Most people picture dust as something you wipe off a shelf. PM2.5 is nothing like that. It can stay suspended in the air for hours or longer, it is usually invisible at the levels most buildings experience, and it is small enough to travel deep into the lungs.
The name means particulate matter 2.5 micrometres across or smaller. A typical human hair is around 20 to 30 times wider. An office or classroom can look spotless while concentrations are elevated because PM2.5 itself provides no reliable visual or sensory warning.
It is also not one pollutant. PM2.5 is a size class: a mixture of solid particles and liquid droplets whose composition depends on what produced it, from organic compounds and elemental carbon to metals, sulphates and nitrates. Two buildings showing the same concentration can therefore contain quite different particle mixtures.
Why it matters
Particles this small can bypass much of the filtering performed by the nose and throat and reach the alveoli deep in the lungs. Some of the smallest particles, and substances carried on their surfaces, may also enter the circulation.
Long-term exposure is associated with cardiovascular and respiratory disease, including ischaemic heart disease, stroke, chronic obstructive pulmonary disease and lung cancer. Short-term increases can worsen asthma and other respiratory conditions.
The World Health Organization recommends staying below 5 µg/m³ as an annual average and 15 µg/m³ over 24 hours. These are health-based guidelines rather than universal indoor compliance limits, but they provide an important reference point for assessing building performance.
Where it comes from
Traffic is a major urban source: exhaust, tyre and brake wear, road dust, and secondary particles that form in the atmosphere from gaseous emissions. Buildings near busy roads, car parks and loading bays may carry a higher outdoor particle burden before anything happens inside.
Cooking can be one of the largest and most underestimated indoor sources of PM2.5. Frying and grilling can push levels up sharply within minutes. Both gas and electric cooking generate particles because heated oils, fats and food are major sources, although the appliance and combustion process also affect the wider pollutant profile. Extraction that is undersized or simply switched off can turn lunch service into a building-wide event.
Wildfire smoke can travel hundreds or thousands of kilometres, so a building nowhere near the fire may still be affected for days. Research suggests wildfire PM2.5 may be more harmful than non-wildfire PM2.5 at the same mass for some respiratory outcomes, although this remains an active area of study. Current regulation generally treats a microgram as a microgram, regardless of where the particle came from.
Outdoor PM2.5 does not stop politely at the front door
Particles enter through ventilation systems, open windows, doors and gaps in the building envelope.
How much reaches occupants depends on outdoor concentration, filtration efficiency, airtightness, ventilation strategy, operating schedules and window use. A well-filtered building may hold indoor levels far below outdoor concentrations. A naturally ventilated or poorly filtered building may follow the outdoor pattern much more closely.
There is no single indoor-to-outdoor ratio that applies everywhere, which is exactly why indoor PM2.5 monitoring is needed rather than relying on assumptions.
What continuous monitoring adds
Because PM2.5 provides no reliable sensory signal, a building without monitoring has no direct, building-specific evidence of what occupants are experiencing.
A one-off test describes a single moment. Continuous data describes how a building actually behaves: traffic peaks, cooking events, smoke infiltration, differences between zones, and whether filtration is doing what the design assumed.
Comparing indoor and outdoor readings helps distinguish infiltration from indoor sources. That can be the difference between changing a filter and fixing an extraction fan.
PM2.5 is not just dust. It is one of the clearest reasons a building needs live air-quality data rather than relying only on periodic assessments.
For questions about PM2.5 or other indoor air quality parameters, contact MoveAir. Our IAQ specialists can help you understand what to measure, interpret the data and identify the most practical monitoring and improvement strategy for your building.
