Is 1,000 ppm of CO₂ bad?

It is not dangerous, but it is the concentration at which measurable effects first appear in the research. Satish et al. (2012) found moderate, statistically significant decrements in six of nine decision-making measures at 1,000 ppm compared with 600 ppm. It also indicates your ventilation rate has fallen below what most standards target.

What CO₂ level makes you sleepy?

Drowsiness, headache and difficulty concentrating are commonly reported above roughly 2,000 ppm, though individual sensitivity varies widely and the symptoms are not caused by CO₂ alone — bioeffluents and humidity accumulate in the same poorly ventilated air. A closed bedroom overnight routinely exceeds 2,000 ppm.

How quickly does CO₂ build up in a closed room?

Faster than most people expect. One adult in a closed 12 m² office at half an air change per hour crosses 1,000 ppm in about seventy-five minutes and settles near 1,700 ppm. A four-person meeting room in the same building crosses 1,000 ppm in under half an hour. The rate depends on room volume, occupancy and air changes per hour.

Does opening a window actually help?

Yes, and much faster than accumulation. A small room with a window open and any cross-breeze usually returns to near-outdoor concentrations within ten to fifteen minutes. The asymmetry matters: the problem takes hours to develop and minutes to fix.

Is high CO₂ the same as bad air quality?

No. CO₂ is a reliable proxy for ventilation rate per occupant, but it says nothing about what is in the air being ventilated. Particulate matter, volatile organic compounds and formaldehyde are independent measurements, and a room can have low CO₂ and poor air quality, or the reverse.

Reference

Indoor CO₂ levels, and what each one means

Outdoor air sits near 425 ppm. Everything above that is you, and how much of it accumulates is set almost entirely by how much outdoor air the room is getting. This page is the reference table we work from, with the source for every figure.

What is a safe CO₂ level indoors?

Below 800 ppm is comfortably safe and is what most ventilation guidance targets. 800–1,000 ppm is acceptable but approaching the threshold where chamber studies first measured decision-making decrements. Above 1,000 ppm indicates ventilation below the rate most standards specify for occupied spaces. CO₂ is not toxic until far higher — around 5,000 ppm as an occupational limit.

The chart

Six ranges, and the evidence behind each.

These bands are anchored to published exposure conditions rather than to internal thresholds. Where a range corresponds to a specific study condition, the study is named.

Indoor carbon dioxide concentration ranges, what the research reports for each, and where each range is typically encountered.
Range	Label	What it means	Typically found in
0–600 ppm	Baseline	Effectively outdoor air. The reference condition against which every cognitive decrement in the literature is measured.	Outdoors, or a room with windows open and cross-ventilation.
600–800 ppm	Optimal	No decision-making decrement reported. This is the band OptiBreath is designed to hold you in.	A well-ventilated office running at or above design airflow.
800–1,000 ppm	Elevated	The threshold zone. Approaching the 1,000 ppm condition at which Satish et al. first measured statistically significant declines in complex decision-making.	A closed home office two hours into the working day.
1,000–1,400 ppm	Degraded	At 1,000 ppm, six of nine decision-making measures fell moderately or substantially versus 600 ppm. Initiative and basic strategy were the most affected.	A meeting room with the door shut and four people in it.
1,400–2,500 ppm	Impaired	Approaching the 2,500 ppm condition, where seven of nine measures fell and effect sizes on strategy and initiative were large.	A packed conference room, or a bedroom with the door closed overnight.
2,500+ ppm	Critical	Large decrements across most decision-making measures, alongside the drowsiness, headache and loss of focus commonly reported at these concentrations.	A sealed room at capacity with mechanical ventilation off or failed.

Band definitions are also available as machine-readable JSON under a permissive licence, with provenance for each figure.

Published thresholds

What the standards bodies actually say.

These are other people's numbers. They are here so you can see where our bands agree with established guidance and where they do not — and because two of them are routinely misquoted.

ASHRAE

United States

~1,100 ppm indoors

Not a health limit. Standard 62.1 sets ventilation rates per person; a steady-state indoor concentration around 700 ppm above outdoors — roughly 1,100 ppm in practice — indicates those rates are being met. ASHRAE is explicit that CO₂ is an indicator of ventilation, not a contaminant of concern at these levels.

ANSI/ASHRAE Standard 62.1, Ventilation for Acceptable Indoor Air Quality ↗

CIBSE

United Kingdom

1,000 ppm daily average

TM40 and the schools guidance treat a daily average at or below 1,000 ppm during occupied hours as the marker of adequate ventilation, with 1,500 ppm as the level requiring action.

CIBSE TM40: Health and Wellbeing in Building Services ↗

REHVA

Europe

800 ppm for infection-risk control

Post-2020 guidance recommends keeping occupied rooms below 800 ppm as a proxy for reducing airborne transmission risk, on the reasoning that CO₂ tracks rebreathed air fraction.

REHVA COVID-19 Ventilation Guidance ↗

OSHA

United States

5,000 ppm, 8-hour TWA

An occupational exposure limit for CO₂ as a toxic gas, set an order of magnitude above anything an office reaches. Frequently misquoted as evidence that indoor CO₂ is harmless below it — the limit is about asphyxiation risk, not cognition.

OSHA Permissible Exposure Limit, 29 CFR 1910.1000 ↗

Global outdoor average

Worldwide

~425 ppm and rising

The floor for any indoor measurement. It has risen roughly 100 ppm in sixty years, which slowly raises the baseline every indoor space starts from.

NOAA Global Monitoring Laboratory, Mauna Loa record ↗

A misreading worth naming

The OSHA figure of 5,000 ppm is frequently cited as proof that indoor CO₂ is harmless below it. That limit exists to prevent asphyxiation in industrial settings and says nothing about cognitive performance. The cognition research operates two orders of magnitude lower — and it is genuinely contested at those levels, which is a separate argument from the occupational limit.

Common questions

Asked plainly, answered plainly.

What is a safe CO₂ level indoors?: Below 800 ppm is comfortably safe and is the range most ventilation guidance targets. Between 800 and 1,000 ppm is acceptable but approaching the threshold where chamber studies first measured decision-making decrements. Above 1,000 ppm indicates ventilation below the rate most standards specify for occupied spaces. CO₂ itself is not toxic until far higher concentrations — around 5,000 ppm as an occupational limit.
Is 1,000 ppm of CO₂ bad?: It is not dangerous, but it is the concentration at which measurable effects first appear in the research. Satish et al. (2012) found moderate, statistically significant decrements in six of nine decision-making measures at 1,000 ppm compared with 600 ppm. It also indicates your ventilation rate has fallen below what most standards target.
What CO₂ level makes you sleepy?: Drowsiness, headache and difficulty concentrating are commonly reported above roughly 2,000 ppm, though individual sensitivity varies widely and the symptoms are not caused by CO₂ alone — bioeffluents and humidity accumulate in the same poorly ventilated air. A closed bedroom overnight routinely exceeds 2,000 ppm.
How quickly does CO₂ build up in a closed room?: Faster than most people expect. One adult in a closed 12 m² office at half an air change per hour crosses 1,000 ppm in about seventy-five minutes and settles near 1,700 ppm. A four-person meeting room in the same building crosses 1,000 ppm in under half an hour. The rate depends on room volume, occupancy and air changes per hour.
Does opening a window actually help?: Yes, and much faster than accumulation. A small room with a window open and any cross-breeze usually returns to near-outdoor concentrations within ten to fifteen minutes. The asymmetry matters: the problem takes hours to develop and minutes to fix.
Is high CO₂ the same as bad air quality?: No. CO₂ is a reliable proxy for ventilation rate per occupant, but it says nothing about what is in the air being ventilated. Particulate matter, volatile organic compounds and formaldehyde are independent measurements, and a room can have low CO₂ and poor air quality, or the reverse.

You cannot fix what you have never measured.

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