Indoor Air Quality Index UKWhat is PM2.5, AQI, DAQI? 24 Terms Explained
Complete indoor air quality index guide for UK homes. Understand PM2.5, AQI, HEPA, MERV vs HEPA, CADR, and other air quality measurements to check indoor air pollution and improve your home.
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Pollutant
PM2.5 (Particulate Matter 2.5)
Fine particles ≤2.5 micrometres (3% width of human hair). Sources: vehicle exhaust, wood burning, industry. Health impact: penetrates lungs/bloodstream causing asthma, heart disease, cancer. WHO guideline: ≤5 µg/m³ annual average.
PM10 (Particulate Matter 10)
Particles ≤10 micrometres. Larger than PM2.5 but still inhalable. Sources: dust, pollen, mould spores, construction. Trapped in nose/throat but can irritate respiratory system. WHO guideline: ≤15 µg/m³ annual.
NO₂ (Nitrogen Dioxide)
Traffic pollution gas from vehicle exhausts, gas stoves, heating. Causes respiratory inflammation, worsens asthma. UK legal limit: 40 µg/m³ annual average. WHO guideline: 10 µg/m³. Measured at roadside monitoring stations.
VOC (Volatile Organic Compounds)
Gases emitted from paints, cleaning products, furniture, carpets, air fresheners. Examples: formaldehyde, benzene, toluene. Health effects: headaches, dizziness, eye/throat irritation, some carcinogenic. Require activated carbon filters to remove.
Ozone (O₃)
Ground-level gas formed when NO2 + VOCs react in sunlight. Respiratory irritant worsening asthma. Peaks on hot summer days. UK threshold: 100 µg/m³ (8-hour average). Some "ionising" air purifiers produce ozone - avoid these.
Measurement
ACH (Air Changes per Hour)
The number of times the complete air volume in a room is filtered and replaced per hour. Target: 5-6 ACH for bedrooms/living rooms. Formula: (CADR × 60) / Room Volume (m³).
AQI (Air Quality Index)
Universal scale 0-500 measuring air pollution levels. 0-50 is Good, 51-100 Moderate, 101-150 Unhealthy for Sensitive Groups, 151-200 Unhealthy, 201+ Very Unhealthy. Based on PM2.5, PM10, O3, NO2, SO2, CO.
Filtration
HEPA (High-Efficiency Particulate Air)
Filter standard removing ≥99.97% of particles ≥0.3 micrometres. True HEPA (H13-H14) certified to EN 1822. Captures pollen, mould, pet dander, PM2.5, bacteria, viruses. Used in hospitals, cleanrooms, air purifiers.
MERV (Minimum Efficiency Reporting Value)
Filter efficiency rating scale 1-20. MERV13 removes 85%+ PM2.5, MERV16 removes 95%+. Higher MERV = better filtration but requires more powerful fan. UK air purifiers typically use MERV13-16 or HEPA H13.
ULPA (Ultra-Low Penetration Air)
Filter grade above HEPA removing ≥99.999% particles ≥0.12 micrometres. Used in semiconductor manufacturing, medical research labs. Overkill for home use - HEPA H13 sufficient for residential air quality. Higher cost and airflow resistance.
Activated Carbon Filter
Porous carbon material absorbing gases, odours, VOCs. Does NOT capture particles - pair with HEPA/MERV for complete filtration. Replace every 3-6 months. Used for cooking odours, smoke, chemical fumes. Not effective for PM2.5.
HEPA H13
European EN 1822 filter grade removing ≥99.95% particles ≥0.3 µm. Higher than USA True HEPA (99.97%). Standard for UK air purifiers. H14 removes ≥99.995% but requires stronger fan. H13 sufficient for home use.
Equipment
Air Quality Monitor
Device measuring indoor/outdoor pollutants (PM2.5, PM10, VOC, CO2, temperature, humidity). Types: laser particle counters (accurate), optical sensors (affordable). Brands: Dylos, Purple Air, Atmotube. Use to determine when to run purifiers.
Ionizer / Ionising Air Purifier
Device releasing negatively-charged ions to attract particles. WARNING: Many produce ozone (harmful). Not recommended by UK health authorities. Less effective than HEPA filtration. Avoid "ionising" feature on air purifiers unless certified ozone-free.
UK Standards
DAQI (UK Daily Air Quality Index)
UK government air quality scale 1-10. Low (1-3), Moderate (4-6), High (7-9), Very High (10). Measures O3, NO2, SO2, PM2.5, PM10. Different from Universal AQI - DAQI uses UK-specific thresholds.
Clean Air Zone (CAZ)
UK urban areas charging non-compliant vehicles to reduce pollution. Categories: C (vans/taxis/buses), D (includes cars). Active in Birmingham, Manchester, Bradford, Sheffield, Newcastle. Daily charge: £8-£12.50. Reduces NO2 by 15-21%.
DEFRA
Department for Environment, Food & Rural Affairs - UK government department managing air quality policy. Operates UK Air Quality Archive (uk-air.defra.gov.uk) with 300+ monitoring stations nationwide. Sets pollution limits and DAQI thresholds.
Standards
WHO Air Quality Guidelines
World Health Organization recommendations: PM2.5 ≤5 µg/m³ annual, PM10 ≤15 µg/m³, NO2 ≤10 µg/m³. More stringent than UK legal limits. Adopted by London and other UK cities. Based on health evidence linking pollution to mortality.
DIY
Corsi-Rosenthal Box
DIY air purifier design using 4-5 MERV13 filters taped to box fan. Invented by Richard Corsi & Jim Rosenthal. Cost: £50-100. Performance: 200-400 m³/h CADR. Popular during COVID-19 pandemic. As effective as commercial units.
Performance
CADR (Clean Air Delivery Rate)
Volume of clean air an air purifier delivers per hour, measured in cubic metres per hour (m³/h) or cubic feet per minute (CFM). Higher CADR = faster room cleaning. Example: 300 m³/h CADR provides 6 ACH in a 50m³ room.
Noise Level (dB)
Air purifier sound measured in decibels. 30-40dB: whisper (sleep-safe), 41-50dB: normal conversation, 51-60dB: background music, 60+dB: too loud for bedrooms. Check CADR-to-noise ratio: aim for 300+ m³/h at <45dB.
Maintenance
Filter Lifespan
Duration before air purifier filter needs replacement. HEPA/MERV13: 6-12 months, Activated Carbon: 3-6 months, Pre-filter: 1-3 months. Varies by usage hours, pollution levels. Replace when airflow drops or filter darkens significantly.
Meteorology
Temperature Inversion
Weather condition trapping pollution near ground level. Warm air layer above prevents pollutants rising/dispersing. Common in winter mornings. Causes pollution spikes in valleys/cities. Check air quality before outdoor exercise during inversions.
UK Weather
Saharan Dust
Fine sand from Sahara Desert blown to UK by southerly winds (March-May). Causes temporary PM10/PM2.5 spikes (50-100 µg/m³). Turns sky orange/hazy. Close windows, run air purifiers on high during episodes. Met Office issues alerts.
Related Air Quality Resources
Live Air Quality Monitor
Check real-time PM2.5, AQI, and pollutant levels near you in the UK.
ACH Calculator
Calculate Air Changes per Hour for your room based on CADR and dimensions.
HEPA vs MERV Filters
Complete comparison of filter types, efficiency ratings, and UK applications.
DIY Air Purifier Guides
Build Corsi-Rosenthal Box and other DIY filtration systems for £50-200.
Understanding UK Air Quality Standards
The UK uses two main air quality scales: the UK Daily Air Quality Index (DAQI) from DEFRA and the Universal AQI used internationally. Understanding these terms helps you interpret pollution data and make informed decisions about ventilation, outdoor activity, and air purifier usage.
| Measurement | UK Standard | WHO Guideline | Why It Matters |
|---|---|---|---|
| PM2.5 | 10 µg/m³ annual | 5 µg/m³ annual | Penetrates lungs causing asthma, heart disease, cancer |
| PM10 | 40 µg/m³ annual | 15 µg/m³ annual | Respiratory irritation, worsens existing conditions |
| NO₂ | 40 µg/m³ annual | 10 µg/m³ annual | Traffic pollution worsening asthma, respiratory inflammation |
Note: UK legal limits are less stringent than WHO guidelines. Many UK cities (London, Birmingham, Manchester) exceed WHO recommendations. Indoor air filtration helps compensate for outdoor pollution exposure.