Why Does CO2 Increase When Windows Are Closed? Malaysia
Why Does CO2 Increase When Windows Are Closed? Malaysia
You close the windows.
The air conditioner is running.
The room feels comfortable.
But after several people stay inside for some time, your CO2 monitor starts showing a higher reading.
Why does this happen?
One of the main reasons is simple:
People continuously produce carbon dioxide through breathing, while closing windows can reduce one pathway for outdoor air exchange.
If CO2 is being produced faster than it is diluted or removed through ventilation, the indoor concentration can increase.
This is particularly relevant in Malaysian bedrooms, offices, meeting rooms and classrooms where air conditioning is commonly used with doors and windows closed.
Where Does Indoor CO2 Come From?
In a normally occupied room, people are an important source of carbon dioxide.
We inhale air and exhale air containing more CO2.
One person produces CO2 continuously.
Two people produce more.
Ten people in a meeting room can change conditions faster.
Thirty students in a classroom create another occupancy situation entirely.
Therefore, indoor CO2 depends partly on:
Number of people
Duration of occupancy
Room size
Fresh-air exchange
Ventilation system
Door and window conditions
This is why CO2 can change substantially throughout the day.
Why Does Closing the Window Matter?
A window can be one route through which indoor and outdoor air exchange occurs.
When windows and doors are closed, natural air exchange may be reduced, although the actual result depends on the building and ventilation system.
People inside continue breathing.
CO2 continues being produced.
If there is insufficient fresh-air exchange, CO2 can accumulate.
The room may still feel cool.
The room may still look clean.
There may be no obvious smell.
But the CO2 trend can still change.
Does a Closed Window Always Mean High CO2?
No.
It is important not to oversimplify the issue.
A building may have mechanical fresh-air ventilation.
Some HVAC systems introduce outdoor air.
Air leakage and other ventilation paths can also affect the room.
Therefore:
Closed windows do not automatically mean CO2 will always become high.
The better approach is to measure the actual room.
This is one reason a CO2 monitor is useful.
Why Air Conditioning Doesn't Answer the Question
A common misunderstanding is:
“My air conditioner is running, so the room must have enough fresh air.”
That should not automatically be assumed.
Many common room air-conditioning systems primarily cool and recirculate indoor air.
Cooling and ventilation are different functions.
An air conditioner can make the room cooler without necessarily providing enough fresh-air exchange for the number of occupants.
This is why CO2 monitoring can be useful even in a very comfortable air-conditioned room.
Example: Two People Sleeping in a Bedroom
Consider a typical bedroom.
At 10:30 PM:
The room is empty.
At 11:00 PM:
Two people enter.
The air conditioner is switched on.
The windows are closed.
The door remains closed.
At midnight:
The room is cool.
At 3:00 AM:
Both people are still sleeping.
At 7:00 AM:
They wake up.
During these eight hours, the occupants continuously produce CO2.
If the bedroom has limited fresh-air exchange, the CO2 trend can increase during the night.
A single daytime measurement would not reveal this overnight pattern.
Why Historical CO2 Data Is Useful
Suppose you check your bedroom at 8:00 AM and see one CO2 number.
That tells you the condition at that moment.
But several more useful questions remain:
What was the CO2 level before sleeping?
When did it start increasing?
How quickly did it increase?
What was the highest reading overnight?
Did it remain stable?
What happened when the bedroom door opened?
How quickly did CO2 decrease after the room became empty?
Historical monitoring helps answer these questions.
Example: Home Office
The same issue can occur during the working day.
Imagine a small home office.
One person enters at 9:00 AM.
The air conditioner is running.
The window is closed.
The door is closed to reduce noise during online meetings.
The person works continuously until 1:00 PM.
The room remains cool.
But occupancy continues for four hours.
A CO2 monitor can show whether the ventilation conditions are keeping up with occupancy.
Example: Meeting Room
Now increase the occupancy.
A meeting room is empty at 8:45 AM.
At 9:00 AM, twelve employees enter.
The door closes.
The meeting continues for 90 minutes.
If the fresh-air supply is insufficient for the occupancy, CO2 can increase much faster than in an empty room.
This is why meeting rooms are useful locations for CO2 monitoring.
Example: Classroom
Classrooms can have even higher occupancy.
A classroom may contain:
20 students.
30 students.
40 students.
Plus a teacher.
When doors and windows are closed for air conditioning, ventilation becomes an important consideration.
Monitoring CO2 can help schools understand how classroom conditions change:
Before students arrive.
During lessons.
During breaks.
After students leave.
Between different classrooms.
The purpose is to measure the actual environment rather than assume all rooms behave the same way.
What Happens When You Open the Door?
Opening a door may change the air exchange between the room and surrounding spaces.
Depending on the conditions, you may observe a change in CO2.
This creates a simple practical experiment.
Monitor the room under normal closed conditions.
Then change one variable.
For example:
Open the door.
Observe the CO2 trend.
Do not focus only on the immediate number.
Look at the direction and rate of change.
This can help you understand how the room responds.
What Happens When You Open the Window?
Opening a window can increase outdoor air exchange under suitable conditions.
This may affect CO2.
However, there is another consideration in Malaysia:
Outdoor air quality.
During haze or periods of elevated outdoor particulate pollution, opening a window may also influence indoor PM2.5.
This creates an important indoor air-quality trade-off.
Closed Windows During Haze
Suppose outdoor PM2.5 is elevated.
You close the windows.
You operate an air purifier.
Indoor PM2.5 begins falling.
This may be desirable from a particulate perspective.
But four people remain inside the room.
Over time, CO2 may increase if fresh-air exchange is insufficient.
Now you have two different measurements moving in different directions:
PM2.5 ↓
CO2 ↑
This is why monitoring both parameters can provide more information than looking at only one air-quality number.
Does an Air Purifier Solve High CO2?
A typical particulate air purifier should not be assumed to solve increasing CO2.
The purifier may reduce:
PM2.5.
Dust.
Smoke particles.
Other filterable airborne particles.
But CO2 is a gas associated strongly with occupancy and ventilation conditions.
Therefore, an air purifier may successfully reduce PM2.5 while CO2 continues to increase.
The purifier is not necessarily malfunctioning.
It is simply addressing a different air-quality problem.
Can a Fan Reduce CO2?
A fan moves air.
Whether it actually improves the CO2 condition depends on where the air is moving.
If a fan simply circulates the same enclosed room air, circulation alone does not necessarily provide fresh outdoor air.
If it supports effective air exchange between spaces or through appropriate ventilation openings, the result may be different.
Again, measurement is more useful than assumption.
Why Temtop M10+ Is Useful for This Application
The Temtop M10+ is useful because CO2 is not the only indoor parameter that may change.
It can monitor parameters including:
CO2
PM2.5
TVOC
AQI
Temperature
Humidity
This is useful for understanding several indoor environmental conditions together.
For example:
Close windows → observe CO2.
Haze outside → observe PM2.5.
Switch on air purifier → observe PM2.5 trend.
Install new furniture → observe TVOC trend.
Operate air conditioning → observe temperature and humidity.
Instead of relying only on whether the room “feels okay,” you have measurement data.
A Simple CO2 Test at Home
You can perform a simple comparison.
Test 1: Empty Room
Leave the room unoccupied for a period and observe the CO2 condition.
Test 2: Occupied Room
Use the room normally with the usual door and window arrangement.
Observe the trend.
Test 3: Change One Condition
Where practical and appropriate, change one ventilation-related condition.
For example, change the door arrangement.
Observe what happens.
The objective is not to create a laboratory experiment.
It is simply to understand how your real room behaves.
Don't Change Five Things at Once
If you want to understand the cause of a change, avoid changing everything simultaneously.
For example, do not:
Open the window.
Open the door.
Switch off the air conditioner.
Move the monitor.
Switch on a fan.
and then try to determine which action changed the reading.
Instead, change one condition at a time where practical.
This makes the result easier to interpret.
Correct Monitor Placement Matters
Avoid placing the CO2 monitor:
Directly beside your mouth.
Beside your pillow.
Immediately beside an open window.
Directly under an air-conditioning outlet.
Inside a cabinet.
In an unusual corner with poor air representation.
For general room monitoring, choose a representative occupied-zone location according to the instrument guidance.
Why Your CO2 Reading May Change Quickly Near Your Face
Exhaled breath contains much more CO2 than normal room air.
If you breathe directly toward the sensor, the reading may rise.
This does not necessarily mean the entire room suddenly reached the same concentration.
Monitor placement therefore matters when interpreting CO2 readings.
Is One CO2 Reading Enough?
Usually, a trend is much more informative.
Instead of asking only:
“What is my CO2 now?”
also ask:
“What was it two hours ago?”
“What happened after people entered?”
“What happened after they left?”
“Does the same pattern happen every day?”
“Which room shows the fastest increase?”
These questions turn a monitor into a useful investigation tool.
CO2 Monitoring for Malaysian Homes and Businesses
CO2 monitoring can be useful in many local applications, including:
Air-conditioned bedrooms.
Home offices.
Meeting rooms.
Conference rooms.
Training centres.
Tuition centres.
Classrooms.
Small offices.
Other occupied enclosed spaces.
The common factor is not the type of building.
It is occupancy combined with ventilation conditions.
Why Does CO2 Increase When Windows Are Closed?
The basic mechanism is straightforward.
People inside the room produce CO2.
If the rate of fresh-air exchange is insufficient to dilute the CO2 being produced, the indoor concentration can increase.
Closing windows may reduce one source of natural air exchange, depending on the building.
But the actual result varies according to the ventilation system, room and occupancy.
That is why the best approach is:
Measure → Observe the trend → Change one condition → Compare
For users who also want to monitor PM2.5, TVOC, temperature and humidity, a multi-parameter indoor air-quality monitor such as the Temtop M10+ provides a broader picture of what is happening inside the room.
Contact MTM Precision
MTM Precision Sdn. Bhd. (744811-A)
Showroom & Service Centre
No. 29-1 & 29-2, Jalan Bandar 18,
Pusat Bandar Puchong,
47160 Puchong, Selangor, Malaysia
Telephone: +603-8080 7172
WhatsApp: +6016-660 7346
Email: mtmpre@yahoo.com / enquiry@mtmpre.com.my
Website: www.mtmpre.com.my
Contact MTM Precision for Temtop M10+ CO2 Monitor Malaysia, quotation and assistance selecting an indoor air quality monitor for homes, bedrooms, offices, meeting rooms, classrooms and other indoor applications.
19 Sep 2026
