One resource projects may use is the World Health Organization’s Global Urban Ambient Air Pollution Database, available at http://www.who.int/phe/health_topics/outdoorair/databases/cities/en/

GBCI will use data from air quality measurements near the project and take 30% of the average value. For example, if the ambient PM2.5 levels average 90 µg/m³ on the day of Performance Verification, the project will use 27 µg/m³ as the threshold for Part 2c. Note that projects using this approach are limited to Silver certification level.