Date
Jan 7, 2026 11:02 AM
Sensors
ENV-CP
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.