CO2 as a Ventilation Proxy
COVID-19, influenza, and many respiratory infections spread primarily through aerosols — tiny particles that linger in the air, especially in poorly ventilated spaces. Since both aerosols and CO2 accumulate when ventilation is poor, indoor CO2 concentration serves as a practical, real-time indicator of airborne infection risk.
The Science
Every time an infected person breathes, talks, or coughs, they release virus-laden aerosols into the room. Simultaneously, they exhale CO2. Both accumulate at similar rates in an enclosed space. Research published in Environmental Science and Technology confirmed a strong correlation between CO2 levels and the estimated probability of airborne transmission.
- Below 700 ppm — ventilation is excellent; aerosol buildup is minimal
- 700-1,000 ppm — moderate risk; typical of many occupied indoor spaces
- Above 1,000 ppm — elevated risk; ventilation is insufficient to adequately dilute respiratory aerosols
- Above 1,500 ppm — high risk; shared air fraction is significant
Limitations of CO2 Monitoring
CO2 is not a perfect proxy. It does not detect the virus itself, and several factors add nuance:
- Masking reduces aerosol emission but not CO2 exhalation
- Air filtration reduces aerosols without affecting CO2
- Outdoor air dilutes both, but the ratio is not always linear
Despite these limitations, CO2 monitoring remains one of the most practical tools available for real-time ventilation assessment in schools, offices, restaurants, and public transport.
Practical Recommendations
- Monitor shared spaces — install CO2 monitors in offices, classrooms, and restaurants.
- Set a 700 ppm action target during respiratory virus season — this is tighter than the standard 1,000 ppm comfort threshold.
- Ventilate aggressively — follow our budget ventilation guide to increase airflow without major expense.
- Layer protections — combine ventilation (reduces both CO2 and aerosols) with HEPA filtration (reduces aerosols only). See our monitors vs purifiers guide.
- Carry a portable monitor — portable CO2 monitors let you assess risk in any indoor setting.
Use the CO2 calculator to estimate steady-state CO2 for different ventilation rates and occupancy levels.