The ozone from cars, industry etc spreads across the country, and because it's not broken down by other pollutants which do not spread in the same way, it collects in rural areas. And this place is apparently one of the worst.
To quote various sources: The pollution is linked to hot sunny weather when industrial and motor vehicle emissions react under these conditions to form ground-level ozone. Light easterly winds sometimes bring this pollution across to the UK from central Europe. If conditions are very still for several days then high pollution can be generated by pollution emissions from UK sources too.
In 2003 two sites (London Marylebone Road and Lullington Heath) experienced at least 100 days of moderate or higher air pollution. Generally speaking, urban areas experienced less air pollution than rural areas in 2005.
63 sites in England were used for measuring ozone in 2005. The highest levels of ozone were at the following sites: Weybourne (East of England) Sibton (East of England) Lullington Heath (South East) Yarner Wood (South West).
Lullington is one of the largest areas of heathland on chalk in Britain, and it gives rise to strange mixtures of plants, and also provides a habitat for Graylings. In breezy conditions we didn't find any of these enigmatic butterflies, but did see a Meadow Brown doing a reasonable impersonation.
Parts of the heath have a good selection of typical chalkland flora, such as Squinancywort......
...and others have acid-loving heathers......
....while parts have the two types close together.