Medical explainer
What is PM2.5—and why does medicine care about it?
PM2.5 is particulate matter with an aerodynamic diameter of about 2.5 micrometres or smaller. The important clinical idea is not the label itself: particles in this size range can penetrate into the small peripheral airways and alveolar region.
1 · Exposure
Fine particles enter with inspired air. The amount inhaled depends on concentration, time and how much air you breathe.
2 · Deposition
Some fine particles deposit in the small airways and alveolar region; deposition varies with particle size and breathing pattern.
3 · Biological response
Particle pollution is associated with respiratory and cardiovascular effects through pathways that include inflammation, oxidative stress and vascular/autonomic effects.
Simplified deposition map
Smaller particles can travel farther into the respiratory tract.
This is a teaching schematic, not a literal map of where every particle ends up. Deposition depends on size, shape, breathing pattern and airway anatomy.
What the evidence supports
Population risk is broader than the lungs.
These are population-level health relationships. A PM2.5 reading cannot diagnose disease, predict whether one person will develop symptoms today, or identify the cause of an individual symptom.
Short-term exposure
Think aggravation and events, not instant diagnosis.
Short-term increases in particle pollution can aggravate respiratory symptoms and asthma and are associated with cardiovascular effects at a population level. Susceptibility differs between people.
Long-term exposure
Risk is about repeated exposure over time.
Long-term particulate exposure is associated with cardiovascular and respiratory morbidity and mortality. WHO's guideline values are therefore health-based targets for population exposure—not personal diagnostic thresholds.
Guidelines are not the same as AQI
WHO concentration guideline ≠ Indian AQI category.
WHO's 2021 air-quality guidelines give health-based concentration levels for pollutants. India's CPCB AQI transforms pollutant concentrations into sub-indices and then one communication index. A concentration in µg/m³ and an AQI value are therefore not interchangeable.
Understand AQI vs PM2.5 →Primary medical references
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