Indoor CO2 Explained: The Number That Tracks Ventilation

The number tracks how much outdoor air is reaching a room, against the headcount inside. Occupants breathe carbon dioxide out; outdoor air dilutes it. The reading settles where those two rates balance, and climbs whenever the second falls behind.

NEA states this plainly. Section 5 of the April 2026 guidance on ventilation and indoor air quality is headed carbon dioxide levels as a proxy for ventilation. The text asks for a ventilation rate measurement where possible, and offers readings in occupied areas as the substitute where it is not.

The framing has sat in Singapore guidance for thirty years. It appears in a 1996 Ministry of the Environment document on good indoor air quality for offices. About 5% of exhaled air is carbon dioxide. Levels rise in poorly ventilated occupied rooms, and the level is often used to judge how well ventilation works.

Two properties make it a good proxy. The source is the occupants themselves, so the signal appears when a room is fullest. And the gas mixes through the space instead of settling, so a desk reading describes the air people breathe.

What it cannot describe is everything else in that air. Dust, cooking smoke, solvent vapour from a renovation and mould spores all move independently. A low number means the air is being replaced; it says nothing about how clean that air was.

A statute, a code of practice and an advisory guideline carry different weight. Quoting one while meaning another is the common error here.

The only statutory figure sits in workplace safety law. The First Schedule to the Workplace Safety and Health (General Provisions) Regulations lists permissible exposure levels for toxic substances. Carbon dioxide appears at 5,000 ppm long term across an eight-hour day, and 30,000 ppm short term across fifteen minutes. Regulation 40 places the duty to stay under it on the occupier.

Read that figure for what it is: a toxicity limit, listed among solvents and metals, more than four times above the level an indoor air quality code advises for an office. No complaint about a stuffy meeting room comes near it.

The figure worth quoting for an air-conditioned office comes from a code of practice. SS 554:2026, published by Enterprise Singapore, deals with indoor air quality. NEA reports that the standard recommends indoor carbon dioxide of not more than 700 ppm above outdoor levels. With ambient outdoor air at about 400 ppm, the resulting limit is approximately 1,100 ppm.

Its reach stops short of the home. The scope covers enclosed air-conditioned premises, then excludes residential premises outright, along with factory production areas, hospitals, polyclinics and laboratories. A flat sits outside it.

Two things about that figure deserve attention. It is a recommendation inside a voluntary standard, binding nobody by itself, though a regulator can make a Singapore Standard mandatory and a lease or contract citing one turns it into an obligation. And it is stated as a difference: the standard asks what the building added to the air it drew in, the honest way to ask whether ventilation keeps pace with the people.

The familiar 1,000 ppm has a genuine Singapore source, and it is old. The 1996 office guidelines set it as a maximum over an eight-hour average, beside carbon monoxide, formaldehyde and ozone. It also says its guidelines carry a wide margin of safety, with occasional exceedance unlikely to produce toxic effects.

An absolute number ages in a way a difference does not. The 1996 document described unpolluted outdoor air as about 0.03%, or 300 ppm. NEA's 2026 guidance puts ambient outdoor carbon dioxide at about 400 ppm. The outdoor baseline moved about a hundred parts per million while the fixed indoor figure stayed put.

One further number circulates. NEA suggests aiming below 800 ppm over the measurement period during an epidemic, following the CDC and REHVA. That is an infection-control target, and the guidance is careful with it. High readings do indicate poor ventilation or overcrowding, but levels may not track exposure risk from infectious pathogens directly.

A workplace can be cool and read badly at the same time. Nothing in the cooling loop touches the number, so the two drift apart without anything breaking.

Start with the box overhead. A split, a cassette or a fan coil pulls room air in, cools it and hands it straight back, so a split system does not ventilate at any fan speed. That mechanism is treated elsewhere. What counts here: the equipment most premises have adds nothing to the display.

Outdoor air was somebody's design decision, taken before the tenancy began. A larger building supplies it through a pre-treated or air handling unit, ducted to an intake grille in the building skin. A strata shop unit frequently has no provision at all. Standing in the room reveals neither arrangement, which is why a first reading surprises people.

Headcount moves and the provision stays put. A floor drawn around one occupancy now holds another, and the outdoor air arriving per person falls in proportion. Meeting rooms show it first and worst: small volume, high density, door shut, and the reading climbs through the hour.

Timing explains a share of bad readings on its own. Base building ventilation plant runs to a schedule, so an early start or a weekend shift can put people in a room before the outdoor air supply comes on. Cooling is often available when ventilation is not.

Conditioning outdoor air costs money, and that pressure never lets up. Air drawn from a Singapore afternoon arrives hot and wet, and the plant pays in electricity to cool and dry it. Trimming that supply lowers the bill and raises the reading. That one was a deliberate choice.

The WSH Council and MOM guidelines on managing indoor air quality name three causes when a level exceeds the limit: over-crowding, insufficient outdoor air intake or distribution, and combustion activity in the space.

Distribution is the one people forget. Air can arrive on a floor in the right quantity and never reach the corner the complaint came from. NEA's guidance suggests readings be used to find pockets of under-ventilated space and overcrowding, meaning a walk round the floor with a meter rather than one fixed point.

A carbon dioxide figure is weak evidence about cooling equipment in either direction. A high reading does not show the unit is faulty, and a low one does not show it is healthy. The two questions answer to different trades.

Nothing in the number reports on coil condition, refrigerant charge, drain fall or filter loading. Those faults announce themselves through other channels: a smell on startup, airflow that weakened across months, water where none belongs, and noise that changed character.

Watch what gets quoted on the back of a bad reading. A chemical wash cleans the coil and adds no outdoor air; a larger unit adds cooling to a room whose shortfall was never heat. If either is proposed as the answer, ask which part of the work brings outdoor air in.

An air cleaner meets the same test. Filtration targets particles, and NEA's guidance recommends filter grades for that job. Carbon dioxide is a gas the occupants keep producing, and a filter standing in the room cannot remove it. A purifier may earn its place on the floor, but it answers a different question.

One overlap between the trades is real. Where a building supplies outdoor air, that air passes through cooling plant on its way in, so plant that is down or throttled takes the ventilation down with it. The reading and the equipment are then connected. Establishing whether the premises has such a provision comes first.

Take the reading properly before spending anything on the strength of it. NEA's guidance says a quick assessment is readings over at least five minutes per sampling location, taken in occupied areas at breathing height. A single spot value from an empty room at eight in the morning settles nothing.

Where the crowd changes through the day, NEA offers two routes: install sensors with visible displays for constant monitoring, or take spot measurements during high occupancy. The peak is what matters; an average across an empty afternoon buries it.

Measure the outdoor air too, because the code's figure is a difference. A reading of 1,100 ppm means one thing where outdoor air sits at 400, and another where an intake overlooks a loading bay or a taxi queue. The 1996 guidelines asked for two outdoor samples at the building entrance or fresh air intake.

Inexpensive meters are now everywhere, and the standard has caught up. SS 554:2026 added an informative annex on low-cost sensors, covering use and data interpretation. Treat a cheap display as a trend instrument: strong for showing a room climbs through a long meeting, weaker as a figure for a report.

NEA names two levers when a reading needs to come down, both with the business: increase ventilation, or reduce occupancy. Neither is a purchase from a cooling contractor, and the second costs nothing beyond a scheduling decision.

A formal route exists once the informal one runs out. WSH Council and MOM guidelines describe an indoor air quality management programme, with a named manager and a three-yearly audit. Further audits are triggered by occupant feedback or by work closing a previous finding. A meter reading is the cheap first pass; an audit by a competent person produces a document a landlord will act on.

Those guidelines carry one definition worth knowing. Acceptable indoor air quality means a substantial majority of occupants, put at 80%, express no dissatisfaction with the air. The meter supports that judgement; it cannot substitute for it. Where readings look fine and people still complain, the room is saying something the meter cannot.

• •Which document and which edition produced any figure quoted at you, since the 1996 office guideline and the current code disagree
• •What the outdoor reading was at the same moment, because the code states its limit as a difference
• •What the headcount was in the room when the reading was taken, and whether the door stayed shut
• •Whether any duct serving the space runs to a grille or louvre facing outdoors, or whether cooling is the whole of it
• •Who holds the operating schedule for that provision, if one exists, and which hours it actually runs