Introduction

During the summer, if you have allergies, you may struggle with sneezing, coughing, and watery eyes. However, even when the sun has disappeared, these symptoms may persist. A less commonly known reason for your symptoms is mould. Mould is an allergen found both indoors and outdoors. Understanding the differences can help you better manage your symptoms and health. This article will explain what mould is, how it affects the body, and how dealing with mould is crucial to managing allergies.

What is mould?

Mould is a type of fungus that reproduces by releasing spores into the air. It thrives in warm, moist environments. These spores can easily move through the air and water, allowing mould to quickly colonise new habitats (homes).¹ Mould can trigger allergies and respiratory symptoms, whether through inhalation or direct contact. These allergy symptoms include:²

• Hay Fever (Allergic Rhinitis)
• Asthma attack (asthma exacerbation)
• Runny or Blocked Nose (Chronic Sinusitis)
• Eczema (Atopic Dermatitis)

These symptoms occur because the immune system mistakenly identifies the spores as harmful invaders. They then release histamines to fight them off, which causes inflammation in the airways.³ Additionally, in vulnerable people with compromised immune systems, the risk of invasive fungal infections can be life-threatening if not treated quickly.⁴

Indoor mould:

In 2023, the English Housing Survey found that 3% of homes experienced serious condensation or mould issues.⁵ Indoor mould can spread rapidly, often becoming visible within 24 to 48 hours of coming into contact with damp surfaces.⁶ Some moulds which produce mycotoxins (harmful chemical compounds) are toxic when inhaled or ingested.⁷ Indoor mould tends to be a year-long problem, especially in damp or poorly ventilated homes.¹

Common indoor mould species:

The most common types of mould found in homes include:⁶

• Aspergillus: Found in air conditioning units or dust
• Penicillium: Commonly found on water-damaged building materials. This also includes wallpaper, insulation, and carpets
• Cladosporium: Commonly found on wood, carpets, or insulation
• Stachybotrys chartarum (black mould): This is found in buildings damaged by water

Risk factors for indoor mould include:

Key factors that can promote mould growth indoors include:¹

• Leaky roofs, pipes, or windows
• Poor ventilation in bathrooms or kitchens
• High humidity indoors
• Condensation around windows and walls
• Flood damage or persistent dampness

Mould must be treated promptly and seriously. Even a small area of mould can trigger symptoms.

Common indoor spots for mould include:

Mould spores can also remain dormant (inactive) in dry conditions. Common household spots for mould include:¹

• Bathrooms: Especially around the shower or sink
• Kitchens: Beneath the sink or behind appliances
• Other surfaces: Window frames, wallpapers, and mattresses

Outdoor mould

Outdoor mould exists in soil, fallen leaves, dead plants, and animal waste.^1,9 Mould is important, as it is a natural decomposer (breaks down organic materials), which helps recycle nutrients back into the soil.¹⁰ Some mould species can produce allergens which severely affect allergy-prone individuals. This is because the spores, which are released into the air, are easy to inhale. Therefore, high spore counts can worsen their respiratory symptoms.

Common outdoor mould species:⁹

• Alternaria: Commonly found in soil, decomposing wood, and rotten food
• Cladosporium: Also found outdoors on decaying organic material
• Aspergillus: Found in soil, crops, or plants

Environmental Factors Influencing Outdoor Mould:¹¹

Outdoor mould growth is affected by the seasonal weather patterns, with spore movement usually at its highest:

• During warm and humid conditions, when temperatures rise above 20°C
• After rainstorms, when there is high moisture
• During gardening, when raking leaves or mowing the grass

Outdoor mould levels fluctuate with the seasons, with higher levels during late summer and early autumn.¹¹ Some studies have suggested that climate change is increasing the duration and intensity of the mould spore season.¹²

Allergy symptoms: indoor vs outdoor mould

While both indoor and outdoor mould can cause similar allergic reactions, the duration and intensity of exposure are different. This can affect the frequency and severity of your symptoms.

Common mould allergy symptoms:^1,13

Symptoms are typically respiratory or dermatological (skin-related). Symptoms can range from mild discomfort to conditions that are painful and chronic. Chronic conditions are common in individuals who have pre-existing respiratory symptoms:

• Sneezing
• Runny nose (rhinorrhoea)
• Stuffy nose (nasal congestion)
• Itchy, red, or watery eyes
• Puffy eyes (periorbital swelling)
• Coughing and wheezing
• Skin rashes or hives (urticaria)

During the winter months, when closed windows reduce ventilation further, indoor mould allergies can lead to persistent or chronic symptoms. One study found that people living in damp, mouldy buildings were at 40% to 80% times greater risk of respiratory symptoms than those in dry environments.¹⁴ In outdoor mould, exposure is typically more acute than chronic.¹ However, in asthmatic patients, these high outdoor mould levels can seriously affect patients' health.

Sensitivity differences:

Interestingly, individual immune responses can vary depending on the mould species. Some people are highly reactive to specific moulds (e.g., Aspergillus, Alternaria) but remain asymptomatic to others.¹⁵

Diagnosis and testing

To diagnose your mould allergy, a doctor may recommend:^13,16

• Skin prick test: This involves placing mould allergens on the skin to check for a physical reaction
• Blood test: Measures immunoglobulin E (IgE) antibodies specific to different mould allergens. This measures the immune system’s response to mould species

Additionally, it is recommended to track when and where symptoms occur to help determine the source of the mould.

For identifying mould in the home, consider:¹

• Visual inspection for dampness or discolouration
• Air quality testing
• Humidity monitoring
• Mould detection kits or professional inspection

Management and prevention

Indoor mould management

To reduce mould exposure and prevent mould growth, you could:^1,2

• Reduce indoor humidity: Ensure places with high humidity, like bathrooms and laundry rooms are well ventilated.
• Improve ventilation: Sufficient airflow in the kitchen and bathroom is also important. If opening the window is not possible, use exhaust fans. Also, avoid drying clothes indoors as this can increase moisture in the air
• Fix leaks promptly: Even a minor leak from pipes, windows, and the roof can provide enough moisture for mould to grow
• Use mould-resistant materials: When developing or renovating your home, make sure to use mould-resistant drywall, insulation, and paint
• Clean regularly: Ensure places such as tile grout, sink basins, and shower walls are regularly cleaned with vinegar or hydrogen peroxide

Outdoor mould management

While outdoor mould cannot be prevented, allergy sufferers should reduce exposure and manage symptoms by following these precautions:^9,11,13

• Wear protective gear: It is recommended to use an N95 respirator mask when gardening. Wearing gloves and long-sleeved clothing to reduce direct contact between the skin and mould
• Shower and change clothes immediately after outdoor activities: As mould spores can stick onto your skin, hair, and clothing, you must change clothes quickly to ensure that no spores contaminate the inside of your house
• Limit outdoor exposure in the evening: Due to humidity and reduced sunlight in the evening and nighttime, mould spores tend to peak during this time. Schedule outdoor work for midday when spore levels are usually lower

When to seek medical help

Seek medical advice if:³

• Symptoms continue even when you manage the environment
• You experience tightness in your chest or difficulty breathing
• You suspect mould exposure is affecting your child or elderly family member

Your GP may recommend medications such as antihistamines, corticosteroid nasal sprays, or, in severe cases, immunotherapy.¹⁶

Summary

Understanding the difference between indoor and outdoor mould is important for people who suffer from allergies. While both indoor and outdoor mould types release spores that can trigger allergies, indoor mould could be more dangerous, as you are exposed to it for an extended period of time. Outdoor mould, while harder to control, is often seasonal and predictable. By identifying your triggers, preventing exposure, and going to the doctor if needed, you can take control of your symptoms and breathe a little easier.