Size Matters: PM10, PM2.5, and Beyond
Learning Objectives
Students will be able to:
- Define PM10, PM2.5, and ultrafine particles
- Explain what the numbers in PM10 and PM2.5 mean
- Describe how particle size relates to health effects
- Interpret particle size measurements in micrometers
Decoding PM: What Do the Numbers Mean?
PM stands for Particulate Matter
The number after "PM" tells you the maximum size of particles in that category, measured in micrometers (μm).
The Three Key Categories
PM10 (Coarse Particles)
Size: 2.5 to 10 micrometers
Examples: Dust, pollen, mold spores
Health: Can reach upper airways (nose, throat, large bronchi). Filtered somewhat by nose hairs and mucus.
PM2.5 (Fine Particles)
Size: Less than 2.5 micrometers
Examples: Combustion particles, smoke, some bacteria
Health: Can reach deep into lungs (alveoli). Harder for body to remove. This is the most health-relevant category.
Ultrafine Particles (PM0.1)
Size: Less than 0.1 micrometers (100 nanometers)
Examples: Viruses, nanoparticles, fresh combustion emissions
Health: Can cross into bloodstream. Highest surface area per mass. Not yet regulated separately.
Visual: The Particle Size Spectrum
Size (μm): 0.01 0.1 1 10 100
|-------|--------|---------|---------|
Ultrafine PM2.5 PM10 Visible
←-- Viruses
←-- Bacteria -->
←-- Smoke -->
←-- Pollen -->
←-- Dust -->
←-- Hair
How Particle Size Affects Where They Go
| Particle Size | Where It Deposits | Body's Response |
|---|---|---|
| >10 μm | Nose, mouth, throat | Sneeze, cough, blow nose |
| 5-10 μm | Trachea, large bronchi | Mucus + cilia move it up |
| 1-5 μm | Smaller bronchioles | Harder to clear |
| <2.5 μm | Alveoli (air sacs) | Can stay for months/years |
| <0.1 μm | Bloodstream | Systemic effects possible |
Why PM2.5 Gets the Most Attention
Small enough to:
- Bypass nose/mouth defenses
- Reach deepest parts of lungs
- Stay suspended in air for hours
- Travel long distances
Large enough to:
- Carry significant mass
- Carry toxic chemicals
- Be measured reliably
- Be filtered (with right equipment)
Practice: Particle Math
Solve These:
- A human hair is 70 μm wide. How many PM2.5 particles could fit across it?
(Answer: 70 ÷ 2.5 = 28 particles) - A PM10 particle is how many times larger than a PM2.5 particle?
(Answer: 10 ÷ 2.5 = 4 times larger in diameter) - If a virus is 0.1 μm, express this as a fraction of a PM2.5 particle.
(Answer: 0.1 ÷ 2.5 = 1/25 or 4% the size) - The EPA sets PM2.5 limits at 35 μg/m³ for 24-hour average. If your room has 50 μg/m³, by what percentage does it exceed the limit?
(Answer: (50-35)/35 = 43% over the limit)
Measuring PM: Units
Particle concentration is measured in:
- μg/m³ = micrograms per cubic meter (mass concentration)
- particles/cm³ = number of particles per cubic centimeter (count concentration)
Why both? Mass tells you total amount; count tells you how many individual particles. One large particle might equal millions of ultrafine particles in mass, but the ultrafine particles have much more surface area.
Key Takeaway
PM stands for Particulate Matter, and the number indicates maximum size in micrometers. PM2.5 (≤2.5 μm) is the most health-relevant because these particles are small enough to reach the deepest parts of our lungs but large enough to carry harmful substances. Smaller is generally worse for health because smaller particles penetrate deeper and are harder for the body to remove.