UK Indoor Air Quality Guide 2026 — Pollen Calendar, HEPA Science & NHS Guidance
Share
Most UK allergy sufferers focus on outdoor pollen counts. But research shows the air inside your home may be doing more damage — and your windows are the problem, not the solution.
This guide brings together the latest science, NHS guidance, and a complete UK pollen calendar so you can understand exactly what you're breathing — and what to do about it.
Why Indoor Air Is Often Worse Than Outdoor Air
The common assumption is that staying indoors during high pollen days protects you. The reality is more complicated.
Research led by King's College London as part of the EXPOSOMICS project found that indoor air pollution in UK homes can reach concentrations 2 to 5 times higher than outdoor levels. Unlike outdoor air, which is constantly diluted and moved by wind, indoor air accumulates pollutants with nowhere to go.
Here's what builds up inside a typical UK home:
- Pollen tracked indoors on hair, clothing and pets — where it settles on surfaces and becomes resuspended by movement
- Dust mite allergens (Der p 1 and Der p 2 proteins) concentrated in carpets, mattresses and upholstery
- Pet dander — protein particles from skin, saliva and urine that remain airborne for hours
- VOCs (volatile organic compounds) from cleaning products, furniture, paint and cooking
- Nitrogen dioxide from gas hobs, at levels that can exceed WHO outdoor limits within 30 minutes of cooking
- PM2.5 fine particles from candles, incense, and even toasting bread
Opening windows helps — but only if outdoor air quality is better than indoor air quality. During peak pollen season, opening windows can dramatically increase indoor pollen concentrations, particularly in the morning (6am–10am) when pollen is released and counts peak.
"Indoor environments concentrate pollutants that outdoor wind disperses. A home with poor ventilation management can expose occupants to higher allergen loads than spending time outdoors."
The Full UK Pollen Calendar — With Particle Sizes
Understanding which pollen is in the air — and how large those particles are — helps you understand both your symptoms and how filtration works against them.
Tree Pollen Season: February – May
| Tree | Peak Period | Particle Size | Allergy Impact |
|---|---|---|---|
| Hazel | February–March | 20–30 microns | Moderate |
| Alder | February–April | 20–35 microns | Moderate |
| Birch | April–May | 20–30 microns | High — major allergen |
| Oak | April–June | 25–35 microns | High |
| Ash | March–May | 25–30 microns | Moderate–High |
| Plane | April–May | 20–25 microns | High (urban) |
Grass Pollen Season: May – August
Grass pollen is the most common cause of hay fever in the UK, affecting an estimated 95% of hay fever sufferers. It's also the most variable in size, which is why some grass pollen can penetrate deeper into the respiratory tract.
| Grass Type | Peak Period | Particle Size | Notes |
|---|---|---|---|
| Timothy grass | June–July | 30–40 microns | Primary UK allergen |
| Rye grass | May–August | 20–35 microns | Common in lawns and fields |
| Meadow grass | May–July | 25–40 microns | Widespread |
| Fescue | June–August | 25–35 microns | Golf courses, parks |
Note on thunderstorm asthma: During thunderstorms, grass pollen grains rupture into sub-pollen particles as small as 0.5–2.5 microns. These particles penetrate deep into the bronchial airways, which is why thunderstorm events can trigger severe asthma in people who don't normally have it. Standard HEPA filtration captures these particles.
Weed Pollen Season: June – September
| Weed | Peak Period | Particle Size | Notes |
|---|---|---|---|
| Stinging nettle | June–September | 15–25 microns | Very common UK weed |
| Plantain | June–August | 18–22 microns | Wayside and lawns |
| Mugwort | July–September | 18–30 microns | Cross-reacts with birch |
| Fat hen | July–September | 20–28 microns | Farmland areas |
All pollen types listed above — from the smallest weed pollen at 15 microns to the largest tree pollen at 100 microns — are captured at 99.97%+ efficiency by true HEPA filters, which are tested and certified at 0.3 microns (the most penetrating particle size).
How HEPA Captures Pollen at the Physics Level
Most people assume air filters work like a sieve — holes small enough to catch particles. That's not how HEPA works, and understanding the actual physics explains why it's so effective against the full pollen size spectrum.
HEPA filters are dense mats of randomly arranged borosilicate glass fibres. Particles are captured through three separate physical mechanisms, each dominant at different particle sizes:
1. Impaction (large particles: >10 microns)
Large pollen grains have too much mass to follow airflow as it curves around filter fibres. They continue in a straight line, impacting directly into a fibre. Efficiency increases with particle size — which means all visible pollen is captured at very high rates.
2. Interception (medium particles: 1–10 microns)
Medium-sized particles do follow the airflow, but pass close enough to a fibre that they make contact and stick. The particle doesn't need to collide — proximity is sufficient due to van der Waals adhesion forces at the fibre surface.
3. Diffusion (tiny particles: <0.1 microns)
Very small particles don't travel in straight lines. Their collisions with air molecules create random Brownian motion — they zigzag unpredictably. This random path dramatically increases their chance of hitting a fibre. Efficiency increases as particles get smaller.
The 0.3-micron rating represents the "most penetrating particle size" (MPPS) — the size where impaction and diffusion are both at their minimum. HEPA's 99.97% rating at this worst-case size means everything larger (all pollen) and smaller (viruses, combustion particles) is captured at higher efficiency.
NHS and NICE CG134 Guidance on Indoor Allergen Reduction
NICE guideline CG134: Allergic Rhinitis in Adults and Children (updated 2023) provides clinical guidance on managing hay fever and perennial allergic rhinitis. Alongside pharmacological treatment (antihistamines, intranasal corticosteroids), NICE recommends:
- Allergen avoidance as a first-line approach — including pollen, dust mite and pet dander reduction
- Keeping windows closed on high pollen count days (particularly mornings)
- Showering and changing clothes after coming indoors to remove pollen
- HEPA air filtration in bedrooms as part of allergen reduction strategy
- Dust mite mattress and pillow encasements combined with HEPA filtration for perennial rhinitis
The NHS also advises hay fever sufferers to check the Met Office pollen forecast daily during season, and to prioritise bedroom air quality — since most people spend 7–9 hours per night there.
Why bedroom air quality matters most: The bedroom is where cumulative allergen exposure is highest. Eight hours of continuous exposure to airborne pollen, dust mite allergens or pet dander overnight produces a more sustained inflammatory response than brief outdoor exposure. A HEPA purifier running overnight consistently reduces the allergen load in bedroom air.
UK Regional Pollen Risk by Area
Pollen levels across the UK vary significantly based on vegetation, climate and geography. This map provides a general guide to relative risk by region:
High Risk Regions
- South East England (Kent, Surrey, Sussex, Hampshire) — long grass pollen season, high oak and birch counts
- East Anglia (Norfolk, Suffolk, Cambridgeshire) — extensive arable land with high grass pollen, early season
- East Midlands (Leicestershire, Northamptonshire, Lincolnshire) — grassland-dominant landscape, high counts
- West Midlands (Warwickshire, Worcestershire) — mixed tree and grass pollen, urban concentration effects
- Greater London — plane tree pollen (April–May) plus urban grass pollen. High due to parks and transport-assisted pollen spread
Moderate Risk Regions
- South West England (Cornwall, Devon, Somerset) — milder, wetter climate reduces peak counts but extends season
- Yorkshire and Humber — mixed; urban areas lower, rural areas moderate to high
- North West England (Lancashire, Cheshire) — moderate grass pollen, lower tree pollen than south
- Wales — predominantly moderate; upland areas significantly lower
Lower Risk Regions
- Scotland — shorter, later season; lower counts overall. Birch pollen significant in central lowlands
- Northern Ireland — Atlantic climate moderates pollen release and dispersal
- Coastal areas nationwide — sea breezes disperse pollen; counts typically lower within 2km of coast
- Upland and moorland areas — sparse vegetation, low pollen diversity and count
Regardless of region, indoor pollen levels track outdoor levels with a 2–4 hour delay in homes without filtration. Filtration eliminates this entirely — maintaining clean indoor air even when outdoor counts are high.
5 Questions About Indoor Air Quality — Answered
Is indoor air really more polluted than outdoor air in the UK?
Yes. Research from King's College London found that indoor air pollution levels can be 2–5 times higher than outdoor air in UK homes and offices. Modern buildings trap pollutants — VOCs from cleaning products, cooking fumes, pet dander, dust mites, and pollen tracked in from outside — with little ventilation to dilute them.
When is pollen season in the UK?
The UK pollen season runs from February through September. Tree pollen (including birch, oak and ash) peaks February–May. Grass pollen — the most common allergy trigger — runs May–August, peaking in June and July. Weed pollen (nettle, plantain, mugwort) follows from June–September. In warm years, seasons can start earlier and last longer.
What does NICE guidance say about managing hay fever indoors?
NICE guideline CG134 recommends reducing exposure to known allergens as a first-line approach alongside antihistamines and nasal steroids. This includes keeping windows closed during high-pollen days, showering after coming indoors, and using HEPA-grade air filtration to remove airborne pollen, dust mite particles and pet dander from indoor air.
How does a HEPA filter actually capture pollen particles?
HEPA filters capture particles through three physical mechanisms. Large pollen particles (10–100 microns) are caught by impaction. Medium particles are caught by interception — they follow airflow but brush against a fibre and stick. Tiny particles are caught by diffusion — Brownian motion causes them to zigzag into fibres. True HEPA captures 99.97% of particles at 0.3 microns, the hardest size to trap — meaning all pollen (10–100 microns) is captured at even higher efficiency.
Which UK regions have the highest pollen levels?
The South East of England, East Anglia, the East Midlands and parts of the West Midlands typically experience the highest grass pollen counts due to extensive farmland and grassland. London sees high grass pollen combined with urban tree pollen. Scotland, Northern Ireland, coastal regions and upland areas generally have lower counts — though birch pollen can be significant in Scottish lowlands.
What to Read Next
Our product recommendation guides on the blog cover the best air purifiers for different room sizes, allergy types and budgets — with honest comparisons and real performance data from UK testing conditions.
