Reality Check: Was pollution worse in London than Beijing?
The claim: Air pollution in London last week was worse than it was in Beijing.
Reality Check verdict: Some one-off readings were higher in London last week, but this was an unrepresentative snapshot and Beijing is generally far worse.
On 22 January, recordings of particulate air pollution were higher in London than in Beijing.
Ruth Cadbury is the Labour MP for Brentford and Isleworth, a part of London that has seen unusually high levels of air pollution recently.
Last week saw the highest level recorded in the capital since April 2011.
The spike was attributed to cold, calm and settled weather, meaning winds were not dispersing local pollutants.
Different countries measure air pollution in different ways.
The UK government uses a one (lowest) to 10 (highest) scale.
Last week's levels in London were a 10.
Another measure is the Air Quality Index (AQI).
Last Monday, according to this measure, some parts of London showed particulate levels a bit higher than in Beijing.
But this was just a snapshot and not the case for most of the week.
On Wednesday afternoon, the overall AQI level in Beijing was about three times higher than in London, and recordings were even higher on the Chinese city's industrial outskirts.
The World Health Organization gathers average particulate levels from cities around the world.
They suggest that Beijing's levels are about five times worse than London's.
The cities with the dirtiest air are Zabol in Iran and Onitsha in Nigeria.
In the UK, overall emissions of all types of air pollution have fallen dramatically since 1970.
Pollution in Beijing is much worse than in London - or in Stockholm, where the same claim was made this week.