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Everything here is built on publicly available data and peer-reviewed methodology. This page explains every number you see: what it means, where it comes from, and where its limitations are.
Primary PM2.5 source. Historical daily aggregates pulled via the /sensors/{id}/days endpoint. Data is pre-computed nightly and stored as static JSON under public/data/cities/. Coverage varies by city (typically 1–5 years of station history).
Annual PM2.5 guideline: 5 µg/m³. This is the primary safety benchmark used across all visualizations. The 2021 revision tightened the previous 10 µg/m³ limit based on updated evidence for long-term cardiovascular and respiratory effects.
Source for the cigarette equivalence conversion factor (22 µg/m³ PM2.5 ≈ 1 cigarette/day). Based on population-level mortality risk comparison.
Framework from the Energy Policy Institute at the University of Chicago (EPIC). Used for life-expectancy impact calculations. Formula documented in the section below.
These five bands drive all colors across the calendar, map, and clock visualizations.
WHO 2021 annual target. Safe for all outdoor activities.
Acceptable for most people. Sensitive groups may notice mild effects.
People with asthma, heart or lung disease should reduce prolonged outdoor exertion.
General population begins experiencing health effects. Reduce outdoor time.
Serious risk for everyone. Avoid outdoor activity. N95 mask if unavoidable.
The 15 µg/m³ good/moderate threshold is set at 3× the WHO annual guideline to reflect realistic short-visit exposure rather than strict annual-average health thresholds.
This conversion, popularized by Berkeley Earth, compares the long-run mortality risk associated with ambient PM2.5 to the risk from active cigarette smoking. A person breathing air at 22 µg/m³ for a full year faces a statistically similar mortality burden increase as someone smoking one cigarette per day.
In this app, the cigarette-equivalence headline uses the annual median PM2.5 to represent a robust typical exposure level and reduce outlier distortion.
The exposure calculator also supports an optional mask scenario: No mask remains the baseline, and users can switch to Surgical, KN95, or N95 to see an estimated range. This is a planning aid only, not a guarantee of protection.
Because fit, humidity, wear time, and activity level vary, the app shows a range rather than a single exact number.
The AQLI framework translates PM2.5 exposure above the WHO baseline into estimated reductions in life expectancy. The coefficient (0.98 years per 10 µg/m³) is derived from epidemiological studies on long-term PM2.5 exposure and mortality.
In this app, years lost is calculated from annual mean PM2.5 (long-term exposure proxy), not annual median.
| Risk | Years lost | Source |
|---|---|---|
| Smoking (1 cigarette/day) | ~1.0 yr | Berkeley Earth / WHO |
| Heavy alcohol use | ~1.3 yr | GBD 2019 |
| Traffic accidents (avg risk) | ~0.3 yr | GBD 2019 |
| This city's air | dynamic | AQLI |
These are population-level estimates. They describe the average statistical impact on a large group, not a prediction for any individual. Always consult a healthcare professional for personal health decisions.
The Lung Clock shows a “typical day” — the hourly median PM2.5 across the most recent 7 days of available sensor data. It is a snapshot of recent patterns, not a historical average. The daylight arc uses a solar declination model (Spencer 1971) calculated from the city's coordinates and current date.