Understanding Air Quality Measurements

An air quality monitor rarely produces a single number. Instead, it reports a set of separate measurements, each describing a different pollutant or comfort factor: fine particles of varying sizes, carbon dioxide, total volatile organic compounds, formaldehyde, temperature and relative humidity. Understanding what a reading actually represents, and what range counts as healthy, is what turns raw data into something useful.

These measurements are not interchangeable. A monitor reporting “45” could mean very different things depending on whether that number refers to relative humidity, PM2.5 in micrograms per cubic metre, or something else entirely. Reading a device correctly starts with knowing which unit and which pollutant is being reported, and what a normal, elevated or concerning value looks like for that specific measurement.

Without a measurement, indoor air quality is a guess. Most pollutants that matter, such as CO₂, PM2.5 and formaldehyde, are invisible and largely odourless at the concentrations found in homes and offices. Two rooms that look and smell identical can have very different pollutant levels depending on ventilation, occupancy and what has recently happened in the space.

Measurements matter because they turn that invisible variation into something actionable. A CO₂ reading that climbs steadily through a meeting suggests ventilation is falling behind occupancy. A PM2.5 spike during cooking points to the extractor fan needing to run longer. Guidance from bodies such as the World Health Organization, the US Environmental Protection Agency and ASHRAE exists because sustained exposure to elevated pollutant levels has measurable health and productivity effects — and those guidelines only work as reference points if there is a reading to compare against them.

Each of the eight common measurements tells you something different:

PM1: The smallest commonly measured particle fraction, covering particles up to 1 micrometre in diameter — smaller than most combustion byproducts. These particles are small enough to penetrate deep into lung tissue. PM1 is a relatively new addition to consumer monitoring and is not yet covered by most national air quality standards.

PM2.5: Particles up to 2.5 micrometres in diameter, and the most widely regulated and researched particulate measurement. It is produced by cooking, candles, wood burning, vehicle exhaust and wildfire smoke, and it penetrates deep into the lungs. The US EPA and WHO both treat PM2.5 as a primary indicator of particle pollution health risk.

PM10: Particles up to 10 micrometres in diameter, including dust, pollen and larger combustion particles. PM10 is generally filtered by the nose and upper airway more effectively than PM2.5, but can still irritate the eyes, nose and throat and contribute to allergy symptoms.

Carbon dioxide (CO₂): A gas exhaled by every person in a room, CO₂ is used as a proxy for ventilation rate rather than as a direct toxin at typical indoor concentrations. As CO₂ rises, it usually means fresh air is not being introduced quickly enough to keep pace with occupancy, and that other pollutants may be accumulating alongside it.

TVOCs (total volatile organic compounds): A combined measurement of gases released by paints, adhesives, cleaning products, air fresheners, furnishings and building materials. TVOC sensors report a single aggregated number rather than identifying individual compounds, so a reading is best treated as a general indicator of chemical off-gassing rather than a precise measure of any one substance.

Formaldehyde (HCHO): A specific VOC common in composite wood products, adhesives, insulation and some textiles. It is measured separately from the broader TVOC reading because of its distinct health profile; it is a well-documented cause of eye, nose and throat irritation at lower levels and a concern at sustained high exposure.

Temperature: Air temperature affects comfort directly and also influences how other pollutants behave; warmer air can increase off-gassing of VOCs from furniture and building materials, for example.

Relative humidity: The amount of moisture in the air relative to what the air can hold at that temperature. Humidity affects comfort, dust mite populations and mould growth risk, and interacts with how severe both heat and cold feel.

These figures summarise commonlycited guidance. Occupational limits, which apply to healthy working-age adultsduring a work shift, are typically higher and less protective than thehealth-based guidance below.

High readings usually trace back to a small number of everyday causes:

Particulate matter (PM1/PM2.5/PM10): Cooking, especially frying and grilling, candles, incense, wood-burning stoves, tobacco smoke, and outdoor sources such as traffic or wildfire smoke entering through windows, gaps or HVAC intakes.

Carbon dioxide: More people in a room than the ventilation rate accounts for, closed windows and doors, and mechanical ventilation systems that are undersized, poorly maintained or switched off.

TVOCs and formaldehyde: New furniture, recent renovations, fresh paint, cleaning sprays, air fresheners, and composite wood products such as MDF and particleboard, particularly in the weeks after installation when off-gassing is highest.

Temperature and humidity swings: Weather changes, HVAC systems that are undersized or poorly balanced, showers and cooking without adequate extraction, and seasonal changes in occupancy patterns.

A short list of responses covers most situations:
1. Ventilate first. Opening windows or running mechanical ventilation is the fastest way to bring down CO₂, TVOCs and many combustion particles at once.
2. Use extraction at the source. Kitchen and bathroom extractor fans remove pollutants before they spread through the rest of the home.
3. Investigate sustained TVOC or formaldehyde readings. A short spike after cleaning is expected; a reading that stays elevated for days points to a specific source worth identifying.
4. Treat PM2.5 spikes as source events. Cooking, candles and smoking cause rapid, short-lived spikes; outdoor smoke events cause slower, longer-lasting elevation that ventilation alone may not fix.
5. Manage humidity within the 40 to 60 percent range. This limits both mould risk and the dry-air discomfort of over-heated indoor air in winter.
6. Track trends, not single readings. A monitor that logs data over days and weeks reveals patterns that one-off checks will always miss.

Myth: PM1, PM2.5 and PM10 are three different pollutants. Reality: they describe the same broad category of particulate matter at different size cut-offs. A PM2.5 reading typically includes everything up to 2.5 micrometres, including the PM1 fraction within it.

Myth: A high CO₂ reading means the air is toxic. Reality: at concentrations typically found in homes and offices, CO₂ itself is not directly toxic. It is a proxy for ventilation adequacy — elevated CO₂ usually means other pollutants are also accumulating and that cognitive performance may be affected, rather than indicating a poisoning risk.

Myth: TVOCs are a precise measurement of one specific chemical. Reality: TVOC sensors report an aggregated estimate across many different gases using a single calibration reference. Two devices from different manufacturers can report different numbers for the same air.

Myth: A government workplace exposure limit is a safe target for a home. Reality: occupational limits such as OSHA's formaldehyde PEL (0.75 ppm) are designed for healthy working-age adults during a defined work shift, not for infants, older adults or people with respiratory conditions over a full day. The WHO indoor air guideline for formaldehyde (0.1 mg/m³, roughly 0.08 ppm) is considerably more protective and is the more relevant reference point for a home.

Myth: If a room smells fine, the air quality must be fine. Reality: most of the pollutants that matter most for health, including CO₂, PM2.5 and formaldehyde at lower concentrations, are odourless. Smell is a poor proxy for air quality; measurement is the only reliable way to know.

1. Air quality is made up of several separate measurements, not one score; each needs to be read against its own healthy range.
2. PM1, PM2.5 and PM10 describe particle size, not different pollutants.
3. CO₂ is primarily a ventilation indicator, not a direct toxicity measure, at home and office concentrations.
4. Guideline figures vary by source; WHO indoor guidelines are generally more protective than occupational limits such as OSHA's.
5. Trends over time matter more than any single reading, which is why continuous monitoring outperforms occasional spot checks.