❓ Frequently Asked Questions
What is AQI (Air Quality Index)?▾
AQI is a standardized 0–500 index that communicates how polluted the air currently is and what health effects might apply. 0–50 Good (minimal risk); 51–100 Moderate (acceptable for most); 101–150 Unhealthy for Sensitive Groups (children, elderly should reduce outdoors); 151–200 Unhealthy (everyone may experience effects); 201–300 Very Unhealthy (serious health effects); 301+ Hazardous (emergency conditions — stay indoors).
What is a safe AQI level to go outside?▾
AQI 0–50 is ideal for all outdoor activities including jogging and cycling. AQI 51–100 is generally fine for healthy adults but sensitive groups should limit prolonged exertion. Above 100, sensitive groups should reduce time outdoors. Above 150, even healthy people should limit extended outdoor activity. Above 200, everyone should stay indoors.
What is PM2.5 and why is it dangerous?▾
PM2.5 is fine particulate matter smaller than 2.5 micrometers — about 30 times smaller than a human hair. These particles bypass the body's natural filters, penetrate deep into the lungs, enter the bloodstream, and can reach organs including the heart and brain. Long-term exposure is linked to heart disease, stroke, lung cancer, and respiratory illness. The WHO guideline is 15 μg/m³ annual average.
How is AQI calculated from pollutant levels?▾
The US EPA formula converts measured concentrations of PM2.5, PM10, CO, SO₂, NO₂, and O₃ to individual sub-index values on the 0–500 scale using piecewise linear equations. The overall AQI is the highest single-pollutant sub-index — so a high PM10 reading during a sandstorm can drive the AQI even if other pollutants are low.
How is breathing polluted air equivalent to smoking cigarettes?▾
Berkeley Earth researchers compared the long-term mortality risk of PM2.5 exposure to cigarette smoking. They found that breathing air with a PM2.5 concentration of 22 μg/m³ for 24 hours produces a similar mortality risk increase as smoking one cigarette per day. The formula is: cigarettes per day = PM2.5 (μg/m³) ÷ 22. This is a mortality risk comparison — the mechanisms of harm differ.