The Invisible World of Aerosols
Learning Objectives
Students will be able to:
- Define "aerosol" and distinguish it from other forms of matter
- Describe the range of particle sizes found in air
- Explain why most particles in air are invisible to the naked eye
- Compare particle sizes using scale models and analogies
Engage: What's Really in This Room?
Think about it: Right now, you're breathing in about 7-8 liters of air every minute. How many particles do you think are in each breath? 10? 100? 1,000?
Answer: In typical indoor air, each breath contains millions of tiny particles!
What is an Aerosol?
Aerosol = Tiny solid particles or liquid droplets suspended in air (or another gas)
The word comes from Greek: aero (air) + sol (solution)
Examples of Aerosols:
Dust, pollen, sea salt, volcanic ash
Smoke, soot, spray products
Bacteria, viruses, mold spores
Size Comparison: How Small Are Particles?
We measure particles in micrometers (also called microns), abbreviated μm.
1 micrometer = 1/1,000,000 of a meter = 1/1,000 of a millimeter
| Object | Size | Visible? |
|---|---|---|
| Human hair | 70-100 μm | Yes |
| Grain of sand | 100-500 μm | Yes |
| Pollen grain | 10-100 μm | Barely (fuzzy) |
| Dust mite | ~200-300 μm | Barely |
| PM10 particle | ≤10 μm | No |
| PM2.5 particle | ≤2.5 μm | No |
| Bacteria | 0.5-5 μm | No |
| Virus | 0.02-0.3 μm | No |
The human eye can see down to about 40-50 micrometers. Most particles that affect our health are invisible!
Activity: Scale Model
If a PM2.5 particle were the size of a tennis ball...
| Tennis ball | = | PM2.5 particle (2.5 μm) |
| Basketball | = | PM10 particle (10 μm) |
| 2-story house | = | Human hair (70 μm) |
| Football field (length) | = | Grain of sand (500 μm) |
This shows how incredibly tiny the particles are that we're concerned about!
Why Can't We See Them?
Visible Light Limitation
Our eyes detect light waves. Objects smaller than the wavelength of visible light (~0.4-0.7 μm) can't be seen directly.
The Dust Beam Effect
When sunlight streams through a window, you see dancing dust motes. These are actually the LARGER particles (10+ μm). The dangerous ones are too small to see even then!
Explore: The Sunbeam Test
Try This:
- Find a room with direct sunlight coming through a window
- Darken the rest of the room if possible
- Look at the beam of light from the side (not directly at the sun!)
- Wave your hand or clap near the beam
- Watch what happens to the visible particles
Discussion Questions:
- What makes the particles visible in the beam?
- If these are the LARGE particles, what does that tell us about the small ones?
- Where do you think these particles come from?
Key Takeaway
Air is full of tiny particles called aerosols. Most particles that affect our health are too small to see with the naked eye. Just because we can't see them doesn't mean they're not there—and understanding particle size is the first step to understanding air quality.