When people hear the words air quality monitor, they often imagine one device measuring one thing.

But the air around us is far more complicated.

A room can have:

• Normal carbon dioxide but elevated carbon monoxide
• Low particle pollution but high formaldehyde
• Comfortable temperature but excessive humidity
• Normal oxygen percentage but lower atmospheric pressure at high altitude
• No noticeable odor even though radon is present

That is why modern environmental monitoring is not about finding one universal “air quality number.”

It is about understanding different gases, particles and environmental conditions separately.

FABISENSE develops monitoring products for parameters including carbon monoxide, carbon dioxide, combustible gas, smoke, radon, oxygen, formaldehyde, PM2.5, PM10, TVOC, benzene, barometric pressure, altitude, temperature and humidity.

But before looking at the technology, it helps to understand what each of these measurements actually represents.

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First: Not Everything an Air Monitor Measures Is a Gas

The terms are often mixed together, but scientifically they belong to several different groups.

This distinction matters because a sensor designed for one pollutant cannot automatically measure another.

That is also why multi-parameter environmental monitors often contain several different sensor technologies.

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1. Carbon Monoxide: CO

Carbon monoxide is one of the most important gases to distinguish from every other measurement in this article.

CO is a colorless, odorless and toxic gas produced when carbon-containing fuels do not burn completely.

Common sources can include:

• Furnaces
• Gas heaters
• Fireplaces
• Gas stoves
• Generators
• Vehicle exhaust
• Fuel-burning equipment

The U.S. Environmental Protection Agency notes that carbon monoxide interferes with the body's ability to deliver oxygen and can cause headache, dizziness, nausea, confusion and, at high enough concentrations, death.

This is why carbon monoxide is fundamentally a safety gas, not simply an indoor comfort measurement.

How is CO measured?

Most consumer monitors display CO in:

ppm — parts per million

A CO detector displaying a live number gives users more information than a simple alarm-only device because they can observe whether concentration is changing.

Learn more about [Carbon Monoxide vs Combustible Gas Safety Guide].

FABISENSE examples include [FABISENSE PTH-24D Carbon Monoxide Monitor] and multi-parameter CO products.

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2. Carbon Dioxide: CO₂

Carbon dioxide sounds similar to carbon monoxide, but the two gases are completely different.

CO₂ is naturally present in the atmosphere and is produced when people and animals breathe.

In occupied indoor spaces, CO₂ often rises as:

• More people enter the room
• Doors and windows remain closed
• Outdoor-air ventilation decreases

CDC/NIOSH states that indoor CO₂ monitoring can provide useful information about ventilation conditions. However, CO₂ alone cannot tell you whether a room is free from other pollutants or infectious particles.

This is an important distinction:

CO₂ is often useful as a ventilation and occupancy indicator, but it is not a complete air-quality score.

CDC mentions 800 ppm as one possible ventilation baseline in some situations, but also explicitly warns that one universal CO₂ number is not appropriate for every building or occupancy type.

How is CO₂ measured?

Usually:

ppm

CO₂ monitoring is particularly useful in:

• Bedrooms
• Offices
• Classrooms
• Meeting rooms
• RVs
• Campers
• Other occupied enclosed spaces

The [FABISENSE PTH-282B+ Portable CO & CO₂ Monitor] combines CO and CO₂ monitoring with temperature and humidity measurements.

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3. Combustible Gas

“Combustible gas” does not describe one single chemical.

Depending on the sensor and application, it may include gases such as:

• Natural gas
• Methane
• Propane
• LPG
• Butane

The important difference from carbon monoxide is that combustible gases are primarily associated with fire and explosion risk, while CO is primarily associated with toxic exposure.

That is why:

A CO detector is not automatically a combustible-gas detector.

And a combustible-gas detector should not automatically be assumed to provide CO protection.

Flammable-gas measurement is often expressed relative to the Lower Explosive Limit, or LEL. NIOSH defines LEL as the lower concentration limit at which a gas or vapor can form a flammable mixture in air.

The [FABISENSE PTH-24E CO & Combustible Gas Detector] monitors carbon monoxide and combustible gases including natural gas, methane, propane and LPG, while also displaying temperature and humidity.

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4. Smoke

Smoke is different again.

Smoke is not one gas.

EPA describes wildfire smoke, for example, as a complex mixture containing gases, volatile organic compounds and particulate matter.

Smoke from combustion can contain:

• Fine particles
• Soot
• Carbon monoxide
• VOCs
• Other combustion byproducts

A smoke alarm or smoke sensor is therefore usually responding to physical characteristics associated with smoke particles rather than measuring a single “smoke gas.”

This is why a smoke detector and a CO detector perform different jobs even though both may respond to the consequences of combustion.

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5. Radon: Rn

Radon is one of the most unusual substances monitored in homes.

It is a naturally occurring radioactive gas produced by the decay of uranium in soil and rock.

Radon can enter buildings through openings in foundations and can accumulate indoors, particularly in:

• Basements
• Ground-floor rooms
• Lower-level spaces

It cannot be seen or smelled.

EPA recommends taking action when long-term home radon levels reach 4 pCi/L, approximately 150 Bq/m³, and says homeowners should also consider mitigation between 2 and 4 pCi/L.

WHO uses 100 Bq/m³ as its preferred reference level where achievable.

Radon units

United States:

pCi/L

Many other countries:

Bq/m³

This is why long-term monitoring matters: radon concentration can fluctuate from hour to hour and day to day.

Learn more about [Radon Monitoring and Indoor Air Safety ].

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6. Oxygen: O₂

Oxygen makes up approximately:

20.9% of normal air

at typical atmospheric conditions.

OSHA considers atmospheres below 19.5% oxygen by volume oxygen-deficient in workplace safety contexts.

Low oxygen can become important in certain:

• Enclosed spaces
• Industrial environments
• Combustion-related environments
• High-altitude situations

But there is another important concept:

Oxygen percentage is not the same thing as oxygen availability at altitude.

At higher elevations, oxygen still makes up roughly the same percentage of the atmosphere, but total atmospheric pressure falls.

That means the partial pressure of oxygen also falls.

This is one reason people can experience altitude effects even though an oxygen sensor still shows a value close to 20.9%.

For more detail, read [Oxygen in the Air: What It Means and Why Monitoring Matters ].

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7. Formaldehyde: HCHO

Formaldehyde is a volatile chemical that can be released from materials such as:

• Pressed wood
• Furniture
• Adhesives
• Cabinets
• Flooring
• Certain building materials

It is especially relevant after:

Renovation

Moving into a new home

Installing new furniture

WHO recommends a short-term indoor formaldehyde guideline of:

0.1 mg/m³ as a 30-minute average

for protection against sensory irritation.

Unlike PM2.5, formaldehyde is not a particle that can simply be described by particle size.

It is a gaseous chemical pollutant and therefore requires an appropriate sensing method.

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8. PM2.5

PM stands for:

Particulate Matter

PM2.5 refers to fine airborne particles with aerodynamic diameters generally:

2.5 micrometers or smaller

EPA notes that the average human hair is around 70 micrometers wide—roughly 30 times wider than the upper size of PM2.5.

Sources can include:

• Wildfire smoke
• Cooking
• Combustion
• Vehicle emissions
• Outdoor pollution
• Certain indoor activities

Because PM2.5 is so small, it can penetrate deeper into the respiratory system than larger particles.

WHO's 2021 air-quality guideline values for PM2.5 are:

5 µg/m³ annual average

15 µg/m³ 24-hour average

These are public-health guideline values—not necessarily the same as the alarm settings of a specific consumer device.

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9. PM10

PM10 includes inhalable particles with aerodynamic diameters generally:

10 micrometers or smaller

This group includes PM2.5 as well as larger coarse particles.

Common contributors can include:

• Dust
• Pollen
• Construction dust
• Soil particles
• Road dust

EPA explains that PM10 and PM2.5 describe particle size ranges, not individual chemical substances.

WHO guideline values for PM10 are:

15 µg/m³ annual average

45 µg/m³ 24-hour average.

To learn more, read [PM2.5 vs PM10: What’s the Difference and Why Indoor Monitoring Matters ].

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10. TVOC

TVOC means:

Total Volatile Organic Compounds

VOCs are organic chemicals that can evaporate into the air under normal indoor conditions.

Potential indoor sources include:

• Paint
• Cleaning products
• Furniture
• Building materials
• Solvents
• Adhesives
• Fragrances
• Consumer products

EPA notes that VOC concentrations indoors can often be higher than outdoors because many everyday materials and products release volatile compounds.

TVOC is useful as a broad screening indicator, but there is an important limitation:

A TVOC number does not tell you exactly which VOCs are present.

Two rooms can have the same TVOC reading while containing very different chemicals.

EPA also warns that simply lowering total VOC concentration does not necessarily guarantee a safer environment because individual VOCs have very different toxicities.

That is why dedicated measurements such as formaldehyde or benzene can provide additional information beyond TVOC alone.

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11. Benzene

Benzene is a volatile aromatic hydrocarbon.

It can be associated with:

• Gasoline
• Vehicle emissions
• Tobacco smoke
• Solvents
• Combustion
• Certain industrial and household sources

CDC describes benzene as a highly volatile chemical that evaporates quickly into air. Long-term high exposure can damage bone marrow, and benzene is a known human carcinogen.

WHO's indoor-air guidance states that benzene is a genotoxic carcinogen and that:

No safe level of exposure can be recommended.

WHO therefore recommends reducing indoor benzene concentrations as low as reasonably achievable.

Benzene is one reason a general TVOC reading should not always be treated as a substitute for identifying individual pollutants.

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12. Altitude

Altitude is not an air pollutant.

But it provides important environmental context.

As elevation increases:

• Atmospheric pressure decreases
• Air density decreases
• Oxygen partial pressure decreases

NOAA explains that as elevation rises, there are fewer air molecules above a given location, so atmospheric pressure becomes lower.

This makes altitude-related information useful for:

• Hiking
• Camping
• Mountain travel
• Outdoor work
• Environmental monitoring

It also helps explain why oxygen conditions can feel different at altitude even when the oxygen percentage remains near normal atmospheric composition.

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13. Barometric Pressure

Barometric pressure tells us how strongly the atmosphere is pressing on a given area.

A commonly used standard sea-level pressure is approximately:

1013.2 hPa

according to NOAA.

Pressure changes with:

• Altitude
• Weather systems
• Temperature
• Atmospheric density

This makes pressure useful for understanding environmental change, but one important limitation should be remembered:

Pressure-based altitude is an estimate.

Weather changes can alter barometric pressure even when you remain at exactly the same elevation.

That is why devices displaying both altitude-related information and barometric pressure provide more useful environmental context than interpreting either number in isolation.

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14. Temperature

Temperature is not pollution.

But it affects how people experience an indoor or outdoor environment and can also influence other environmental processes.

For example, temperature can influence:

• Relative humidity
• Material emissions
• Thermal comfort
• Sensor performance
• Atmospheric pressure relationships

EPA notes that high indoor temperature and humidity can also influence concentrations of some indoor pollutants.

This is why temperature commonly appears alongside gas and air-quality readings on multi-parameter monitors.

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15. Relative Humidity

Humidity describes water vapor in the air.

Most consumer environmental monitors display:

Relative Humidity — %RH

The National Weather Service explains that relative humidity describes how much moisture is present relative to the amount the air could hold at its current temperature.

This means relative humidity depends strongly on temperature.

Humidity is important because excessive moisture can support:

• Mold growth
• Dust mites
• Moisture damage
• Condensation

EPA generally recommends keeping indoor relative humidity below 60% and ideally around 30–50% where practical.

Humidity that is too low can also contribute to dry eyes, dry skin and mucous-membrane irritation.

So humidity is not simply a “comfort number.”

It is part of the overall indoor environment.

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Why One Sensor Cannot Tell You Everything

This is perhaps the most important lesson.

Consider these examples:

A room could have:

CO₂: elevated
but
CO: zero

That may point toward occupancy and ventilation rather than incomplete combustion.

Another room could have:

PM2.5: low
but
formaldehyde: elevated

A particle monitor alone would not reveal that gaseous pollutant.

A basement might have:

normal PM

normal CO₂

normal temperature

but still have a radon problem.

And at high altitude:

O₂ percentage may remain near 20.9%

while atmospheric pressure and oxygen partial pressure are significantly lower.

This is why:

Air quality is not one number.

It is a collection of different measurements that answer different questions.

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Matching the Measurement to the Question

A useful way to think about environmental monitoring is to start with the risk or question.

If you are concerned about fuel-burning appliances or generators, monitor:

CO

If you want to better understand occupancy and ventilation, monitor:

CO₂

If you are concerned about natural gas, propane or LPG leaks, monitor:

Combustible gas

For fire conditions, use:

Smoke detection

For basements and ground-level rooms, consider:

Radon

For high-altitude or environmental observation, monitor:

O₂ + Pressure + Altitude

After renovation or new furniture, consider:

Formaldehyde + TVOC

For wildfire smoke, dust or particulate pollution, monitor:

PM2.5 + PM10

For overall environmental context:

Temperature + Humidity + Pressure

Different environments require different measurements.

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How FABISENSE Approaches Multi-Parameter Monitoring

FABISENSE develops dedicated and multi-parameter monitors because no single sensor technology can accurately describe every condition in the air.

Examples across the FABISENSE ecosystem include:

[PTH-24E | /products/24-e]
CO + Combustible Gas + Temperature + Humidity.

[PTH-282B+ | /products/282b]
CO + CO₂ + Temperature + Humidity.

[FABISENSE 10F | /products/10f]
Oxygen + Barometric Pressure + Temperature + Humidity.

[PTH-3MW | /products/pth-3mw]
Formaldehyde + TVOC + Temperature + Humidity.

FABISENSE also develops monitoring solutions covering radon, smoke, particulate matter and broader indoor-air parameters.

Explore [FABISENSE Air Quality Monitors].

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Frequently Asked Questions

Are CO and CO₂ basically the same thing?

No.

CO is carbon monoxide, a toxic product of incomplete combustion.

CO₂ is carbon dioxide, a normal part of exhaled breath and is often useful as an indicator of occupancy and ventilation.

They require different sensors and should never be treated as interchangeable.

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Are PM2.5 and PM10 gases?

No.

They are categories of airborne particles classified primarily by aerodynamic size.

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Is smoke a gas?

Not exactly.

Smoke is a complex mixture containing particles, gases and chemical compounds.

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Can TVOC tell me exactly which chemical is in the room?

No.

TVOC is a combined measurement of volatile organic compounds. It is useful for spotting changes, but it does not identify every individual chemical.

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Is formaldehyde a VOC?

Yes.

EPA includes formaldehyde among volatile organic compounds relevant to indoor air.

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Is benzene the same as TVOC?

No.

Benzene is one specific volatile organic compound.

TVOC represents a combined signal from multiple VOCs.

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Does high CO₂ mean the air is dangerous?

Not automatically.

Indoor CO₂ can help indicate occupancy and ventilation, but it should be interpreted in context. A single CO₂ reading is not a complete measurement of indoor air quality.

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Does oxygen percentage fall dramatically at high altitude?

Usually not.

The percentage remains close to normal atmospheric composition, but atmospheric pressure decreases, meaning fewer oxygen molecules are available in each breath.

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Can one air-quality monitor detect everything?

Usually not.

Different pollutants require different sensing technologies. A monitor should be selected according to the hazards or environmental conditions you actually need to understand.

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The Bottom Line

The air around us is not made up of one “good” or “bad” measurement.

It contains gases, particles and changing environmental conditions.

Carbon monoxide tells us about toxic combustion risk.

Carbon dioxide can provide useful ventilation context.

Combustible gases relate to fire and explosion hazards.

Smoke signals combustion products.

Radon is a radioactive gas that requires dedicated monitoring.

Oxygen and pressure help describe atmospheric conditions.

Formaldehyde, TVOC and benzene provide information about volatile chemical pollutants.

PM2.5 and PM10 describe airborne particle pollution.

And temperature, humidity, pressure and altitude give the environmental context needed to interpret many of those readings properly.

There is no single sensor that tells the whole story.

The goal of environmental monitoring is not to produce more numbers.

It is to help those numbers answer a more useful question:

What is happening in the air around me?

Learn more about [FABISENSE air safety and environmental monitoring solutions ].

FABISENSE — Smarter Sensing. Safer Living.

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Authoritative Sources

U.S. Environmental Protection Agency — Indoor Air Quality
Information on CO, particulate matter, VOCs, humidity, combustion pollutants and indoor-air management.

World Health Organization — Air Quality and Indoor Air Guidelines
WHO guideline information for PM2.5, PM10, CO, formaldehyde, radon and benzene.

CDC / NIOSH — Ventilation and Carbon Dioxide
Guidance on interpreting CO₂ as a ventilation indicator.

Occupational Safety and Health Administration — Oxygen Safety
Normal oxygen monitoring and oxygen-deficient atmosphere guidance.

U.S. EPA — Radon
EPA action-level guidance and radon measurement units.

NOAA / National Weather Service
Scientific explanations of atmospheric pressure, altitude and relative humidity.