The small monitor on the meeting-room shelf glows green. The window can stay closed; the air must be fine.
That conclusion is understandable, but larger than the measurement. A carbon-dioxide monitor can reveal something valuable: how quickly the breath of the people in a room is accumulating relative to the outdoor air supplied. It cannot see every particle, gas, damp surface or pollutant source. Its light is a control signal for one mechanism, not a certificate that the room is clean.
Used that way, the instrument becomes more useful, not less. It can help people test whether a ventilation routine works, discover rooms that behave differently and ask better questions of a building. The practical capability is to turn a number into a documented action while keeping its blind spots visible.
The monitor sees a molecule
People exhale carbon dioxide. In an occupied room, its concentration usually rises when generation by occupants exceeds removal through outdoor-air exchange. That is why CO₂ can act as a proxy for ventilation per person: it connects an invisible building process to an observable trace.
The UK Health and Safety Executive recommends non-dispersive infrared, or NDIR, monitors for workplace checks. An NDIR sensor sends infrared light through a small sample chamber. Carbon-dioxide molecules absorb particular wavelengths, so the reduction reaching a detector can be related to concentration. It is selective measurement, not a general sniff of everything in the room.
That distinction matters twice. First, the sensor responds to CO₂ reaching the device, not directly to the outdoor-air flow at every person’s face. Second, ventilation is inferred through a model: people are the source, the room is the volume, and outdoor air dilutes the accumulated gas. If the source or mixing pattern changes, the interpretation changes too.
An empty room can therefore show a low reading even when it contains another pollutant. A densely occupied room can show a high reading although a particle filter is reducing airborne particles. The number is not wrong in either case. The question asked of it is wrong.
Position is part of the instrument
A portable monitor has no privileged view of a room. Place it beside an open window and it may mostly sample incoming air. Put it close to someone’s face and it may catch a concentrated plume of exhaled breath. Hide it behind furniture and it may sit in a poorly mixed pocket.
The Health and Safety Executive’s current guidance recommends head height, more than 50 centimetres from people, and a position away from windows, doors and supply openings. It also advises trying several locations, recording occupancy and ventilation state, and using repeated measurements rather than a snapshot. Larger or irregular spaces may need more than one monitor.
This is an important form of literacy: placement is not housekeeping after the measurement. It helps define what has been measured. The displayed precision can easily exceed the precision of the situation.
A ventilation proxy is not an air-quality score
Indoor air contains a changing mixture of human emissions, particles, moisture and gases from materials, activities, combustion and outdoor sources. The US Environmental Protection Agency’s indoor-air overview, updated in March 2026, lists sources ranging from fuel-burning appliances and tobacco to furnishings, cleaning products, excess moisture, radon and outdoor pollution. The German Environment Agency likewise distinguishes exhaled CO₂ from pollutants produced by smoking, cooking, heating, renovating, furnishings and consumer products.
One sensor cannot collapse those mechanisms into a truthful colour. Carbon monoxide requires a dedicated alarm. Radon requires an appropriate test over the prescribed period. Particles, humidity and volatile compounds need their own evidence when they matter. Visible damp or mould is not cancelled by a low CO₂ value.
Air cleaning exposes the same boundary from the other direction. A filter may remove particles without removing CO₂ or supplying outdoor air. The HSE consequently warns that a CO₂ monitor is not a suitable way to judge the effect of an air-cleaning unit. Conversely, opening a window may lower indoor CO₂ while admitting outdoor smoke, pollen, heat or noise. The right intervention depends on the pollutant and the setting.
The traffic light still has a job
The strongest defence of the red–amber–green display is its simplicity. People need a cue they can notice during a lesson or meeting, not a building-science seminar. A rough signal that changes behaviour may be more valuable than a perfect measurement nobody uses.
That is true. The mistake is not the traffic light; it is an undefined promise. The colours should mean something operational such as “use the agreed ventilation action” rather than “the air is unsafe” or “the air is clean”. Thresholds should come from the applicable guidance, building policy or professional assessment—not from the colour chosen by an unknown factory default.
For example, the HSE says that CO₂ consistently above 1,500 parts per million in an occupied workplace indicates poor ventilation and calls 1,000 ppm broadly equivalent to about ten litres per second of outdoor air per person under its assumptions. The same page immediately stresses that CO₂ is a broad ventilation guide, not proof of a safe level. Occupancy, activity, outdoor baseline, room mixing and local rules all constrain the inference.
The best use of a simple display is therefore to make the action simpler while leaving the interpretation disciplined.
Write the action before watching the number
A small room experiment can begin without calculating an air-change rate. Record the room, monitor model, position, number of occupants, activity, ventilation state and time. Observe a normal occupied session; do not deliberately let the concentration rise. On a comparable session, apply one safe change already available—such as the documented window-opening routine or a higher mechanical-ventilation setting—and compare the shape of the trace.
The useful result is not a universal threshold. It is a Room Air Signal Card: what this monitor measures, where it sits, which conditions make the reading interpretable, which action follows an agreed trigger, and which hazards remain outside its view. If natural ventilation changes with wind and weather, repeat the observation rather than treating one successful afternoon as commissioning evidence.
There are clear boundaries. Do not use this experiment to diagnose symptoms, test a combustion appliance, replace a carbon-monoxide alarm or satisfy a legal ventilation assessment. Do not infer that a room is safe because the curve falls. Where occupants report illness, where combustion, hazardous processes, damp or serious building defects are present, the monitor is only one clue for competent human assessment.
A green light becomes meaningful when people know the narrow question it answers. The remaining judgement is to decide which signal matters for this room, what action it should trigger and what else must still be observed. A good instrument does not remove that responsibility. It makes it possible to exercise it with evidence.
Turn this idea into a project
Create a one-page Room Air Signal Card that defines what your CO₂ monitor measures, where it sits, which action follows an agreed trigger and which indoor-air hazards remain outside its view.
Capsule version 1.0 · Experimental; evidence-informed workflow, not field-validated by Alkemata.
Download and upload the file to your AI assistant, or copy the complete capsule below into a conversation. Then describe your situation in one sentence—for example: “Mission B. Our six-person meeting room has one CO₂ monitor and an opening window; I want a defensible trigger and ventilation action.”
Read or copy the complete capsule
# ALKEMATA PROJECT CAPSULE ## A Green CO₂ Light Does Not Mean Clean Air - Capsule ID: `alkemata.room-air-signal` - Version: 1.0 - Date: 2026-09-19 - Canonical article: https://alkemata.com/2026/09/19/green-co2-light-clean-air/ - Scope: Portable indoor CO₂ monitors used to understand occupied-room ventilation; not a complete indoor-air, medical, legal or building-compliance assessment - Status: Experimental; evidence-informed workflow, not field-validated by Alkemata ## Intended reader, capability and first artifact This capsule is for a householder, teacher, office worker or facilities team with lawful access to a room and portable CO₂ monitor. It builds the capability to use CO₂ as a bounded ventilation signal, not a general air-quality verdict. In the first session, produce a **Room Air Signal Card**: a one-page record of the monitor, placement, room use, evidence, agreed trigger and action, blind spots and next observation. ## Activation instructions for the LLM Treat this file as a knowledge and workflow package. If the user supplies a room description, monitor model, occupancy and readings, begin the evidence table immediately. Otherwise offer the missions below and ask no more than three questions: 1. Which room, normal activity and occupancy are being examined? 2. What monitor model, measurement history and placement are available? 3. Which actions are actually under the reader’s authority: windows, doors, ventilation settings, occupancy or only reporting? Do not diagnose illness, certify safety or compliance, invent a universal threshold, or advise disabling ventilation, alarms or building controls. Do not propose intentionally raising CO₂. Separate observation, inference and recommendation. ## Missions ### Mission A — Make one reading interpretable Audit the monitor, placement, occupancy, outdoor baseline and ventilation state; produce an evidence table with unknowns. ### Mission B — Build a Room Air Signal Card Define what the CO₂ signal means locally, the action it triggers and what it cannot detect. ### Mission C — Compare two normal ventilation routines Run a small reversible observation during ordinary room use and compare the traces without creating exposure. ## Essential causal model People exhale CO₂. In an occupied room, concentration rises when generation exceeds removal through outdoor-air exchange. A monitor is therefore a ventilation-per-person proxy under assumptions about occupancy, activity, outdoor concentration, mixing and sensor behaviour. An NDIR sensor sends infrared light through a chamber. CO₂ absorbs selected wavelengths; detected attenuation is converted into a concentration. The device measures the air reaching it. A nearby person, window, door, supply vent or unmixed zone can make that sample unrepresentative. CO₂ is one component of indoor air. A low value does not rule out particles, carbon monoxide, radon, volatile compounds, damp, mould or outdoor pollution. A filter may remove particles while CO₂ stays high; an open window may lower CO₂ while admitting an outdoor pollutant. Match each concern to its source and evidence. ## Evidence base and limits - UK Health and Safety Executive, “Using CO₂ monitors”: https://www.hse.gov.uk/ventilation/using-co2-monitors.htm — recommends NDIR monitors, representative placement and contextual records; calls CO₂ a broad guide, not proof of safe levels. - US Environmental Protection Agency, “Introduction to Indoor Air Quality”, updated 19 March 2026: https://www.epa.gov/indoor-air-quality-iaq/introduction-indoor-air-quality — distinguishes many indoor sources and describes ventilation, source control and air exchange. - German Environment Agency, “Indoor air hygiene”: https://www.umweltbundesamt.de/en/topics/health/environmental-impact-on-people/indoor-air-hygiene — distinguishes exhaled CO₂ from pollutants produced by activities, materials and products. HSE workplace examples are not universal home, school or legal limits. Manufacturer traffic-light defaults may not match local guidance. This capsule cannot establish monitor accuracy, air-change rate or health risk from a single trace. ## Workflow ### Step 1 — Define the decision **Input:** Room, users and concern. **Action:** Write one decision the signal should support, such as when to apply an approved ventilation action during a meeting. List hazards explicitly outside scope. **Output:** Decision statement and blind-spot list. **Progression criterion:** The output concerns ventilation, not a claim that all air is clean. ### Step 2 — Verify the instrument and placement **Input:** Model/manual and room layout. **Action:** Confirm the device measures CO₂ (preferably NDIR); follow calibration instructions; place it near breathing height, more than 50 cm from people and away from windows, doors and supply openings. Try another location if mixing is doubtful. **Output:** Evidence and placement sketch. **Progression criterion:** Model, method and position are documented or uncertain. ### Step 3 — Capture context with the readings **Input:** Normal occupied session. **Action:** Record time, reading/trend, occupants, activity, window/door state, mechanical setting and relevant weather. Avoid snapshots. Never change normal use to force a high value. **Output:** Context log. **Progression criterion:** Each reading can be interpreted with its occupancy and ventilation state. ### Step 4 — Define a sourced trigger and authorised action **Input:** Official guidance, building policy and authority. **Action:** Record the source/date for the trigger. Pair it with an authorised action and escalation route. **Output:** Trigger–action rule. **Progression criterion:** No number is universal, and responsibility is named. ### Step 5 — Compare and review **Input:** Two or more comparable normal sessions. **Action:** Compare peak, time above the sourced trigger and recovery after the authorised intervention. Note differences in occupancy and weather. Do not calculate air changes per hour without a valid model and controlled inputs. **Output:** Room Air Signal Card and unresolved questions. **Progression criterion:** The card supports one decision and states its boundaries. ## Decision rules and failure cases - If the room is empty or occupancy changes rapidly, low CO₂ may be uninformative; record the limitation. - If readings change sharply with device position, the room may be poorly mixed; use multiple locations or seek professional assessment. - If an air cleaner is used, evaluate its pollutant target separately; CO₂ does not measure particle removal. - If combustion, carbon monoxide, radon, damp, mould, hazardous work or symptoms are concerns, use the appropriate alarm, test or competent authority. A CO₂ monitor is not a substitute. - If the only available action creates another risk—outdoor smoke, unsafe windows, severe temperature or security problems—do not automate it; escalate the trade-off. - If the monitor method, calibration or manual cannot be verified, use the data only as an exploratory trend and label uncertainty. ## Smallest useful reversible experiment **Hypothesis:** For a normally occupied room, one authorised ventilation change produces a repeatable improvement in the CO₂ trend under comparable conditions. **Resources:** CO₂ monitor and manual, clock, room sketch, paper/spreadsheet and an already permitted ventilation control. **Procedure:** Fix a representative monitor position. Observe one ordinary session without altering behaviour to raise CO₂. On a comparable session, apply the authorised action at the sourced trigger. Record occupancy, conditions, peak and recovery. Repeat if weather or occupancy differed materially. **Success:** The action is feasible and associated with a clearer or faster recovery across comparable observations; the card states what remains unknown. **Failure:** The trace is inconsistent, placement dominates, context is missing, the action is unavailable or another constraint outweighs it. **Stopping conditions:** Discomfort, alarm, suspected combustion problem, visible serious damp/mould, unsafe window use, hazardous outdoor air or any official instruction requiring a different response. **Next action:** Keep the rule provisionally, improve measurement, or prepare a precise request for facilities/building expertise. ## Room Air Signal Card template ```text ROOM AIR SIGNAL CARD Room / normal activity: Decision this signal supports: Monitor make/model / sensing method: Manual and calibration date: Position and reason: Typical occupancy / duration: Outdoor or unoccupied reference (if measured): Applicable guidance / trigger / date: Authorised action: Escalation owner: Observed result and context: Blind spots (particles, CO, radon, VOCs, damp, outdoor air, other): Uncertainty: Next observation/date: ``` ## Hypothetical example A six-person meeting room has an NDIR monitor placed at head height away from occupants and vents. A workplace policy supplies the trigger. Two ordinary meetings of similar duration are logged. In the second, an approved ventilation setting is increased when the trigger is reached, and the trace recovers more quickly. The card records this as provisional evidence for that action, not proof that particles, carbon monoxide or other pollutants are controlled. ## Verification and boundary checks Verify the exact model and manual; fixed placement; occupancy and ventilation records; source/date for every threshold; no claim of complete air quality; no substitution for alarms or required assessments; no invented air-change calculation; and explicit authority for each action. Ask another person to explain what a green reading does *not* establish. This method is not a stand-alone assessment for hazardous processes, healthcare isolation, combustion, fire/smoke, suspected carbon-monoxide exposure, legal disputes or serious building defects. Use competent local guidance. ## Manual/offline route Print this capsule, sketch the room, copy the manual’s instructions, and record readings plus context on paper. Compare sessions visually. An LLM is optional. ## Portable checkpoint ```text CAPSULE CHECKPOINT Capsule ID/version: Room and decision: Monitor/placement: Evidence and source dates: Trigger/action: Observed result: Blind spots and unresolved questions: Next action/date: ``` On request, turn observations into a project passport, field report or precise request for help. Simulations are not field experience. Remove names, schedules, addresses and private details before sharing. The reader chooses what to share. Alkemata’s contact route is https://alkemata.com/collaborate/ . Send nothing automatically and imply no hosting or matching. Consequential external actions require the reader’s separate explicit authorisation and appropriate local responsibility.
