A tourist looks through the thick fog towards Tower Bridge on January 23, 2017 in London, United Kingdom. [Photo/VCG]

Readings at 3pm on Monday showed that some locations in the capital showed higher particulate levels than in China's capital. London's air on Monday afternoon contained 197 micrograms per cubic meter for particulate matter (PM2.5) on the index. In Beijing, the reading was 190.

On Tuesday morning, the UK Department for Environment Food and Rural Affairs said that air pollution levels for the Greater London area were still "very high". The reading in London for fine particulate matter was 157, higher than Shanghai's but lower than Beijing's.

In London, the culprits for the bad air are wood-burning stoves and automobile traffic, exacerbated by mild wind conditions, according to King's College in London.