Safe CO₂ levels in a room:
the complete ppm chart

Almost every question about indoor air quality in India gets answered with a PM2.5 number. That is understandable — outdoor particulate pollution here is genuinely severe, and it is what the news reports. But it means most people have never seen the other number, the one that describes whether the room they are sitting in is being ventilated at all.

That number is carbon dioxide, measured in parts per million. This page is the chart, and the reasoning behind it.

The chart

Every band below is a range that a real room passes through over the course of an occupied day. The descriptions are drawn from ASHRAE Standard 62.1, WHO indoor air guidance, and the controlled-exposure literature on CO₂ and cognition.

Why the number is not really about carbon dioxide

Here is the part that changes how you read the chart. At the levels in the table above, carbon dioxide is not poisoning anyone. Its value is as a proxy.

CO₂ indoors comes almost entirely from the people in the room. A seated adult exhales roughly 18 to 20 litres of it an hour — about 0.005 litres per second, the figure ventilation standards are built on. So the concentration in a room is a direct measure of how much of the air you are breathing has already been through somebody else's lungs — and therefore of everything else that accumulates alongside it: exhaled moisture, body odour, volatile compounds from furnishings and cleaning products, and airborne respiratory particles.

This is why CO₂ became the standard indicator for ventilation adequacy in building codes worldwide, and why public-health guidance during and after COVID-19 converged on CO₂ monitors for shared indoor spaces. A room at 700 ppm is getting plenty of outdoor air. A room at 2,200 ppm is not — whatever the air conditioner is doing.

What the standards actually say

Two documents get cited constantly and are worth separating.

ASHRAE Standard 62.1 — the international reference for ventilation in commercial and institutional buildings — does not set a CO₂ limit as such. It specifies minimum outdoor-air rates per person and per unit floor area, and it treats CO₂ as an indicator that those rates are being achieved. The widely quoted 1,000 ppm figure comes from the observation that steady-state indoor CO₂ roughly 650 ppm above outdoor corresponds to the ventilation rate at which most visitors judge a space acceptable. With Indian outdoor air around 420 ppm, that puts the practical ceiling near 1,050 ppm.

WHO indoor air guidance approaches it from a health rather than a comfort direction, and lands in a similar place for ordinary occupied rooms.

The practical consequence is the same either way: hold an occupied room below 1,000 ppm, and treat 1,500 ppm as the point where ventilation must already be running. We cover the differences in more detail in ASHRAE 62.1 vs WHO guidelines explained for Indian buildings.

Why Indian rooms sit high on this chart

Three things compound here in a way they do not in a temperate country with mild outdoor air.

Sealed construction. Modern Indian apartments and offices are built airtight on purpose — to keep out heat, dust, street noise and outdoor particulates. Every one of those is a good reason. The side effect is that the building's natural air exchange, which used to happen through leaky windows whether anyone thought about it or not, is now close to zero.

Split air conditioning. A split AC recirculates. It takes the air already in the room, removes heat from it, and returns it. It brings in no outdoor air at all. So the more comfortable the room becomes, the more firmly people keep the windows shut, and the faster CO₂ climbs. We wrote about this specific trap in why modern Indian homes have a ventilation problem.

Occupancy density. Indian classrooms, offices and function halls generally hold more people per square metre than the design assumptions in the standards being cited. CO₂ scales directly with people.

Put those together and a sealed 4 m × 4 m bedroom with two sleepers and the door closed will pass 2,000 ppm before morning. A meeting room with ten people crosses 2,500 ppm within about 90 minutes. Neither is unusual — they are the default.

What the levels feel like

The unhelpful thing about CO₂ is that it has no smell, no colour and no immediate sensation. What people notice is second-order, and they almost always attribute it to something else:

• 1,000–1,500 ppm — the meeting that should have taken 30 minutes and took 70. Attention drifting. Nobody feels unwell.
• 1,500–2,500 ppm — yawning that spreads around a room. People stepping out “for some air”. The AC gets turned down, which does nothing.
• 2,500–4,000 ppm — a heavy, close feeling. Mild headache by the end of the afternoon. Guests leaving a function early without being able to say why.
• Above 4,000 ppm — clear discomfort: headache, fatigue, occasionally nausea. Usually blamed on the food, the heat, or a coming illness.

Because every one of those attributions is wrong, the problem persists indefinitely in buildings where nobody measures. That is the actual argument for a monitor: not that CO₂ is dangerous, but that its effects are invisible and consistently misattributed.

What lowers CO₂ — and what does not

Only one thing removes carbon dioxide from a room: replacing the air with outdoor air. An open window, an exhaust fan, a fresh-air unit, an ERV. That is the complete list.

Things that do not lower CO₂, despite very widespread belief:

• Air purifiers. A HEPA filter captures particles. Carbon dioxide is a gas molecule far smaller than anything a particulate filter can catch; it passes straight through. An excellent purifier running at full speed changes the CO₂ in a room by nothing at all. This is worth its own article: CO₂ vs AQI vs PM2.5 — what an air purifier does not fix.
• Air conditioners. Split and window units recirculate. They change temperature, not composition.
• Houseplants. The arithmetic is brutal. A person exhales roughly 18 to 20 litres of CO₂ an hour; the quantity of foliage needed to offset one sleeping adult overnight is measured in hundreds of plants, and at night plants respire and emit CO₂ themselves. Plants are lovely. They are not ventilation.
• Exhaust fans that are switched off. Most rooms in India already have the hardware. The failure is not equipment, it is control — nobody knows when to run it, so it runs on a guess or not at all.

How to find out what your own room does

Two options, in increasing order of usefulness.

Estimate it. The physics is a straightforward mass balance: CO₂ generation from occupants, dilution from outdoor air exchange, and the room volume that buffers both. Our free CO₂ build-up calculator runs that model — enter your room's floor area, ceiling height, number of people and rough ventilation rate, and it plots the curve over an occupied day and tells you where it settles.

Measure it. An estimate tells you what a room like yours should do. Only a sensor tells you what yours actually does, and rooms consistently surprise people — a door left ajar, a gap in a window frame, a fan that someone has quietly disconnected. How to check and measure CO₂ levels in your room covers what to buy, where to put it and how to read the result.

The short version

Below 1,000 ppm, your room is fine. Between 1,000 and 1,500, ventilation has stopped keeping up. Above 1,500, it is not working, and above 2,500 it is measurably costing you concentration and comfort. The fix is always the same — move air — and the only real decision is whether that happens because somebody remembered, or because something measured it.