How Much CO2 Is Dangerous

You can’t see carbon dioxide, but your body notices it. As levels climb, thinking dulls, headaches creep in, and sleep suffers. This guide distills what counts as “too much” CO2 indoors, what symptoms to watch for, and the fastest fixes—grounded in occupational limits (OSHA/NIOSH), building‑science guidance (ASHRAE), and public‑health sources

How Much CO2 Is Dangerous: quick answers

• Aim to keep indoor CO2 below about 1,000 ppm for comfort and cognition; it’s a widely used benchmark for acceptable ventilation (ASHRAE background; see Persily 2021).
• Workplace limit: 5,000 ppm as an 8‑hour time‑weighted average (OSHA/NIOSH). Short‑term (15‑min) exposure limit: 30,000 ppm (NIOSH). IDLH: 40,000 ppm.
• Typical outdoor today: ~425 ppm (NOAA Mauna Loa weekly average, early Sep 2025).
• Key rule of thumb used by building pros: indoor CO2 ≈ outdoor + 700 ppm when ventilation matches ~7.5 L/s‑person; that lands around ~1,100 ppm with today’s outdoor air.

What is CO2?

CO2 is the colorless, odorless gas we exhale. It isn’t the same as CO (carbon monoxide). CO is poisonous at tiny concentrations; CO2 becomes risky mainly at higher levels, where it displaces oxygen and causes hypercapnia (elevated blood CO2).

What levels of CO2 are typical indoor and outdoor?

CO2 (ppm), What it typically means/feels like, Key sources

• ~400–450 - Typical outdoor air in 2025 (source: NOAA GML)
• 600–1,000 - Well‑ventilated indoors; most people feel fine (source: ASHRAE FAQ/Background)
• 1,000–1,500 - Stale/“sleepy” air; attention and decision‑making can dip (source: EHP 2012/2015; Harvard Healthy Buildings)
• 1,500–2,500 - Headaches, drowsiness more common; performance drops more clearly (source: EHP 2012; MDH; industry summaries)
• 2,500–5,000 - Sleepiness, reduced concentration; some experience palpitations (source: MDH; industry/occupational notes)
• 5,000 (8‑hr TWA) - Regulatory workplace limit for an 8‑hour shift (source: NIOSH/OSHA)
• 30,000 (15‑min) - Short‑term exposure limit; avoid unless controlled (source: NIOSH  (STEL))‍
• 40,000 (4%) - Immediately Dangerous to Life or Health (IDLH) (source: NIOSH/OSHA)

At what level does CO2 in the room become dangerous?

Practically: if your monitor sits near or above 1,000–1,200 ppm during normal use, the space is under‑ventilated for how many people and activities you have. ASHRAE no longer sets a hard CO2 limit, but their historic appendix linked comfort ventilation to indoor levels about 700 ppm above outdoors—roughly ≤1,100 ppm today. Above ~1,500 ppm, many occupants report stuffiness, headaches, or fatigue; by ~2,000+ ppm, measurable performance losses become more likely.

Symptoms of high CO2 indoor

• Dull thinking, slower reaction time, trouble focusing
• Sleepiness, headaches, faster heart rate
• At tens of thousands of ppm: serious hypercapnia, loss of consciousness (emergency)

Special situations: bedrooms, babies, pregnancy

Sleep rooms often spike: closed doors, several hours, two people. Try to stay under ~1,000–1,200 ppm by cracking a window, boosting mechanical ventilation, or running a timed supply fan. There are no separate CO2 standards for infants or pregnancy in general guidance, so use conservative targets and prioritize ventilation, especially in small rooms and nurseries.

The best way to decrease high CO2 indoor

1. Bring in outdoor air: open opposing windows/doors for a cross‑breeze or increase your system’s outdoor‑air setting.
2. Reduce occupancy for the room size, or move meetings to larger, better‑ventilated spaces.
3. Use demand‑controlled ventilation (DCV) tied to a reliable NDIR CO2 sensor where possible.
4. Check return/supply paths and filters; a blocked return can trap exhaled air even with a strong fan.
5. Know the limits: standard HEPA purifiers do not remove CO2; plants have modest, slow effects in real rooms.

FAQ about CO2

How much CO2 is too much in a room?

‍Regularly above ~1,000–1,200 ppm suggests insufficient ventilation for the current occupancy and activity. For workplaces, 5,000 ppm is the legal 8‑hour limit, not a comfort target.

Can you breathe 3% CO2 (30,000 ppm)?

‍That’s the NIOSH 15‑minute short‑term exposure limit. It can cause symptoms and is not appropriate outside of controlled industrial settings.

What is a dangerously high CO2 level?

‍40,000 ppm (4%) is IDLH—conditions immediately dangerous to life or health.

What is considered high CO2 in a house?

‍Sustained readings above ~1,000–1,200 ppm point to under‑ventilation; aim lower when sleeping or with vulnerable occupants.

Is 800 ppm CO2 safe?

‍Yes—typically indicates good ventilation for most sedentary activities.

Is 1,000 ppm CO2 dangerous?

‍Not an emergency, but many studies find cognitive performance starts dipping around here; it’s a cue to add fresh air.

Is 1,200–1,500 ppm bad for health?

‍Expect stuffiness and headaches for some; improve ventilation to bring it down.

Is 2,000–2,500 ppm harmful?

‍Short stints happen in crowded rooms; plan to ventilate. Prolonged exposures can worsen symptoms and performance.

Is 5,000 ppm dangerous?

‍It’s the 8‑hour legal limit for workers, not a comfort target. Keep typical indoor levels far below this.

What happens around 10,000 ppm?

‍Likely pronounced drowsiness and faster breathing; address immediately and investigate the cause.

 What CO2 level causes headaches or drowsiness?

‍Many people notice symptoms above ~1,200–1,500 ppm; sensitivity varies.

What CO2 level affects concentration?

‍Experimental work shows measurable declines from ~1,000 ppm upward, with larger drops by ~2,500 ppm.

What’s safe for babies/children or during pregnancy?

‍No separate numeric limits; keep rooms well below ~1,000 ppm when possible and ensure steady fresh air.

What CO2 level is unsafe for sleep with windows closed?

‍Bedrooms that drift above ~1,200–1,500 ppm overnight are common; crack a window or add supply air to stay nearer ~800–1,000 ppm.

How long is it safe to stay in a high‑CO2 room?

‍Treat it like a spill: ventilate now, then fix the cause. Avoid staying for hours above ~1,500–2,000 ppm.

Do indoor plants reduce CO2 enough to make a room safe?

‍Not quickly. In typical homes you’d need a wall of plants for a noticeable effect; ventilation is the reliable lever.

Do air purifiers lower CO2?

‍Standard HEPA units do not remove CO2 gas. They’re great for particles, not for CO2.

How can I quickly lower high CO2?

‍Cross‑ventilate, reduce occupancy, boost outdoor air on HVAC, and verify returns aren’t blocked.

What are official exposure limits?

‍OSHA/NIOSH: 5,000 ppm (8‑hr TWA); NIOSH: 30,000 ppm (15‑min STEL); IDLH: 40,000 ppm.

What’s normal outdoor vs. typical indoor?

‍Outdoors ~420–430 ppm in 2025; well‑ventilated indoor spaces typically run ~600–1,000 ppm depending on crowding and ventilation.

Choosing a CO2 monitor (quick buying note)

Opt for a monitor featuring a true NDIR (Non-Dispersive Infrared) sensor, with at least a 0–5,000 ppm measurement range (or higher if needed), built-in data logging capabilities, and a recent factory calibration. Steer clear of “eCO2” estimates derived from VOC sensors — they’re not actual CO2 readings.

If you’re also interested in broader air-quality tracking, consider the Atmotube PRO — a portable monitor that delivers real-time readings of particulate matter (PM1, PM2.5, PM10), total VOCs, temperature, humidity, and barometric pressure. It requires no calibration, offers long battery life, Bluetooth connectivity, data export (CSV/cloud), and a mobile app with alerts and mapping features.

Though it doesn’t include a dedicated CO2 sensor, it's a compelling companion if you're monitoring more than just CO2.