The Aerosol Factor
Learning Objectives
Students will be able to:
- Explain why indoor environments are higher risk for airborne transmission
- Describe how aerosols accumulate in poorly ventilated spaces
- Use CO2 as a proxy for infection risk
- Identify factors that increase aerosol concentration
The "Stuffy Room" Feeling
Have you ever walked into a crowded room and thought "it's stuffy in here"? That feeling often means CO2 is building up from people breathing. And if CO2 is accumulating, so are any infectious particles!
The Accumulation Principle
Think of a bathtub with a partially closed drain:
- Water flowing IN = Infectious particles being exhaled
- Drain = Ventilation removing particles
Level stays low (good ventilation)
Level rises (accumulation!)
Five Risk Factors for Indoor Transmission
| Factor | Higher Risk | Lower Risk |
|---|---|---|
| Ventilation | None or poor | Excellent (fresh air) |
| Time | Hours together | Brief contact |
| Crowding | Packed room | Few people |
| Activity | Singing, exercising, shouting | Quiet, still |
| Space volume | Small, cramped | Large, open |
CO2 as a Risk Proxy
We exhale CO2 with every breath. We also exhale aerosol particles. So CO2 levels tell us about "rebreathed" air:
| CO2 Level (ppm) | What It Means | Infection Risk |
|---|---|---|
| <600 | Nearly outdoor air quality | Lowest |
| 600-800 | Well-ventilated indoor space | Low |
| 800-1,200 | Moderate ventilation | Moderate |
| 1,200-2,000 | Poor ventilation | Higher |
| >2,000 | Very poor—stuffy! | High |
The Exposure Equation
Low risk
Medium risk
High risk!
Activity: Venue Risk Assessment
Rate each venue's infection risk from 1 (lowest) to 5 (highest). Consider all five factors!
| Venue | Risk Factors to Consider | Your Rating |
|---|---|---|
| Outdoor park | Excellent ventilation, open space | |
| Crowded subway car | Small, packed, limited ventilation | |
| Packed nightclub | Crowded, loud (shouting), hours together | |
| Classroom with windows open | Good ventilation, moderate crowding | |
| Small meeting room, door closed | Small volume, no fresh air |
Superspreader Events
Most documented "superspreader" events share characteristics:
Common Features
- Indoor setting
- Poor ventilation
- Extended time (1+ hours)
- Activity that increases aerosols
Real Examples
- Choir practices
- Indoor restaurants
- Fitness classes
- Crowded parties
Key Takeaway
Indoor air quality directly affects infection risk. When CO2 levels rise, so does the concentration of any infectious particles. The five key factors are: ventilation, time, crowding, activity level, and space volume. By monitoring CO2, we can estimate how "shared" the air is—and take action before risk gets too high.