SCO2, PM25, ENV-CP Air Quality Level & Value Mapping

We have integrated a simple numerical description system into ENV-CP, which may aid in understanding general circumstances compared to cold, hard figures. The defining details of this system are explained here.

CO₂ (Carbon Dioxide)

The sensor evaluates indoor CO₂ concentration and maps it to clear air-quality levels and descriptive statuses.

• < 600 ppm
• Level: Excellent
• Status: Excellent air quality
• 600 – 799 ppm
• Level: Good
• Status: Good air quality
• 800 – 999 ppm
• Level: Fair
• Status: Fair air quality
• 1000 – 1499 ppm
• Level: Mediocre
• Status: Ventilation recommended
• 1500 – 1999 ppm
• Level: Bad
• Status: Ventilation required
• ≥ 2000 ppm
• Level: Bad
• Status: Heavily contaminated indoor air

Notes

• Outdoor fresh air is typically around 400 ppm.
• Sustained CO₂ levels above 1000 ppm indicate insufficient ventilation.

PM2.5 (Fine Particulate Matter)

PM2.5 concentration is mapped to qualitative dust levels based on commonly used air-quality thresholds.

• ≤ 12.0 µg/m³
• Dust Level: Good
• 12.1 – 35.4 µg/m³
• Dust Level: Moderate
• 35.5 – 55.4 µg/m³
• Dust Level: Unhealthy for Sensitive Groups
• 55.5 – 150.4 µg/m³
• Dust Level: Unhealthy
• 150.5 – 250.4 µg/m³
• Dust Level: Very Unhealthy
• > 250.4 µg/m³
• Dust Level: Hazardous

Notes

• PM2.5 refers to particles smaller than 2.5 µm, which can penetrate deep into the lungs.
• Short-term spikes may occur during cooking, smoking, or poor outdoor air conditions.

Rebreathed Fraction (%)

Rebreathed Fraction is the estimated percentage of indoor air that has been previously exhaled by people, derived from CO₂ concentration.

• Unit: %
• Typical Range: 0–5% (can be higher in poorly ventilated spaces)
• Theoretical Range: 0–100%
• How it’s calculated:
• Outdoor CO₂ reference: 400 ppm
• Exhaled human breath CO₂: 38,000 ppm
• Formula:
• Rebreathed (%) = clamp((CO₂_indoor - 400) / (38000 - 400), 0..1) × 100
• Interpretation:
• 0% → Air is close to outdoor air (very fresh, well ventilated)
• Higher values → More rebreathed air, indicating poorer ventilation

CO₂ vs Rebreathed Fraction (Reference)

• 400 ppm → 0.0% (Outdoor / fresh air)
• 600 ppm → ~0.5%
• 800 ppm → ~1.1%
• 1000 ppm → ~1.6%
• 1500 ppm → ~3.0%
• 2000 ppm → ~4.3%

Notes

• Values are approximations, calculated using the formula above.
• Rebreathed Fraction is most useful for comparing ventilation quality, not as a health threshold.
• A sustained value above ~2% usually indicates that ventilation should be improved.