certain components, particularly around coils, drain pans and air-handling units.

Regular inspection and cleaning of ventilation systems helps prevent conditions where mould can develop and circulate through the air supply.

If you're interested in learning more about maintaining ventilation systems to reduce microbial build-up, you can read our guide: Step-by-Step HVAC Deep Clean: Guide to Using SAN-AIR HVAC Cleaning Products

Monitor for early warning signs

Mould problems often start small before becoming visible. Being aware of early warning signs can help prevent larger issues.

Watch for:

• Persistent musty odours
  
• Visible mould growth on walls or ceilings
  
• Condensation on windows or surfaces
  
• Recurring damp patches or water stains

Addressing these signs early can help prevent mould from spreading throughout the building.

While many small mould issues can be addressed with basic cleaning and moisture control, some situations may require professional assessment. Minor surface mould caused by condensation or small leaks may be manageable with proper cleaning and ventilation improvements. However, larger infestations, recurring mould growth, or contamination inside building systems such as walls or HVAC units often require specialised expertise. 

In these cases, professionals such as mould remediation specialists, HVAC technicians or building inspectors may be needed to properly identify and resolve the source of the problem.

Read more about the different specialists involved in diagnosing and resolving mould-related issues here: Who to Call for Mould Problems: The Experts You Need for Every Type of Mould Issue.

Conclusion

Mould is something many people rarely think about until it becomes visible or begins to cause problems. Yet fungi such as Aspergillus are a natural part of our environment and are present in the air around us every day. Recent news stories have simply reminded us how important it is to understand these microorganisms and the conditions that allow them to grow.

Greater awareness of mould and indoor air quality can help people recognise potential risks earlier. Damp environments, persistent condensation, water damage and musty odours are often early warning signs that something may be wrong inside a building. Paying attention to these signals allows issues to be addressed before mould spreads or affects indoor air quality.

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