Measurements · Particulate Matter
PM4 (Respirable)
Also known as respirable dust, respirable fraction
Respirable particulate ≤4 µm (respirable convention); the fraction reaching the gas-exchange region, used in occupational dust exposure.
What it is
What is PM4 (respirable dust)?
PM4 is the respirable fraction of airborne dust: particles of 4 microns and smaller, defined by health scientists as the portion fine enough to penetrate beyond the upper airways and reach the gas-exchange region deep in the lungs. It is the size fraction that matters most for occupational lung disease, because it is where the most damaging dusts, above all crystalline silica, do their harm. Wherever materials are cut, ground, crushed, drilled, or handled (construction, mining, quarrying, stone fabrication, foundries, and manufacturing), respirable dust is generated, and PM4 is the metric used to assess how much of it workers can actually inhale into the deep lung.
Health and operational effects
Respirable dust exposure causes a range of serious, often irreversible lung diseases: silicosis from crystalline silica, coal workers' pneumoconiosis, and other dust-related fibrosis and impairment, along with an elevated risk of lung cancer and chronic obstructive pulmonary disease. The diseases develop over years and can progress even after exposure ends, which is why occupational limits target the respirable fraction specifically. Measuring total or coarse dust is not enough. The health-relevant particles are the fine ones that reach the alveoli.
For employers in dusty industries, respirable dust is a frontline occupational-health and compliance obligation, backed by exposure limits and mandatory monitoring.
Limits
PM4 (Respirable) Limit Values
The published values, by averaging period; each links to the standard that sets it.
Decay test, start and end
- ISO 23875 25 µg/m³: respirable particulate; decay time 120 s at most
Respirable dust is reported as a mass concentration in mg/m³ or µg/m³, expressed as an 8-hour time-weighted average for occupational compliance. Limits depend on the dust: general respirable-dust limits apply broadly, while crystalline silica within the respirable fraction has a far stricter limit of its own. Because the value is a shift average, brief peaks add to the cumulative dose. Real-time monitoring shows where those peaks come from, while gravimetric sampling with silica analysis determines the compliance figure.
How it is measured
How PM4 (Respirable) Is Measured
Two approaches work together. Gravimetric sampling draws air through a size-selective cyclone that isolates the respirable (PM4) fraction onto a filter, which is weighed and, where silica is a concern, analyzed in a laboratory for crystalline content. This is the reference method for compliance and gives a time-weighted average. Real-time respirable-dust monitors use optical sensing with a respirable-fraction inlet to show concentration continuously, revealing which tasks and moments drive exposure so controls can be applied as work proceeds. The two are complementary: real-time for control, gravimetric for compliance.
An instrument on Thiamis2
A third-party instrument connected to a Thiamis gateway, reporting into the same Environet account.
Placing respirable-dust monitoring
Personal sampling places the cyclone and filter in the worker's breathing zone to capture actual inhaled exposure over the shift. Area and real-time monitors sit near dust-generating activities and at the boundary between work and occupied zones to verify that controls (water suppression, extraction, enclosure) are working. Because respirable-dust exposure is strongly task-driven, monitoring is most useful when tied to specific operations, so that the tasks contributing most to the cumulative dose can be identified and controlled.
Where it is monitored
Solutions That Monitor PM4 (Respirable)
Each one links to the solution page showing what that industry measures it for.
Cabin Air Quality
Core parameterRespirable dust (PM4) is the worker-health frontline in mining and quarrying, because it is the size fraction that reaches deep into the lungs and, where it carries crystalline silica, causes silicosis.
Occupational Safety & Industrial Hygiene
Often addedWorker exposure, confined space, and heat stress
Perimeter Monitoring
Often addedSite boundary dust, silica, and noise compliance
Article Library
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Questions
PM4 (Respirable) FAQ
What does "respirable" dust mean?
Respirable dust is the fine fraction, around 4 microns and smaller (PM4), that penetrates beyond the upper airways into the deep, gas-exchange region of the lungs. It is defined this way because that is where the most damaging dusts, especially crystalline silica, cause disease. Coarser dust is largely trapped higher up and is less hazardous.
How is PM4 different from PM2.5 and PM10?
The size fractions serve different purposes. PM10 and PM2.5 are environmental (ambient) metrics for community air quality, while PM4 (respirable dust) is an occupational-health metric defined by how deeply particles reach into the lungs. They overlap in size but are used, sampled, and regulated in different contexts: workplace exposure versus outdoor air.
Can a real-time monitor replace gravimetric sampling for compliance?
No. Compliance with respirable-dust and silica limits relies on gravimetric sampling with laboratory analysis of the crystalline content. Real-time respirable-dust monitors are extremely useful for immediate control and for finding high-exposure tasks, but they complement rather than replace the reference method.
How do I monitor respirable dust on site?
Combine gravimetric personal sampling for compliance with a real-time respirable-dust (PM4) monitor for control and boundary checks. Use the configurator to pair a Thiamis gateway with a respirable-dust monitor and add PM10, silica sampling, and noise for a full construction, mining, or quarrying panel.
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