Is the UK's air getting cleaner or dirtier?

The UK has been suffering unusually bad smogs. But in the long run, its air is getting cleaner. One of the most harmful constituents of smog is PM2.5, particles that are less than 2.5 micrometres across. The World Health Organization recommends that PM2.5 levels should not exceed 10 micrograms per cubic metre, averaged over a year. The graph below shows that the proportion of the UK population exposed to these harmful levels of PM2.5 has been falling steadily since 2003. (Source: Environmental Performance Index, Yale University)