What Is a Good CO2 Level Indoors? A Clear Guide to the Numbers
What CO2 level is safe indoors? How it affects focus, sleep, and health — with clear thresholds and when you need to act.
Table of Contents
- The CO2 Scale: What the Numbers Mean
- How Fast CO2 Builds Up
- One Person in a Bedroom (150 sq ft, door closed)
- Two People in a Bedroom
- Classroom (25 Students, 900 sq ft)
- Open-Plan Office (20 People, 2,000 sq ft)
- The Science: Why CO2 Affects Your Brain
- What Affects Your Indoor CO2
- Things That Raise CO2
- Things That Lower CO2
- Things That Do NOT Lower CO2
- How to Monitor CO2 at Home
- Where to Place Your Monitor
- What to Do With the Data
- CO2 Standards and Guidelines
- The Bottom Line
TL;DR
A good indoor CO2 level is below 800 ppm. Between 800-1,000 ppm you may feel stuffy and lose focus. Above 1,000 ppm ventilation is needed. Above 2,000 ppm causes headaches and drowsiness. Outdoor air is about 420 ppm — the closer your indoor air is to outdoor levels, the better your ventilation.
Every breath you take adds CO2 to the room. In a well-ventilated space, it dissipates. In a sealed room — and most modern homes and offices are well-sealed — CO2 builds up fast, and you feel the effects before you realize why.
This guide explains what CO2 numbers mean, where the thresholds are, and what to do when levels climb too high.
The CO2 Scale: What the Numbers Mean
Here is the practical scale for indoor CO2 levels. These thresholds are based on ASHRAE standards, the Harvard COGFX study, and occupational health guidelines.
| CO2 Level | Rating | What You Will Notice |
|---|---|---|
| 400-600 ppm | Excellent | Fresh, outdoor-like air. Ideal for focus and sleep. |
| 600-800 ppm | Good | Well-ventilated indoor space. No noticeable effects. |
| 800-1,000 ppm | Moderate | Some people notice stuffiness. Ventilate soon. |
| 1,000-1,500 ppm | Poor | Reduced concentration, drowsiness. Ventilate now. |
| 1,500-2,000 ppm | Bad | Clear cognitive impairment. Open windows immediately. |
| 2,000-5,000 ppm | Very Bad | Headaches, fatigue, poor decision-making. |
| 5,000+ ppm | Dangerous | Occupational exposure limit. Should not occur indoors. |
The key threshold to remember: 800 ppm. Below 800, most people have no symptoms. Above 800, ventilation should improve. This is a practical action point that balances air quality with energy efficiency.
How Fast CO2 Builds Up
CO2 accumulation depends on three things: how many people are in the room, the room size, and how much fresh air enters.
One Person in a Bedroom (150 sq ft, door closed)
Starting at 400 ppm (fresh air), one adult at rest exhales about 15 liters of CO2 per hour:
- After 1 hour: ~700 ppm
- After 2 hours: ~1,000 ppm
- After 8 hours (overnight): ~1,500-2,000 ppm
This is why you wake up feeling groggy in a closed bedroom. Your air was significantly degraded by morning.
Two People in a Bedroom
Double the people, roughly double the rate:
- After 1 hour: ~1,000 ppm
- After 4 hours: ~2,000 ppm
- After 8 hours: ~2,500-3,000+ ppm
Couples sleeping with the door and windows closed almost always have CO2 above 1,500 ppm by morning.
Classroom (25 Students, 900 sq ft)
- After 30 minutes: ~1,000 ppm
- After 1 hour: ~1,500 ppm
- After 2 hours: ~2,500+ ppm
This is why CO2 monitoring in classrooms is becoming standard practice. Students lose focus not because the lesson is boring — but because the air is stale.
Open-Plan Office (20 People, 2,000 sq ft)
Even with mechanical ventilation, busy offices often hover at 800-1,200 ppm depending on occupancy and HVAC settings. Meeting rooms are worse — 6 people in a 200 sq ft room can hit 2,000 ppm in 30 minutes.
The Science: Why CO2 Affects Your Brain
The Harvard COGFX study (2015) tested cognitive function in controlled environments at different CO2 levels. The results were striking:
- At 600 ppm: Baseline cognitive performance
- At 1,000 ppm: 15% reduction in cognitive scores across 9 domains
- At 2,500 ppm: 50% reduction, with the largest drops in strategic thinking and information usage
A follow-up study in 2021 confirmed these findings in real office buildings. Workers in well-ventilated offices (low CO2) scored significantly higher on cognitive tests than those in poorly ventilated spaces.
This is not about comfort — it is about how well your brain works. If you or your team cannot think clearly in the afternoon, check the CO2 before blaming the workload.
What Affects Your Indoor CO2
Things That Raise CO2
- People. Each person exhales 200 ml of CO2 per minute at rest, more during exercise
- Gas appliances. Gas stoves, ovens, and unvented heaters produce CO2 directly
- Closed doors and windows. Sealing the room traps exhaled CO2
- Poor HVAC. Insufficient fresh air supply in mechanical ventilation
Things That Lower CO2
- Open windows. The single most effective and free solution
- Mechanical ventilation (ERV/HRV). Exchanges stale air for fresh while retaining heat or cooling
- Exhaust fans. Bathroom and kitchen fans draw out stale air if a fresh air path exists
- Fewer people. Reducing occupancy in a room reduces CO2 generation
Things That Do NOT Lower CO2
- Air purifiers. HEPA filters remove particles, not gases. Your purifier does nothing for CO2.
- Plants. The effect is real but far too small to matter in a room.
- Air fresheners. They mask smells. They do not add oxygen or remove CO2.
How to Monitor CO2 at Home
A CO2 monitor with an NDIR sensor is the only reliable way to measure indoor CO2 levels. Prices range from $60 for a SwitchBot CO2 Detector to $200 for an Aranet4 HOME.
Where to Place Your Monitor
- Bedroom. Place it on your nightstand. You will likely be surprised by morning readings.
- Home office. Track CO2 during work hours. Open a window when it passes 800 ppm.
- Living room. Monitor during family time, especially in winter when windows are sealed.
- Kitchen. Gas cooking adds CO2 — a monitor shows whether your range hood is keeping up.
What to Do With the Data
The simplest approach: watch the number. When it crosses 800 ppm, increase ventilation. When it drops below 600, you can seal up again to save energy. This demand-based ventilation is more efficient than running fans on a fixed schedule.
For a more comprehensive approach, see our guide on how to test air quality at home, which covers PM2.5, radon, and VOCs alongside CO2.
CO2 Standards and Guidelines
| Standard | Recommended Limit | Context |
|---|---|---|
| ASHRAE 62.1 | 1,100 ppm (700 above outdoor) | Commercial building ventilation |
| ASHRAE 62.2 | Ventilation rate based | Residential buildings |
| Harvard Healthy Buildings | 800 ppm | Schools and offices for cognitive performance |
| Germany (DIN 1946) | 1,000 ppm | Classroom air quality standard |
| OSHA | 5,000 ppm | Occupational 8-hour exposure limit |
The trend is toward tighter standards. Post-pandemic awareness of indoor air quality has pushed schools and offices toward the 800 ppm threshold recommended by the Harvard Healthy Buildings program. Most experts agree this is the right target for spaces where people need to think clearly.
The Bottom Line
Good indoor CO2 is below 800 ppm. Acceptable is below 1,000 ppm. Above 1,000, you are losing cognitive performance whether you feel it or not.
The fix is simple: more fresh air. A $60-200 CO2 monitor shows you exactly when and where you need it. For most people, cracking a window when the number turns yellow is all it takes.
Related reading:
- Best CO2 Monitors for Home — our top picks
- Best CO2 Monitors for Classrooms and Offices — school and workplace deployment
- Aranet4 vs Airthings View Plus — the two leading premium monitors compared
- Air Quality Monitor vs Air Purifier — understand which tool solves which problem