Pollutionwatch: Ozone, the flipside to sunny days

The record temperature this month also brought problems with summertime smog. Ground-level ozone reached between 7 and 9 on the UK's 10-point air-quality index. Millions of people across the eastern half of England were exposed to pollution levels that were about twice World Health Organisation (WHO) guidelines. This was the worst summertime smog across southern England since 2012.

Ozone is synonymous with Los Angeles in the 1960s and 1970s, when concentrations could be assessed by measuring how fast the smog rotted rubber.