Inhalable dust is the fraction of airborne dust that can be breathed in through the nose and mouth. It includes larger particles that deposit in the upper airways. The general workplace exposure limit is 10 mg/m3 (8-hour TWA), sampled with an IOM head.
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What inhalable dust is
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What respirable dust is
Respirable dust is the much finer fraction (around 4 microns median) that penetrates deep into the gas-exchange region of the lung, where it can cause the most serious disease. The general limit is 4 mg/m3 (8-hour TWA), sampled using a cyclone that separates out the fine fraction.
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Why the distinction matters
The fine respirable fraction drives diseases such as silicosis and pneumoconiosis, so for hazards like crystalline silica the respirable limit (and a substance-specific limit of 0.1 mg/m3 for RCS) is what counts. Sampling the wrong fraction can seriously misjudge risk.
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Frequently asked questions
Can you sample both fractions at once?
Yes — workers can wear separate inhalable and respirable samplers simultaneously to capture both fractions in one survey.
Which fraction is more dangerous?
The respirable fraction generally causes the most serious lung disease because it reaches the deepest part of the lung.