WHAT THE NUMBERS MEAN

Small parts,
big story.

A few hundred parts in a million can tell you a lot.

Parts per million is how scientists count carbon dioxide in the sky, and how a room monitor tells you when the air needs changing. Here is what the numbers mean, from NOAA and UK guidance.

PPM parts per million

Read a monitor

A bright meeting room with a high window tilted open, morning light falling across an empty table and chairs — AI-generated image, not a photograph of a real person or place.

Outdoors: about 426
Under 800: fine
Over 1500: act
One unit, two uses

THE SHORT ANSWER

One unit,
three stories.

The same small unit measures the planet's air,
the air of the past, and the air in your office.

OUTDOORS

About 426 ppm now

NOAA's monthly average at Mauna Loa, Hawaii, was 426.07 ppm in September 2026, up from 424.37 ppm a year earlier.

See the numbers

THE PAST

280 before industry

Air bubbles trapped in ice cores show that before the Industrial Revolution, carbon dioxide was 280 ppm or less, and never above 300 ppm in the ice-age cycles of the past million years or so.

What ppm means

INDOORS

A ventilation gauge

People breathe out CO2, so it builds up in stuffy rooms. UK guidance treats readings consistently over 1500 ppm as a sign of poor ventilation.

Read a monitor

A QUICK MONITOR CHECK

What does
the reading mean?

Pick a CO2 reading from an occupied room. The bar shows how much action UK guidance suggests.

Monitor check

Choose a readingFar below 400 ppmUnder 800 ppm800 to 1500 ppmOver 1500 ppm, consistentlyOne sudden high reading

Far below 400 ppmCheck the monitor

That is lower than outdoor air. The Welsh guidance says a very low reading may mean the monitor is in the wrong place, so move it and measure again.

Under 800 ppmNo action

A consistent value under 800 ppm suggests the room is well ventilated. Keep some background ventilation going anyway.

800 to 1500 ppmOpen up

A sign that background ventilation is not enough. Opening windows and doors should usually bring it back under 800 ppm.

Over 1500 ppm, consistentlyImprove ventilation

HSE says levels consistently above 1500 ppm in an occupied room indicate poor ventilation and you should take action to improve it.

One sudden high readingWait and retake

Single snapshot readings can mislead. The Welsh guidance suggests waiting 5 minutes before retaking a high reading.

Sources: HSE, Ventilation in the workplace: using CO2 monitors; Welsh Government, Carbon dioxide monitors in education settings.

Based on HSE's guidance on using CO2 monitors and the Welsh Government's guidance for education settings. Readings are a broad guide to ventilation, not a measure of safe levels.

TWO NUMBERS WORTH KNOWING

The sky's share
keeps rising.

One figure from this autumn,
one from before the factories.

426 ppm

Mauna Loa monthly average, September 2026

NOAA's Global Monitoring Laboratory reported 426.07 ppm for September 2026, against 424.37 ppm in September 2025.

OUTDOOR CO2 · NOAA

≤ 280 ppm

Carbon dioxide before the mid-1700s

When continuous measurements began at Mauna Loa in 1958, global carbon dioxide was already 315 ppm.

BEFORE INDUSTRY · NOAA CLIMATE.GOV

NOAA Climate.gov says carbon dioxide rose by about 0.8 ppm a year in the 1960s and by 2.6 ppm a year on average from 2015 to 2024.

FOUR TIPS FROM HSE

Get a reading
you can trust.

A monitor is only as good as where you put it and how you read it. These come from HSE's workplace guidance.

Place it well
Head height, away from windows, doors and air supply openings, and over 50 cm from people, whose breath contains CO2.
Watch the day, not a moment
Take several readings while the room is in use, as numbers of people, activities and ventilation change.
Write it down
Record the reading, how many people were in the room and what ventilation was in use at the time.
Know its limits
Monitors do not work well with air cleaning units, rooms with only one or two people, or very large, high-ceilinged spaces.

Gloved hands holding a clear cylinder of ancient ice, full of tiny trapped air bubbles, in a cold laboratory — AI-generated image, not a photograph of a real person or place.

A small atmospheric observatory with white domes and instrument masts on a bare volcanic mountain slope at dawn — AI-generated image, not a photograph of a real person or place.

WHAT PPM MEANS

Count the
molecules.

NOAA reports carbon dioxide as a mole fraction: the number of CO2 molecules divided by all the molecules in dry air. A fraction of 0.000400 is written as 400 ppm.

Indoors, the same unit stands in for airflow. HSE says 1000 ppm is equivalent to about 10 litres of fresh air per second for each person, the rate many guides recommend for most commercial buildings. Its Approved Code of Practice says the supply should not normally fall below 5 to 8 litres per second per person.

Questions

Questions people ask

What does ppm stand for?

Parts per million. For carbon dioxide in air, NOAA defines it as the number of CO2 molecules per million molecules of dry air, so 0.000400 is written as 400 ppm.

What is a normal CO2 level outdoors?

NOAA's monthly average at Mauna Loa, Hawaii, was 426.07 ppm in September 2026. Before the Industrial Revolution it was 280 ppm or less.

What CO2 reading is good indoors?

Welsh Government guidance for schools says a consistent value under 800 ppm suggests a well-ventilated space, and readings consistently over 1500 ppm indicate poor ventilation.

Does a low reading mean the air is safe?

No. HSE says CO2 measurements are only a broad guide to ventilation rather than a demonstration of safe levels, and air cleaners remove other contaminants but not CO2.

Why does my monitor jump when I lean over it?

Your breath contains CO2. HSE advises placing monitors over 50 cm away from people, as a monitor that is too close may give a misleadingly high reading.

Small parts,
big story.