Home › Tools › CO PPM Levels
📊 FREE REFERENCE TOOL — BASED ON UL 2034 & EPA DATA

Carbon Monoxide PPM Levels:
What's Safe, What's Deadly

Every CO level on one chart — from safe background air to immediately fatal concentrations. Based on UL 2034, EPA standards, and NIOSH data.

⚠️ Critical context: Standard CO alarms only sound at 70 ppm (after 1–4 hours) or 150 ppm (after 10–50 minutes) under UL 2034 requirements. They do not alert at the lower chronic levels where sustained exposure still causes lasting cardiovascular and neurological harm. A detector that shows live PPM readings — like AirShield — gives you visibility before any alarm threshold is crossed.

Full CO PPM Reference Table

Each row represents a distinct concentration level with its regulatory classification, alarm behavior, symptoms, and recommended response.

CO Level Classification UL 2034 Alarm Requirement Time to Effect Symptoms
0–9 ppm Safe No alarm N/A
  • No symptoms
  • Normal outdoor background CO is 0.1–0.2 ppm
  • Indoor air with no combustion sources typically 0.5–5 ppm
10–34 ppm Elevated UL alarm activates after 30 days at this level Effects emerge after hours of sustained exposure
  • Subtle headache after several hours of exposure
  • Slight fatigue — easily attributed to tiredness
  • No acute symptoms in healthy adults at short exposures
35 ppm EPA Action Level EPA maximum 8-hour workplace exposure limit (OSHA PEL) 6–8 hours of sustained exposure
  • Headache and dizziness within 6–8 hours of sustained exposure
70 ppm UL 2034 Alarm Threshold Standard CO alarm activates within 1–4 hours at this level 1–2 hours for noticeable symptoms
  • Headache, fatigue, nausea within 1–2 hours
  • Children and elderly may show symptoms sooner
150–200 ppm Danger — Alarm Within 35 Min UL 2034: alarm within 10–50 minutes at this level 2 hours for severe symptoms; loss of consciousness possible
  • Severe headache, dizziness, disorientation within 2 hours
  • Judgment begins to impair — self-rescue becomes harder
400 ppm Life-Threatening UL 2034: alarm within 15 minutes at this level Life-threatening within 3 hours
  • Life-threatening within 3 hours
  • Severe headache, vomiting, convulsions
  • Loss of consciousness
800 ppm Fatal — 45 Minutes Extreme — rapid alarm response required Death within 2–3 hours
  • Dizziness, nausea, convulsions within 45 minutes
  • Unconsciousness within 1 hour
  • Death within 2–3 hours
1,600 ppm Fatal — 20 Minutes Extreme emergency Death within 1 hour
  • Headache, dizziness within 20 minutes
  • Unconsciousness within 1 hour
  • Death within 1 hour of sustained exposure
6,400+ ppm Immediately Fatal Extreme emergency Death within 15–30 minutes
  • Headache and dizziness within 1–2 minutes
  • Convulsions within 15 minutes
  • Death within 25–30 minutes

Sources: UL 2034 Standard for Single and Multiple Station Carbon Monoxide Alarms; EPA Indoor Air Quality Guidelines; NIOSH Pocket Guide to Chemical Hazards; CDC Carbon Monoxide Poisoning Data.

The Alarm Gap: Why Standard Detectors Leave You Exposed

Most residential CO alarms are designed to meet UL 2034 thresholds — which means they only sound after 1–4 hours at 70 ppm, or after 10–50 minutes at 150 ppm. They produce no alert at the 10–35 ppm range where prolonged chronic exposure causes lasting cardiovascular harm, or at low concentrations that accumulate during sleeping hours.

The 10–35 ppm range is sometimes called the "chronic exposure zone." Occupants can breathe CO at these levels for hours — during sleep — without any alarm ever sounding. Over months, this correlates with elevated rates of cardiovascular events, cognitive decline, and neurological symptoms that are typically misattributed to other causes.

A detector that shows live PPM readings closes this gap. Instead of waiting for a threshold alarm, you can see rising CO concentrations in real time and respond before any regulatory threshold is crossed.

Key CO Concentration Reference Points

35 ppm
EPA Action Level
OSHA 8-hour PEL for workplace exposure
70 ppm
UL 2034 Threshold
Standard alarm activates after 1–4 hours
400 ppm
Life-Threatening
Alarm within 15 min; fatal within 3 hours
6,400+ ppm
Immediately Fatal
Death within 15–30 minutes

Related Articles

Real-Time PPM Display

See Your CO Level Before Any Alarm Sounds

AirShield's electrochemical sensor shows live CO concentration on an OLED display. Know your level at 1 ppm — not 70. UL 2034 certified, plug-in, no batteries.

See AirShield's Live Display →

Journalists and safety educators: you're welcome to cite or link to this chart. For press inquiries, visit airshield.store/press