PM2.5 Monitoring
Learning Objectives
Students will be able to:
- Explain how optical PM2.5 sensors work
- Interpret PM2.5 readings and relate them to health guidelines
- Identify common sources of indoor and outdoor PM2.5
- Use the Air Quality Index (AQI) to understand PM2.5 levels
Why Monitor PM2.5?
PM2.5 is the most health-relevant air pollutant.
Unlike CO2, which tells us about ventilation, PM2.5 directly affects our health. These tiny particles can penetrate deep into our lungs and even enter our bloodstream. Monitoring PM2.5 helps us know when we need to take protective action.
How Optical PM Sensors Work
The Process
- Air intake: A small fan draws air into the sensor
- Laser beam: Air passes through a focused laser beam
- Light scattering: Particles scatter the laser light in all directions
- Detection: A photodetector measures scattered light
- Calculation: Software estimates particle count and size
Light Scattering Principle
Larger particles scatter more light at different angles than smaller particles. By measuring the intensity and pattern of scattered light, the sensor can estimate both the size and number of particles in the air.
Understanding PM2.5 Units
PM2.5 is measured in micrograms per cubic meter (μg/m³)
What does this mean?
- 1 microgram = 1 millionth of a gram
- 1 cubic meter ≈ the air in a phone booth
- 10 μg/m³ = 10 millionths of a gram per cubic meter
- Even at 100 μg/m³ (very unhealthy), particles are invisible
Putting it in perspective
- A single grain of salt ≈ 60,000 micrograms
- At 12 μg/m³, you inhale ~170 μg per day
- At 150 μg/m³, you inhale ~2,100 μg per day
- Small amounts add up over time!
The Air Quality Index (AQI)
The AQI converts PM2.5 concentrations into a standardized scale from 0-500.
| AQI Range | PM2.5 (μg/m³) | Level | Health Advisory |
|---|---|---|---|
| 0-50 | 0-12 | Good | Air quality is satisfactory |
| 51-100 | 12.1-35.4 | Moderate | Acceptable; sensitive individuals may be affected |
| 101-150 | 35.5-55.4 | Unhealthy for Sensitive Groups | Children, elderly, and those with lung/heart conditions should limit prolonged outdoor exertion |
| 151-200 | 55.5-150.4 | Unhealthy | Everyone may experience health effects; sensitive groups more seriously affected |
| 201-300 | 150.5-250.4 | Very Unhealthy | Health alert; everyone may experience serious effects |
| 301-500 | 250.5-500 | Hazardous | Health emergency; entire population affected |
Sources of PM2.5
Outdoor Sources
- Vehicle exhaust — Cars, trucks, buses
- Wildfires — Major seasonal source
- Industrial emissions — Factories, power plants
- Construction — Dust and equipment
- Secondary formation — Chemical reactions in atmosphere
Indoor Sources
- Cooking — Especially frying, broiling
- Candles and incense — Significant particle sources
- Smoking/vaping — Very high PM2.5
- Cleaning activities — Dusting, vacuuming
- Outdoor infiltration — Particles entering from outside
Reading a PM2.5 Display
Typical Display
This reading of 23 μg/m³ is in the "Moderate" range. It's acceptable for most people but sensitive individuals might want to limit prolonged exposure.
During Wildfire Smoke
This reading shows "Unhealthy" air quality. Everyone should reduce outdoor activities, and indoor spaces should use filtration.
Sensor Limitations
What to Keep in Mind
- Humidity effects: High humidity can cause water droplets to be counted as particles, inflating readings
- Particle type: Sensors assume average particle density; smoke particles vs. dust may read differently
- Calibration: Low-cost sensors may drift over time and need recalibration
- Response time: Readings may lag behind rapid changes in air quality
- Comparison: Different sensors may give slightly different readings for the same air
Activity: Exploring PurpleAir
PurpleAir is a network of thousands of low-cost PM2.5 sensors worldwide, with real-time data available online.
Try This:
- Visit map.purpleair.com
- Find your school's location on the map
- Click on nearby sensors to see current PM2.5 readings
- Compare readings from different sensors in your area
- Check if any patterns relate to roads, industry, or geography
Discussion Questions:
- What is the typical PM2.5 level in your area today?
- Do sensors near busy roads show higher readings?
- How do indoor vs. outdoor readings compare?
- What time of day shows the highest readings? Why might that be?
Key Takeaway
PM2.5 sensors use light scattering to count particles in the air. Unlike CO2, PM2.5 directly impacts health, making it crucial to monitor. The Air Quality Index (AQI) helps translate raw PM2.5 numbers into actionable health guidance. Understanding both the capabilities and limitations of these sensors helps us use the data wisely.