PM2.5 are fine particles — reduce them with particle filtration and source control. VOCs are gases released by products, materials and activities — reduce them with source control and ventilation, with activated carbon as a helper. CO₂ is exhaled by people and is mainly an indicator of how much outdoor air a room receives — reduce it only with ventilation. An air purifier addresses the first well, the second partly and the third not at all.
Three different problems
| PM2.5 | VOCs | CO₂ | |
|---|---|---|---|
| What it is | Particles ≤2.5 µm in diameter | Organic chemicals that evaporate at room temperature | Carbon dioxide gas |
| Main indoor sources | Cooking (especially frying, grilling, toasting), candles, smoking, outdoor air, wood stoves | Paints, adhesives, new furniture, cleaning products, fragrances, cooking | People breathing; gas appliances without flues |
| Main control | Filtration, source control, range hood extraction | Source control, ventilation, sorbent filters | Ventilation with outdoor air |
| HEPA filter effect | Strong | None | None |
| Activated carbon effect | None | Partial, compound-dependent | None |
| Typical consumer sensor | Optical (laser scattering) | Metal-oxide (MOx) index | NDIR |
PM2.5: fine particles
PM2.5 refers to the mass concentration of particles with a diameter of 2.5 micrometres or less — about thirty times thinner than a human hair. Their small size lets them remain airborne for hours and penetrate deep into the lungs. The World Health Organization's 2021 air quality guidelines set a guideline level of 5 µg/m³ as an annual mean and 15 µg/m³ as a 24-hour mean for ambient air. There is no separate indoor guideline, but these values are a sensible benchmark for indoor readings averaged over time.
Indoors, PM2.5 comes from two directions: outdoor air that leaks or is ventilated in, and indoor activities. Frying and toasting can briefly push readings far above outdoor levels. That is normal; what matters is how quickly levels recover and the average over the day.
What helps: a cooker hood vented outdoors during cooking, avoiding smoking and candles indoors, a particle filter sized for the room (see sizing), and keeping windows closed during outdoor pollution episodes while running filtration.
VOCs: volatile organic compounds
VOC is an umbrella term for hundreds of compounds — from harmless-smelling terpenes in citrus cleaners to formaldehyde and benzene. Indoor concentrations of many VOCs are often higher than outdoors, because so many sources are inside: new furniture and flooring, paints and varnishes, adhesives, cleaning and personal-care products, air fresheners and cooking.
Some VOCs have specific guideline values. For example, the WHO indoor air guidelines set 0.1 mg/m³ (30-minute average) for formaldehyde. For most others there is no single "safe number", and "total VOC" (TVOC) is a rough indicator rather than a health metric because it lumps together compounds of very different toxicity.
What helps: choosing low-emission products, airing rooms thoroughly after painting or installing new furniture, storing solvents and fuels outside living areas, and using sorbent filtration such as activated carbon as a supplement. Carbon works for many VOCs but poorly for formaldehyde.
CO₂: an indicator of ventilation
Outdoor air currently contains roughly 420 ppm of carbon dioxide. Indoors, every occupant adds CO₂ with each breath, so the concentration rises unless outdoor air replaces room air. That makes CO₂ an excellent, inexpensive proxy for how well a room is ventilated relative to the number of people in it.
| Indoor CO₂ | What it usually indicates |
|---|---|
| ≈ 420–600 ppm | Close to outdoor levels; generous ventilation or few occupants |
| 600–1,000 ppm | Typical of adequately ventilated occupied rooms |
| 1,000–1,500 ppm | Ventilation is modest for the occupancy; stuffiness becomes noticeable to many people |
| Above 1,500 ppm | Ventilation is likely insufficient for the number of occupants; review airing or mechanical ventilation |
These bands are practical rules of thumb used widely in building practice, not health limits. Many ventilation guidelines treat indoor levels persistently above about 1,000 ppm as a signal to review ventilation.
What helps: only more outdoor air — opening windows, trickle vents, or mechanical ventilation, ideally with heat recovery. No HEPA or carbon filter lowers CO₂ in any meaningful way.
Reading consumer air quality monitors
- PM2.5 sensors count particles optically and estimate mass. They are good at showing changes and relative levels but can over-read in high humidity (water droplets look like particles) and differ between brands by a noticeable margin.
- VOC sensors in consumer devices are mostly metal-oxide elements that respond to many gases at once and report an index. Use them to spot events — cleaning, cooking, a new sofa — not as concentration measurements of any specific compound.
- CO₂ sensors using NDIR (non-dispersive infrared) technology are generally reliable. Cheaper "eCO₂" readings calculated from a VOC sensor are estimates, not measurements.
A monitor that combines true NDIR CO₂ with an optical PM2.5 sensor gives the most useful picture for everyday decisions: the PM2.5 reading tells you when to run filtration, the CO₂ reading tells you when to ventilate.
Putting it together: a simple decision flow
- Stuffy room, high CO₂? Ventilate. A purifier will not help.
- Smoke, cooking particles, outdoor pollution, pollen? Use particle filtration sized for the room, plus source control.
- Chemical or new-material smell? Ventilate and remove the source where possible; carbon filtration can reduce odor meanwhile.
- Several at once? Combine ventilation and filtration. During outdoor pollution episodes, filtered mechanical ventilation or short, intense airing followed by filtration is a practical compromise.
Key takeaways
- PM2.5, VOCs and CO₂ come from different sources and need different controls.
- WHO guideline levels for PM2.5 are 5 µg/m³ (annual) and 15 µg/m³ (24-hour).
- Carbon helps with many VOCs but not formaldehyde; HEPA does not help with gases.
- CO₂ is a ventilation indicator and only ventilation reduces it.
Frequently asked questions
What is a good indoor PM2.5 level?
The WHO 2021 guideline for outdoor air is 5 µg/m³ as an annual mean and 15 µg/m³ as a 24-hour mean. There is no separate indoor limit, but these values are a sensible benchmark: short spikes during cooking are normal, sustained high readings are worth investigating.
Can an air purifier lower CO₂?
No. HEPA and carbon filters do not remove carbon dioxide in any meaningful amount. CO₂ comes from people breathing and is controlled by supplying outdoor air through ventilation.
Are cheap VOC sensors accurate?
Most consumer VOC sensors use metal-oxide elements that react to many compounds at once and report a relative index rather than a true concentration of a specific chemical. They are useful for spotting changes, such as after cleaning or painting, but not for compliance measurements.
Sources and further reading
- World Health Organization — WHO global air quality guidelines (2021)
- WHO Regional Office for Europe — WHO guidelines for indoor air quality: selected pollutants (2010)
- U.S. EPA — Guide to Air Cleaners in the Home
- U.S. EPA — Air Cleaners and Air Filters in the Home (incl. Residential Air Cleaners: A Technical Summary)