Executive Summary
Every day, we make decisions based on what we can see. We wipe dust off our shelves, notice smoke rising from a barbecue, or avoid a room that smells musty. Yet some of the most important pollutants in the air are completely invisible.
One of these is PM2.5—fine airborne particles measuring 2.5 micrometres (µm) or smaller. These particles are about 30 times smaller than the width of a human hair and are small enough to remain suspended in the air for long periods.
PM2.5 can come from many sources, including vehicle emissions, cooking, industrial activities, cigarette smoke, candles, incense and transboundary haze. Because the particles are so small, they can be carried by air currents throughout a room before eventually settling.
Understanding what PM2.5 is, where it comes from and how it behaves is an important first step in creating healthier indoor environments.
A Morning in Singapore
Imagine waking up on what appears to be a perfectly clear morning.
You open your curtains.
Blue sky.
No haze.
No smoke.
Nothing unusual.
You prepare breakfast, fry an egg, toast some bread and make coffee.
The kitchen smells wonderful.
The house looks clean.
Yet, during those few minutes of cooking, millions of microscopic particles may have been released into the air. Most are far too small for your eyes to detect.
This is the challenge with PM2.5.
It doesn’t announce its presence.
It has no colour.
No obvious smell.
No warning sign.
Understanding PM2.5 begins with accepting a simple truth:
The cleanest-looking air is not always the cleanest air.
Science Minute
What Does PM2.5 Mean?
“PM” stands for Particulate Matter.
The number 2.5 refers to the particle’s diameter.
A PM2.5 particle is 2.5 micrometres or smaller.
For comparison:
Object | Approximate Size |
Human hair | 70 µm |
Fine beach sand | 90 µm |
PM10 | 10 µm |
PM2.5 | 2.5 µm |
A PM2.5 particle is so small that around 30 could fit across the width of a single human hair.
Illustration suggestion: A size comparison showing a human hair, grain of sand, PM10 and PM2.5 at the same scale.
Why Size Matters
Imagine trying to stop footballs with a fishing net.
Easy.
Now imagine trying to stop grains of flour with the same net.
Impossible.
Air behaves in a similar way.
Large particles settle quickly onto floors and furniture because gravity acts on them more strongly.
PM2.5 particles are different.
Because they are extremely small and light, they can remain suspended in the air for extended periods. Even gentle air movement from a fan, an air-conditioner or someone walking through a room can keep them circulating.
This is one reason why PM2.5 receives so much attention from scientists and public health agencies.
Where Does PM2.5 Come From?
Many people associate PM2.5 only with haze.
In reality, haze is just one source.
Common sources include:
Outdoor Sources
- Vehicle exhaust
- Industrial emissions
- Construction activities
- Wildfires
- Transboundary haze
- Road dust
Indoor Sources
- Frying and stir-frying
- Burning candles
- Incense
- Cigarette smoke
- Fireplaces (in some countries)
- Certain hobbies such as woodworking or sanding
- Dust stirred up during cleaning
One important point is that PM2.5 can enter homes from outside through open windows, doors and ventilation, but it can also be generated indoors during everyday activities.
The Kitchen: An Unexpected Source
Many people think of kitchens as places where delicious food is created.
From an air quality perspective, they are also places where airborne particles are produced.
When cooking oil is heated to high temperatures, tiny droplets and combustion particles are released into the surrounding air.
The hotter the cooking process, the greater the potential for particle generation.
This doesn’t mean people should stop cooking. Rather, it highlights the importance of using a cooker hood where available, maintaining good ventilation and recognising that cooking is an important contributor to indoor air quality.
How Long Does PM2.5 Stay in the Air?
Imagine dropping a marble into a swimming pool.
It sinks almost immediately.
Now imagine sprinkling flour into the air.
Instead of falling straight down, the tiny particles drift, swirl and remain suspended.
PM2.5 behaves much more like flour than a marble.
Because these particles are so small and light, they can stay airborne for many hours, depending on factors such as:
- Air movement
- Ventilation
- Humidity
- Particle size
- Indoor activities
Every time a person walks across a room, opens a door or turns on a fan, invisible air currents are created. These currents can keep fine particles circulating far longer than most people expect.
This is why a room can still contain elevated particle levels long after cooking has finished.
Science Minute
Air Never Really Stops Moving
Many people picture the air in a room as still.
It isn’t.
Warm air rises.
Cool air sinks.
Air-conditioners create circulation.
Ceiling fans generate airflow.
Even people walking around disturb the air.
Think of a room as a slow-moving river rather than a box filled with motionless air.
Can You See PM2.5?
Usually, no.
This surprises many people.
When sunlight shines through a window, you may notice tiny specks floating in the beam of light.
Most of those visible particles are actually larger than PM2.5.
The finest particles are generally too small for the human eye to detect individually.
In other words:
Just because the air looks clean doesn’t mean it is free of PM2.5.
PM2.5 During Haze
When haze affects Singapore, PM2.5 becomes a familiar term in weather reports and air quality updates.
During these periods, outdoor concentrations can rise significantly.
If windows and doors are left open, some of these particles may enter indoor spaces.
However, haze is only one part of the story.
For most of the year, indoor PM2.5 often comes from everyday activities such as cooking, burning candles or smoking.
Thinking about PM2.5 only during haze season means overlooking many common sources present throughout the rest of the year.
Measuring PM2.5
Because PM2.5 is invisible, specialised sensors are needed to estimate its concentration.
Modern air quality monitors use optical technology to detect and estimate the amount of fine particulate matter suspended in the air.
Many smart homes now include indoor air quality monitors that display PM2.5 levels in real time.
These devices help homeowners understand how everyday activities influence indoor air.
For example, you may notice:
- PM2.5 rises while frying food.
- Levels gradually decrease after cooking.
- Opening windows can either improve or worsen indoor air quality depending on outdoor conditions.
- Vacuuming without an efficient filter may temporarily stir particles into the air.
Monitoring helps transform indoor air quality from something invisible into something measurable.
Myth vs Fact
Myth:
PM2.5 only exists during haze.
Fact:
PM2.5 is present all year round. Haze is simply one of many possible sources.
Myth:
If my home looks clean, there can’t be PM2.5.
Fact:
PM2.5 particles are far too small to be seen individually.
Myth:
Only factories produce PM2.5.
Fact:
Cooking, smoking, candles and incense are common indoor sources.
Myth:
PM2.5 disappears immediately after cooking.
Fact:
Fine particles can remain suspended long after cooking has ended.
Practical Ways to Reduce PM2.5 Indoors
No single solution removes every source of PM2.5.
Instead, good indoor air quality comes from combining several practical habits.
While Cooking
- Use the cooker hood whenever possible.
- Avoid overheating cooking oil.
- Increase ventilation when outdoor air quality is favourable.
During Haze
- Monitor outdoor air quality.
- Minimise unnecessary opening of windows if outdoor PM2.5 levels are elevated.
Around the Home
- Avoid indoor smoking.
- Maintain your air-conditioning system.
- Clean regularly to reduce settled dust.
- Use an appropriately sized air purifier for occupied spaces.
These simple measures work together to reduce the overall amount of airborne particles in the home.
Why PM2.5 Matters in Singapore
Singapore is a modern, densely populated city with heavy reliance on air-conditioning and apartment living.
This creates a unique indoor environment.
Homes are often kept closed to maintain comfortable temperatures.
Cooking frequently takes place in relatively compact kitchens.
Outdoor pollution levels can change due to traffic, construction or seasonal haze.
As a result, understanding PM2.5 is not only useful during major haze events—it is relevant throughout the year.
Why Scientists Pay So Much Attention to PM2.5
Among the many types of air pollutants, PM2.5 has been extensively studied because of its size and prevalence. Public health agencies around the world routinely monitor PM2.5 as one of the key indicators of air quality.
What makes PM2.5 different is not that it is the only pollutant of concern—it isn’t—but that it is small, widespread and produced by many everyday activities.
This is why you frequently hear PM2.5 mentioned in weather reports and air quality indexes.
PM2.5 Is Only Part of the Indoor Air Story
One of the biggest mistakes people make is focusing only on PM2.5.
Imagine judging the quality of a meal by looking only at the amount of salt.
Salt matters.
But so do sugar, protein, vitamins and freshness.
Indoor air works the same way.
A healthy indoor environment isn’t defined by PM2.5 alone.
Other important factors include:
- VOCs
- Formaldehyde
- Mould spores
- Allergens
- Carbon dioxide
- Odours
- Humidity
- Ventilation
This is why indoor air quality should always be viewed as a complete system rather than a single number.
Did You Know?
A typical family may generate more PM2.5 from cooking over the course of a year than they realize, simply because cooking is a daily activity. While the amount varies depending on cooking methods and ventilation, it highlights why kitchens deserve attention when thinking about indoor air quality.
Frequently Asked Questions
Is PM2.5 the same as dust?
Does rain remove PM2.5?
Rain can help remove some airborne particles from the atmosphere outdoors.
However, indoor PM2.5 generated by cooking or other activities still needs to be managed within the home.
Can PM2.5 enter my home even when the windows are closed?
Yes.
Although closed windows reduce the amount of outdoor air entering a home, no building is completely airtight. Small amounts of outdoor particles may still enter through gaps, ventilation systems or when doors are opened.
Why does my air quality monitor show a spike when I cook?
Cooking—particularly frying, grilling and stir-frying—is one of the most common indoor sources of PM2.5.
Does vacuuming increase PM2.5?
It can temporarily increase airborne particles by disturbing settled dust, especially if the vacuum cleaner has limited filtration or if dust is stirred into the air during cleaning.
Engineering Insight
One of the lessons we’ve learned from years of studying indoor environments is that airflow is just as important as filtration.
Imagine placing a heater in one corner of a room.
The entire room doesn’t become warm instantly. Heat gradually spreads through air movement.
Air purification works in a similar way.
Particles cannot be removed unless the air containing them eventually reaches the purification system. That’s why room layout, furniture placement, airflow patterns and purifier positioning all influence overall performance.
When evaluating any air purifier, it’s worth considering not only how well it captures particles, but also how effectively it treats the air throughout the entire room.
Key Takeaways
- PM2.5 refers to airborne particles 2.5 micrometres or smaller.
- These particles are about 30 times smaller than the width of a human hair.
- PM2.5 comes from both outdoor sources (such as traffic and haze) and indoor sources (such as cooking, candles and smoking).
- Because PM2.5 is invisible, clean-looking air is not always clean air.
- Good indoor air quality combines source control, ventilation, humidity management and air purification.
- PM2.5 is important, but it is only one component of indoor air quality.
