How Indoor Air is Measured – Tools and Metrics

To take control of indoor air quality, it’s important to understand how key contaminants are measured and what the numbers mean. Many modern air quality monitors display real-time data, but not all use the same units or provide clear guidance on what’s safe. Here’s a breakdown of the most common measurements, safe ranges, and when to take action.

1. PM2.5 (Fine Particulate Matter)

  • Unit: Micrograms per cubic meter (µg/m³)
  • Healthy Range: 0–12 µg/m³
  • Caution: 12.1–35 µg/m³ (may affect sensitive individuals)
  • Unhealthy: 35+ µg/m³ (everyone may be affected)
  • Note: Some devices display PM2.5 in an “AQI scale” instead. You can convert:
    • 0–50 AQI ≈ 0–12 µg/m³
    • 51–100 AQI ≈ 12.1–35 µg/m³

2. PM10 (Coarse Particulate Matter)

  • Unit: Micrograms per cubic meter (µg/m³)
  • Healthy Range: 0–50 µg/m³
  • Caution: 50.1–150 µg/m³
  • Unhealthy: 150+ µg/m³

3. TVOCs (Total Volatile Organic Compounds)

  • Units: Milligrams per cubic meter (mg/m³) or parts per billion (ppb)
  • Healthy Range: 0–0.5 mg/m³ (0–220 ppb)
  • Caution: 0.5–2.0 mg/m³ (220–880 ppb)
  • Unhealthy: >2.0 mg/m³ (880+ ppb)
  • Conversion Tip: Multiply mg/m³ by ~440 to estimate ppb (for common VOCs like formaldehyde or toluene)

4. Formaldehyde (HCHO)

  • Units: mg/m³, ppb, or µg/m³
  • Healthy Range:
    • ≤0.08 mg/m³
    • ≤80 µg/m³
    • ≤65 ppb
  • Caution:
    • 0.08–0.25 mg/m³ (65–200 ppb)
  • Unhealthy:
    • >0.25 mg/m³ (200+ ppb)

5. Carbon Dioxide (CO₂)

  • Unit: Parts per million (ppm)
  • Healthy Range: 400–800 ppm
  • Caution: 800–1,200 ppm (mild drowsiness, poor focus)
  • Unhealthy: 1,200+ ppm (headaches, fatigue, cognitive effects)
  • Critical: 5,000+ ppm (OSHA time-weighted exposure limit for 8 hours)

6. Carbon Monoxide (CO)

  • Unit: Parts per million (ppm)
  • Healthy Range: 0–3 ppm
  • Caution: 4–8 ppm
  • Unhealthy: 9+ ppm
  • Dangerous: 35+ ppm (EPA 1-hour limit); 100+ ppm = Emergency
  • Important: Always install CO alarms with a digital display for real-time readings.

7. Radon

  • Unit: Picocuries per liter (pCi/L) or Becquerels per cubic meter (Bq/m³)
  • Safe Range:
    • 0–2.0 pCi/L
    • 0–75 Bq/m³
  • Caution Zone:
    • 2.1–3.9 pCi/L (75–150 Bq/m³)
  • Action Level:
    • 4.0+ pCi/L (150+ Bq/m³)
  • Conversion Tip:
    • 1 pCi/L ≈ 37 Bq/m³

8. Relative Humidity (RH)

  • Unit: Percentage (%)
  • Optimal Range: 40–50%
  • Caution Zones:
    • 30–39% (dryness issues)
    • 51–60% (mold risk increases)
  • Unhealthy:
    • <30% (dry air, respiratory irritation)
    • >60% (ideal for mold, dust mites, bacteria)

🛠 Pro Tip:

When using a multi-sensor air quality monitor, make sure:

  • You know which unit it’s displaying (some allow toggling between µg/m³ and AQI or ppb and mg/m³).
  • You check for calibration options and data logs, especially if monitoring for sensitive individuals.