What Is a Good CO2 Level Indoors? (A Simple Guide to PPM)

In Summary

• Outdoor CO2 is roughly 420 ppm, while ideal indoor levels are kept below 800-1,000 ppm.
• Levels above 1,000 ppm are common in closed rooms and may subtly impact focus and energy.
• Continuous monitoring gives you the data to ventilate your space exactly when you need it.

If you've ever searched:

"What is a good CO2 level indoors?"

"Normal CO2 levels in house"

"Indoor CO2 levels ppm"

You're asking the right questions.

Most people don't think about indoor air quality until they see a number on a screen.

And when they do, the immediate question is: what does this actually mean?

Here is a simple breakdown of carbon dioxide (CO2) levels, measured in parts per million (ppm).

The Baseline: Outdoor Air

Before we talk about indoors, we have to talk about outdoors.

Currently, global outdoor CO2 levels sit around 420 ppm.

This is the baseline.

Unless you live next to a major industrial site or heavy traffic, the fresh air outside your window is roughly 420 ppm.

When you open a window to "let some fresh air in," this is the air you are inviting inside.

The Ideal Zone: Under 800 ppm

In an ideal world, indoor air quality should match outdoor air quality as closely as possible.

But because we breathe, and because buildings are sealed for energy efficiency, indoor CO2 naturally rises.

Most building science and health organisations suggest keeping indoor CO2 below 800 to 1,000 ppm.

In this range:

• Air feels fresh
• Cognitive function remains optimal
• Odours and other pollutants are typically well-diluted

If your room sits between 400 and 800 ppm, your ventilation is working well.

The Compromise Zone: 1,000 to 1,500 ppm

This is where things start to shift.

In many modern homes, home offices, and meeting rooms, CO2 easily drifts into the 1,000 to 1,500 ppm range when doors and windows are closed.

It happens quietly.

You won't feel suffocated. You won't struggle to breathe.

But research suggests you might feel:

• Subtly tired
• Less focused
• Slightly foggy

When people complain about a room feeling "stuffy" or "stale," CO2 is often hovering in this zone.

The Problem Zone: Above 1,500 ppm

When CO2 climbs above 1,500 ppm, the effects become more pronounced.

At these levels, air exchange is poor.

Studies indicate that sustained exposure to levels above 1,500 ppm may significantly influence decision-making, reaction times, and perceived fatigue.

In bedrooms with doors and windows closed, it is incredibly common for CO2 to spike past 2,000 ppm overnight.

This can lead to restless sleep, waking up feeling groggy, or morning headaches.

Why Continuous Monitoring Matters

You can't manage what you don't measure.

Without a sensor, you are just guessing based on how you feel.

And because CO2 accumulation is slow and invisible, we often blame our fatigue on a lack of sleep, or our brain fog on a lack of coffee.

Continuous monitoring changes the equation.

When you can see the number climbing past 1,000 ppm at 2pm, the afternoon slump suddenly makes sense.

It gives you the signal to act.

Open a window. Leave the door ajar. Turn on the ventilation.

It's a simple intervention that pays immediate dividends.

Because focus and energy don't just happen in your brain.

They happen in a room.