How the conversion works
- Relative units (ppm / ppb) describe concentration by volume
- Absolute units (µg/m³ / mg/m³) describe concentration by mass
Conversion depends on the gas molecular weight and environmental conditions. In MyAir, we use:
- Temperature: 20 °C
- Pressure: 1013 hPa
These are standard reference conditions used in industry guidance.
Conversion factors used
Pollutant | Guidance | Conversion |
Carbon Monoxide (CO) | DEFRA | 1 ppm = 1.1642 mg/m³ |
Ozone (O₃) | DEFRA | 1 ppb = 1.9957 µg/m³ |
Sulphur Dioxide (SO₂) | DEFRA | 1 ppb = 2.6609 µg/m³ |
Nitrogen Dioxide (NO₂) | DEFRA | 1 ppb = 1.9125 µg/m³ |
Nitric Oxide (NO) | EH40 | 1 ppb = 1.25 µg/m³ |
Hydrogen Sulphide (H₂S) | EH40 | 1 ppb = 1.4 µg/m³ |
Carbon Dioxide (CO₂) | EH4O | 1 ppm = 1.83 mg/m³ |
TVOC (Isobutylene equivalent) | Calculated (MW 56.11) | 1 ppb = 2.33 µg/m³ |
Reference sources
- DEFRA conversion factors: https://uk-air.defra.gov.uk/reports/cat06/0502160851_Conversion_Factors_Between_ppb_and.pdf
- EH40 workplace exposure limits: https://www.hse.gov.uk/pubns/books/eh40.htm
Summary
- MyAir converts all gases using fixed factors at standard conditions
- Factors are based on DEFRA and EH40 guidance
- This ensures consistent and transparent unit conversion across the platform