You saved me writing a comment (almost). The map show huge countries such as Canada, Russia, China, and most countries in South America as not having the highest air quality rating even though the air pollution is specific to certain relatively small regions that experience such pollution, while vast regions of those countries are basically undeveloped wilderness with very pure air. This map is so misleading that it is unhelpful.

From a health perspective you have to measure average air quality experienced by a person under typical circumstances. No point measuring air no one breaths.