Only 7 countries currently meet the World Health Organization's clean air guidelines. What are they doing right?

The guidelines set out a safe limit for PM2.5 – tiny particles of air pollution. The limit is 5 micrograms or fewer of PM2.5 per cubic metre of air. There are the only countries that meet it. PM2.5 particles come from a range of sources such as vehicles, power plants and industrial emissions and they pose one of the greatest environmental threats to human health worldwide. So what can we learn from the countries that meet the WHO guidelines?