A passneger train travels through fog on the Docklands Light Railway (DLR) network in east London on January 23, 2017.
A thick fog in London forced the cancellation of around 100 flights at Heathrow Airport on Monday and revived concern about air quality in the capital. There were also dozens of cancellations at London City Airport and road users were warned about driving conditions after freezing temperatures overnight. / AFP / Daniel LEAL-OLIVAS        (Photo credit should read DANIEL LEAL-OLIVAS/AFP/Getty Images)