Measurements · Particulate Matter
PM2.5
Also known as fine particulate matter, fine dust, particulate matter 2.5
Fine particulate matter ≤2.5 µm that penetrates deep into the lungs and can enter the bloodstream. A primary ambient air-quality and health metric.
What it is
What is PM2.5?
PM2.5 is airborne particulate matter with an aerodynamic diameter of 2.5 microns or less, roughly thirty times smaller than the width of a human hair. Because these particles are so fine, they bypass the body's natural defenses in the nose and throat and travel deep into the alveoli of the lungs, where the finest fraction can cross into the bloodstream. PM2.5 comes from combustion above all: vehicle exhaust, wood and coal burning, wildfires, industrial processes, and power generation, along with secondary particles that form in the atmosphere from gaseous precursors such as sulfur dioxide and nitrogen oxides.
It is the most consequential air pollutant for human health worldwide, which is why it anchors almost every ambient and indoor air quality program.
Health and operational effects
Long-term exposure to PM2.5 is causally linked to cardiovascular disease, stroke, reduced lung function, chronic respiratory illness, and premature death; short-term spikes trigger asthma attacks, hospital admissions, and measurable drops in cognitive performance. The World Health Organization tightened its annual guideline to 5 µg/m³ in 2021 precisely because harm continues well below levels once thought safe. There is no established threshold under which PM2.5 is free of risk.
For building operators and employers the operational stakes are concrete: elevated indoor PM2.5 correlates with absenteeism and complaints, drives HVAC filtration costs, and increasingly appears in lease terms and healthy-building certifications.
Limits
PM2.5 Limit Values
The published values, by averaging period; each links to the standard that sets it.
24-hour, notification threshold
24-hour, top of Good (AQI 50)
24-hour, top of Moderate (AQI 100)
24-hour, top of Unhealthy for Sensitive Groups (AQI 150)
24-hour, top of Unhealthy (AQI 200)
24-hour, top of Very Unhealthy (AQI 300)
A01 Air Quality
Current AQI 151 (NowCast)
Current AQI above 500
Daily average, continuous
Acceptable
High performance
- EPA NAAQS 9 µg/m³: 2024 primary
- EU 2024/2881 10 µg/m³: 2030 limit value (previously 25)
- EPA AQI 225.4 µg/m³: Hazardous above; AQI 500 is 325.4 µg/m³
- Cal/OSHA 5141.1 55.5 µg/m³: hazard communication, training, controls, and respirators for voluntary use
- Cal/OSHA 5141.1 500.4 µg/m³: respirator use required; per the regulation's Appendix A table
- LEED v5 15 µg/m³: 3 points at or below
- LEED v5 12 µg/m³: 4 points at or below; also the one-time test limit
- RESET Air 35 µg/m³: below
- RESET Air 12 µg/m³: below
PM2.5 is reported as a mass concentration in µg/m³, averaged over a defined period, and the averaging time matters as much as the number. Compare annual averages against annual guidelines and 24-hour averages against short-term limits; a single high instantaneous reading during cooking or a passing truck is expected and not itself a compliance breach. The parameter's limit-value table on this page lists the applicable EPA, WHO, EU, and WELL values with their averaging periods.
How it is measured
How PM2.5 Is Measured
Most continuous monitors use one of three approaches. Optical (light-scattering) sensors shine a laser at the airstream and infer mass concentration from scattered light; they are compact, real-time, and cost-effective, and dominate distributed networks and indoor monitoring. Beta attenuation monitors (BAM) measure the absorption of beta radiation by particles collected on a filter tape and are common in regulatory-grade fixed stations. Gravimetric samplers physically weigh particles collected on a filter over a fixed period and remain the reference method for calibration.
Optical instruments are sensitive to humidity and particle composition, so reference-grade programs either use BAM or gravimetric methods or apply correction factors. It is a distinction worth understanding when comparing a low-cost sensor reading against a regulatory value.
Built into Aethair PRO and Aethair IAQ2
Measured by the monitor's own sensors, calibrated before it ships: nothing to add.
An instrument on Thiamis3
A third-party instrument connected to a Thiamis gateway, reporting into the same Environet account.
Placing PM2.5 sensors
For indoor air quality, place sensors in the breathing zone (1–1.7 m above the floor) in occupied areas, away from doors, windows, and supply diffusers that would give unrepresentative readings. For fence-line and ambient monitoring, position instruments at the site boundary with an intake clear of local obstructions, and pair them with wind speed and direction so that dust events can be attributed to their source rather than merely detected.
Every device and option that measures PM2.5 5
| Device / option | Type | Base platform | Measures |
|---|---|---|---|
Aethair PRO built-in sensors | Built-in sensors | Aethair PRO | |
 Aethair IAQ built-in sensors | Built-in sensors | Aethair IAQ | |
 TSI · DustTrak 8530, DustTrak 8530EP, DustTrak 8532… | Particulate & Dust Monitors | Thiamis | |
 Met One · E-BAM, E-BAM Plus | Particulate & Dust Monitors | Thiamis | |
 Trolex · Air XD (TX8005), Air XD (TX8015), XD ONE (TX8060)… | Particulate & Dust Monitors | Thiamis |
Where it is monitored
Solutions That Monitor PM2.5
Each one links to the solution page showing what that industry measures it for.
Healthcare Air Monitoring
Core parameterIn clinical settings, PM2.5 is a marker of both infection-control performance and the effectiveness of the high-grade filtration that hospital ventilation relies on.
Indoor Air Quality
Core parameterIndoor PM2.5 is driven by a mix of infiltration from outdoor air and internal sources such as cooking, printing, candles, and occupant activity, which means indoor levels can be lower than outside on a polluted day or far higher during a cooking peak.
Outdoor Monitoring
Core parameterPM2.5 is the single most health-consequential pollutant in ambient air monitoring, which is why it anchors regulatory networks and public air-quality indices alike.
Perimeter Monitoring
Core parameterWhile PM10 dominates construction dust conditions, the finer PM2.5 fraction matters for community health exposure and for demolition, cutting, and combustion sources on site.
School Air Monitoring
Core parameterIn schools, PM2.5 reaches classrooms from outdoor traffic and wildfire smoke as well as indoor activity, and children, who breathe more air relative to body weight than adults, are more exposed to its effects.
Wildfire Air Monitoring
Core parameterDuring wildfire smoke episodes, PM2.5 is the pollutant that matters most for public health, and levels can climb from clean to hazardous within an hour as a plume arrives.
Cleanrooms, Labs & Manufacturing
Often addedParticle counting and controlled environments
HVAC & Building Management
Often addedMonitor ventilation and filtration, with data for your BMS
Article Library
Reading on PM2.5
PM2.5 Exposure Limits & Monitoring: OSHA and NAAQS Standards
What are the OSHA and NAAQS exposure limits for PM2.5? Fine particulate thresholds explained, plus how continuous monitoring keeps workplaces compliant.
Ambient Air Quality Monitoring: What Nonattainment Changes
Ambient air quality monitoring explained: how PM2.5 attainment is measured, what a nonattainment designation triggers, and what site-level monitoring adds.
Fenceline Air Quality Monitoring: Equipment and Approaches
Compare fenceline air quality monitoring approaches: standalone stations, pelican-case kits, and connected 4G sensor networks.
How Building Certifications Use Air Quality Monitoring
How WELL and LEED v5 incorporate air quality monitoring, why both frameworks are moving toward continuous performance, and what that means for building teams.
Workplace Air Quality Monitoring for Offices and Employers
How to monitor workplace air quality in offices, what to measure for employee health and focus, and how continuous connected monitoring documents the result.
Indoor Air Quality in Schools: Standards & Monitoring 2026
Indoor air quality in schools: which standards apply, what to monitor, and how CO2 and ventilation affect student attendance, health, and test scores.
Related
More in Particulate Matter
Questions
PM2.5 FAQ
What is the difference between PM2.5 and PM10?
PM2.5 covers particles up to 2.5 microns; PM10 covers everything up to 10 microns, so PM10 includes the coarser dust fraction as well as the fine one. PM2.5 penetrates deeper into the lungs and carries greater health risk per unit mass, while PM10 is more associated with coarse sources like road and construction dust. Most monitors report both.
What is a safe level of PM2.5?
The WHO 2021 guideline is 5 µg/m³ as an annual average and 15 µg/m³ over 24 hours. The WHO position is that there is no threshold below which PM2.5 is entirely safe, so lower is always better; regulatory limits such as the US EPA NAAQS and the EU 2030 limit values are set higher than the guideline for feasibility reasons.
Can a low-cost optical sensor be trusted?
Optical sensors give reliable trends and are excellent for spatial coverage and alerting, but their absolute accuracy depends on humidity and particle type. For regulatory or legal purposes, use a reference (BAM or gravimetric) method, or a low-cost sensor that has been co-located with one and corrected against it.
How do I measure PM2.5 for my application?
Any of the options on this page will report PM2.5. Aethair IAQ and Aethair PRO measure it with a built-in laser optical sensor, and several external monitors connected through Thiamis offer optical, beta-attenuation, and gravimetric methods. Use the configurator to combine PM2.5 with the other parameters your site needs.
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