China's national AQI scale has six grades:
优 (excellent, 0-50),
良 (good, 51-100),
轻度污染 (light, 101-150),
中度污染 (moderate, 151-200),
重度污染 (heavy, 201-300), and
严重污染 (severe, 301+).
重度污染 alerts trigger mandatory school closures and vehicle restrictions in many cities.
