Winter Mould vs Summer Spores: Tracking Cladosporium vs Alternaria

Published: Mon Mar 23 2026 00:00:00 GMT+0000 (Coordinated Universal Time)

When people think about allergies, pollen and dust mites tend to dominate the conversation. Yet Cladosporium vs Alternaria — two of the most prevalent airborne mould spores in the UK — can trigger persistent respiratory and allergic symptoms that many individuals struggle to identify. Understanding the seasonal behaviour of these mould allergens is an important step towards proactive wellbeing, and specific IgE blood testing can help clarify whether mould spore sensitivity may be contributing to ongoing symptoms.

Mould spore allergies affect a significant proportion of the UK population, and London's urban environment — with its combination of older housing stock, variable humidity, and dense vegetation — creates conditions where both Cladosporium and Alternaria thrive at different times of the year. This article explores their differences, seasonal patterns, and how allergy screening may offer useful insight.

What Are Cladosporium and Alternaria?

Cladosporium and Alternaria are two genera of environmental fungi that release microscopic spores into the air. These spores are among the most common outdoor aeroallergens worldwide. When inhaled, they can sometimes trigger immune responses in sensitised individuals, potentially contributing to symptoms such as nasal congestion, sneezing, itchy eyes, and respiratory discomfort. Specific IgE blood testing can help identify whether an individual has developed sensitivity to one or both of these mould types.

Both fungi belong to the Deuteromycetes group and are found extensively across the UK, though their growth patterns, peak seasons, and preferred environments differ considerably.

Key Characteristics

Cladosporium is one of the most abundant airborne moulds globally. It thrives in both indoor and outdoor environments, with spore counts detectable throughout much of the year — including cooler months.
Alternaria is predominantly an outdoor mould associated with decaying plant matter, soil, and agricultural environments. Its spore release peaks during warmer, drier months.
Both produce lightweight spores that travel easily on air currents — and can even be carried across continents during events like Saharan dust plumes — meaning exposure is often unavoidable even without visible mould growth nearby.

Practical Insight: Many individuals with year-round nasal or respiratory symptoms may not immediately consider mould spore exposure as a potential contributing factor, particularly during winter months when Cladosporium remains active.

Seasonal Patterns: When Do Spore Counts Peak?

One of the most clinically relevant differences between Cladosporium and Alternaria is their seasonal behaviour. Understanding these patterns can help individuals — and their healthcare providers — contextualise symptoms more effectively.

Cladosporium: The Year-Round Presence

Cladosporium spores are detectable in UK air samples from early spring through to late autumn, with some counts persisting into winter, particularly indoors. Damp bathrooms, window frames with condensation, and poorly ventilated kitchens can all harbour Cladosporium colonies during colder months.

In London specifically, winter condensation in older properties and flats without adequate ventilation may sustain indoor Cladosporium levels even when outdoor counts are relatively low.

Alternaria: The Summer Specialist

Alternaria spore counts in the UK typically rise from late May and peak between July and September. Hot, dry, and windy conditions favour spore dispersal — and thunderstorm events can further amplify airborne mould concentrations. Alternaria is particularly associated with agricultural areas, gardens, compost heaps, and fallen leaves — though urban parks and green spaces in London also contribute to local counts.

Cladosporium vs Alternaria: Comparison Table

Feature	Cladosporium	Alternaria
Peak season (UK)	Spring through autumn; some winter indoor presence	Late spring through early autumn (June–September peak)
Primary environment	Indoor and outdoor; damp surfaces, soil, decaying matter	Predominantly outdoor; soil, decaying vegetation, crops
Spore size	Small (2–10 µm); penetrates lower airways more easily	Larger (20–60 µm); tends to affect upper airways
Indoor relevance	High — especially in damp, poorly ventilated UK homes	Lower — primarily enters through open windows and doors
Common symptom pattern	May contribute to persistent, year-round symptoms	May contribute to seasonal symptom flare-ups in summer
UK prevalence	Very common; highest airborne mould spore count in UK	Common; second most prevalent outdoor mould allergen
IgE testing available	Yes — specific IgE (m2 Cladosporium herbarum)	Yes — specific IgE (m6 Alternaria alternata)

Practical Insight: Individuals who notice that their symptoms persist outside the traditional grass or tree pollen season may wish to consider whether mould spore sensitivity could be relevant — particularly if symptoms worsen in damp conditions or during warm, dry spells.

Who Should Consider Mould Allergy Testing?

Mould spore allergy testing may be worth considering for individuals who experience:

Persistent nasal symptoms (congestion, sneezing, runny nose) that do not clearly align with pollen seasons
Year-round respiratory discomfort that worsens in damp or humid environments — a pattern explored further in our guide on mould allergy symptoms and damp homes
Seasonal symptom patterns that peak in late summer or persist into autumn and winter — the so-called “second allergy season”
Symptoms that improve when away from home or usual indoor environments
A history of sensitivity to other environmental allergens, as co-sensitisation to moulds is not uncommon
Living or working in London properties with known damp issues, condensation, or limited ventilation

A specific IgE blood test can help identify whether the immune system has produced antibodies against Cladosporium, Alternaria, or both. This information can sometimes be helpful when discussing symptom management with an appropriate healthcare professional.

Our clinic provides allergy blood testing to help individuals understand their specific sensitivities. We offer testing and reporting — results can then be shared with a GP or relevant healthcare provider for further guidance.

Practical Insight: Mould sensitisation sometimes co-exists with other allergies. Individuals who already know they are sensitised to house dust mites or pet dander may find it useful to check for mould-specific IgE as part of a broader allergy screening panel.

How Often Should Mould Allergy Testing Be Considered?

For most individuals, a single baseline mould allergy blood test can provide useful information about current sensitisation status. However, there are circumstances where periodic re-testing may be considered:

After significant environmental changes — such as moving to a new property, experiencing a damp problem at home, or changes to workplace conditions
If symptoms change — for example, if previously seasonal symptoms become year-round, or if new symptom patterns emerge
As part of broader allergy review — some individuals choose to include mould panels when repeating other environmental allergy checks

There is no single recommended frequency that applies to everyone. The decision to retest is best guided by individual circumstances and, where appropriate, discussion with a healthcare professional.

Understanding Your Results: What Mould-Specific IgE Levels May Indicate

When a specific IgE blood test for Cladosporium or Alternaria returns a result, it is typically reported as a concentration level (kU/L) alongside a classification:

Negative or very low levels may suggest that mould spore sensitisation is unlikely to be a significant factor in current symptoms
Mildly elevated levels can sometimes indicate low-level sensitisation, which may or may not be clinically relevant depending on symptom patterns
Moderately or highly elevated levels may suggest more significant sensitisation, which could be worth discussing with a GP or allergy-focused healthcare service

It is important to note that a positive IgE result indicates immune sensitisation — it does not, on its own, confirm that mould exposure is the cause of symptoms. Clinical correlation with symptom history, timing, and environmental context is essential, and this assessment is best carried out by an appropriate healthcare professional.

Practical Insight: IgE blood testing provides objective data that can complement a symptom diary. Recording when and where symptoms occur — alongside test results — may help build a more complete picture for discussion with medical advice providers.

London and UK Context: Why Mould Matters Here

The UK climate creates particularly favourable conditions for mould growth. High average humidity, significant rainfall, and the prevalence of older housing stock with inadequate ventilation all contribute to elevated indoor mould exposure for many residents.

In London specifically:

Victorian and Edwardian terraced properties, which make up a large proportion of the housing stock, are especially susceptible to condensation and damp
Basement and ground-floor flats in areas across London often experience persistent moisture issues
Urban green spaces and parks — while valuable for wellbeing — also contribute to outdoor Alternaria and Cladosporium spore counts during warmer months
The NHS provides allergy services, though waiting times for specialist mould allergy assessment can vary. Private allergy testing can offer a timely way to gather initial information about sensitisation, with results available to share with NHS or private healthcare providers

According to UK-based aerobiology monitoring, Cladosporium consistently accounts for the highest proportion of airborne fungal spores across most UK monitoring stations, while Alternaria — though present in lower concentrations — is considered more clinically potent per spore in terms of allergenic activity.

Practical Steps for Mould-Aware Wellbeing

While our clinic focuses on testing and reporting rather than providing medical advice or treatment, the following general environmental awareness points are widely recognised in UK public health guidance:

Ventilation — ensuring adequate airflow in kitchens, bathrooms, and bedrooms may help reduce indoor mould spore levels
Monitoring humidity — a hygrometer can help track indoor humidity levels; many sources suggest keeping levels below 60%. Our article on humidifiers in London flats discusses how indoor moisture management affects allergy triggers
Awareness of peak seasons — being mindful that Alternaria peaks in summer and Cladosporium persists year-round can help contextualise symptoms
Pollen and mould calendars — several UK resources provide daily spore count information during the main season

For individuals who have identified mould sensitisation through blood testing at our clinic, sharing these results with a GP or appropriate healthcare professional can be a constructive next step.

Frequently Asked Questions

What is the difference between Cladosporium and Alternaria allergies?

Cladosporium and Alternaria are both common airborne mould fungi, but they differ in seasonal patterns and environmental behaviour. Cladosporium is present year-round — including indoors during winter — while Alternaria predominantly peaks in summer months. Specific IgE blood testing can identify whether an individual is sensitised to one or both, which may help explain symptom timing and triggers.

Can mould spores cause allergy symptoms in winter?

Yes, certain mould types — particularly Cladosporium — can remain active indoors during winter months. Condensation on windows, damp in bathrooms, and poorly ventilated rooms may sustain mould growth. Individuals experiencing persistent winter nasal or respiratory symptoms that do not align with typical cold or flu patterns may wish to consider mould spore sensitisation as a potential factor.

How is mould allergy tested in the UK?

Mould allergy testing in the UK typically involves a specific IgE blood test, which measures immune antibodies directed against particular mould allergens such as Cladosporium herbarum (m2) or Alternaria alternata (m6). This blood test can be conducted at private clinics and does not require a referral. Results can then be discussed with an appropriate healthcare professional.

Is Alternaria more dangerous than Cladosporium?

Neither mould should be described as inherently dangerous, though research suggests Alternaria alternata may have higher allergenic potency per spore compared to Cladosporium. Alternaria sensitisation has been associated in some studies with more significant respiratory responses. However, individual reactions vary, and sensitisation status is best assessed through appropriate testing.

Why are my allergy symptoms worse in a damp house?

Damp indoor environments can support the growth of multiple mould species, including Cladosporium, Aspergillus, and Penicillium. Higher indoor humidity and visible damp or condensation may increase airborne spore concentrations. If symptoms appear to worsen in damp conditions, mould-specific IgE testing may help identify whether mould sensitisation is a contributing factor.

Can I be allergic to Cladosporium and Alternaria at the same time?

Yes, co-sensitisation to multiple mould allergens is not uncommon. An individual may show elevated specific IgE levels to both Cladosporium and Alternaria, as well as other environmental allergens. Comprehensive allergy blood testing can screen for multiple sensitisations simultaneously, offering a broader view of potential triggers.

Should I get tested if I only have symptoms in summer?

Summer-predominant symptoms that persist beyond the grass pollen season — particularly in July through September — may sometimes be linked to Alternaria spore exposure rather than, or in addition to, pollen allergy. Specific IgE testing for Alternaria can help clarify whether mould sensitisation may be relevant to late-summer symptom patterns.

Where can I get a mould allergy blood test in London?

Private allergy clinics in London, including The Allergy Clinic, offer specific IgE blood testing for common mould allergens. This does not require a GP referral. Results are provided in a clear report format that can be shared with a healthcare professional for further discussion and guidance.

How long does it take to get mould allergy test results?

Turnaround times for specific IgE blood tests typically range from a few days to around two weeks, depending on the laboratory and the breadth of the panel requested. Our clinic provides clear timescale information at the point of booking, ensuring transparency throughout the testing process.

Can children be tested for mould allergies?

Yes, specific IgE blood testing for mould allergens can generally be carried out for children. If a child is experiencing persistent or seasonal nasal, eye, or respiratory symptoms, and mould exposure is considered a possibility, testing may provide useful information. Any results should be discussed with the child's GP or appropriate paediatric healthcare service.

About This Article

This content has been produced following UK medical editorial best practice, drawing on established aerobiological and immunological literature. It is written to support public understanding of mould spore allergies and the role of specific IgE blood testing as part of a proactive approach to wellbeing.

The Allergy Clinic provides allergy blood testing and reporting services. We do not provide diagnosis, treatment, or prescriptions. Our role is to deliver clear, accurate test results that individuals can use in discussions with their chosen healthcare provider.

Medical Disclaimer

This article is provided for educational and informational purposes only. It does not constitute medical advice, diagnosis, or treatment recommendations. The content is not intended to replace consultation with a qualified healthcare professional.

Individual symptoms, health concerns, and test results should always be assessed by an appropriate healthcare professional who can take into account personal medical history and clinical context. No guarantees regarding health outcomes are made or implied.

AllergyClinic.co.uk provides nurse-led venous blood sample collection and laboratory reports only. We do not offer doctor or GP consultations, clinical interpretation of results, prescribing, or treatment planning as part of our test bookings. Always consult a qualified healthcare professional — such as your GP or an NHS/private allergy specialist — for interpretation of your results, clinical guidance, and any treatment decisions.

If you are experiencing severe or worsening symptoms, call 999 (UK) or 112 (EU) immediately, or contact the NHS 111 service.

This content complies with GMC advertising guidance, CQC patient communication standards, ASA standards, and UK medical editorial best practice.

This content complies with UK GMC advertising guidance, CQC patient communication standards, and ASA advertising standards.

Written Date: 23 March 2026 Next Review Date: 23 March 2027