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CO2 Measurement Edge Cases

Where the CO2 number fools you — cooking, calibration, and placement.

A CO2 monitor is only as good as its placement and calibration — and CO2 does not catch every air quality problem.

A CO2 reading of 600 ppm sounds safe. It might be right or might be 200 ppm off. CO2 monitors drift, get fooled by cooking, and miss everything that is not CO2. Here is what to watch for when interpreting the number and when other metrics matter more.

Quick answer

A CO2 monitor is only as good as its placement and calibration — and CO2 does not catch every air quality problem.

What you are trying to do
Where the CO2 number fools you — cooking, calibration, and placement.
Best next step
CO₂ Cognitive Tax
Limit to remember
Treat this as a practical aid for the task, not a replacement for professional judgment.

Key points

  • ▸ Sensor drift: cheap NDIR sensors drift 50-200 ppm per year. Most have an ABC (automatic baseline calibration) mode that auto-recalibrates to 400 ppm during quietest part of day.
  • ▸ Placement: put the monitor at head height where you breathe, not on the floor or ceiling. CO2 stratifies slightly.
  • ▸ Cooking and combustion: gas stoves, fireplaces, and smoking dump both CO2 and much nastier compounds. A normal CO2 reading during cooking is a false-alarm; NO2 and particulates are the real concern.
  • ▸ VOCs and particulates: CO2 does not track formaldehyde, radon, PM2.5, or volatile organics. A second monitor covers these.
  • ▸ Ventilation without fresh air: recirculating AC moves air without reducing CO2. Only actual fresh air intake or open windows drop CO2.

Examples

  • Cheap monitor reading 450 ppm outdoors
    Actual outdoor is ~420. 30 ppm high — low enough to trust relative changes but not absolute. Re-expose to outdoor air weekly to retain calibration drift awareness.
  • CO2 normal but air feels bad
    Likely VOCs from new furniture, cleaning products, or a gas stove. CO2 is not the indicator — get a VOC or PM2.5 reading.
  • Heating season · sealed house · CO2 ~2000 ppm
    Classic winter problem. HRV or cracked windows are the only fixes. Recirculating HVAC makes no difference.

When to use which tool

Related

Frequently asked questions

› Which CO2 monitor should I buy?

Aranet 4 and Airthings View Plus are the two most consistently accurate consumer options as of 2025. Avoid anything under $80 — cheap sensors drift enough to be misleading.

› Is low CO2 always good? Trust & accuracy

Low CO2 means good ventilation for human metabolic byproducts. It does not guarantee low radon, low VOCs, or low particulates. CO2 is necessary but not sufficient for clean indoor air.

› How should I use this guide with a Kefiw tool? How-to

Use the guide as the plan and the linked Kefiw tool as the check. Read the steps first, try the move manually, then use the tool to compare outputs, catch edge cases, and decide whether the result actually fits your task.

› What mistake do tool guides help avoid? Troubleshooting

Tool guides help avoid using a utility mechanically without understanding what you are trying to accomplish. Most word, writing, and text utilities are fast, but speed can hide context mistakes. Know whether you are solving a puzzle, cleaning copy, drafting a line, or checking a rule.

› Can a tool guide help me learn the skill? How-to

A tool guide can help you learn if you pause before accepting the output and ask why it worked. Compare your first guess with the tool result, look for the rule or pattern, and repeat that review. Passive copying solves one task; active review builds the skill.