PM2.5 refers to fine particulate matter with a diameter of 2.5 micrometers or less — roughly one-thirtieth the width of a human hair. Particles that small bypass the nose and throat, enter the deep lung, and cross into the bloodstream. Long-term exposure is associated with heart disease, stroke, chronic lung disease, lung cancer, and measurable reductions in life expectancy. Children, older adults, and people with existing cardiovascular or respiratory conditions are disproportionately affected.

Mixed-waste burning — particularly of plastics — emits PM2.5 alongside dioxins, furans, polycyclic aromatic hydrocarbons, and heavy metals. Standard low-cost PM sensors capture only the particulate portion. The toxic organic compounds released in the same plume are not measured by the instrumentation described in this document, and their absence from the record should not be read as their absence from the air.

Two scales are commonly quoted. The WHO Air Quality Guidelines (2021) set health-based targets: 5 µg/m³ annual mean, 15 µg/m³ 24-hour mean. These were revised downward in 2021 to reflect updated evidence of harm at lower concentrations. The US EPA Air Quality Index translates PM2.5 into a 0–500 scale with six public-health categories; its breakpoints are looser than WHO's, reflecting regulatory rather than purely health-based limits.

Throughout this document, "above WHO 24-hour" means the daily average exceeded 15 µg/m³ — a threshold that roughly 68% of days in the Kerobokan record cross, and roughly 92% of days in the Kopernik record cross.