Toxic smog and blood rain are to take over London skies as a Saharan dust cloud gets blown across to Britain .
The capital has been placed on a Level 6 warning by the Department for Food, Agriculture and Rural Affairs ( Defra ).
But what is it all about? Here's all you need to know.
Where has the "toxic smog" come from?
According to forecasters, a cloud of dust is being swept across Europe from the Sahara desert and will hit Britain on Thursday, carried by turbulent weather systems created above the Atlantic.
A Level 7 red warning - or "high" - has been placed for areas such as Norfolk and places like London were placed on level-6.
What is the smog made up of?
Smog is formed when air pollution levels are high and there isn’t much wind, so a combination of particles and ground level ozone builds up to create a yellowish or black fog.
How can it affect you?
Smog can cause your lungs to work less well because the particles can cause the linings in the airways to become inflamed, leading to breathing problems.
Pollution can make people with asthma or heart and lung diseases more sensitive to triggers that make their condition worse.
And children with asthma are particularly vulnerable to pollution as when they play outdoors they have faster breathing rates and their lungs are still developing.
What are the short-term effects?
The temporary short-term effects of being exposed to smog can include illnesses such as pneumonia or bronchitis. They also include discomfort such as irritation to the nose, throat, eyes, or skin and headaches, dizziness, and nausea.
Has this happened before?
This weather follows a similar rise in pollution in March, when a build-up of smog in Paris blew across the Channel and settled in the UK.
Showers of rain then brought down the dust in what is known as "blood rain".
Before that, in spring 2014 the UK was once again affected by dust clouds. On that occasion the emergency services reported a surge in call-outs to patients with breathing problems.
At the time, 1.6 million people suffered an asthma attack as smog filled the air, Asthma UK said.
Every year, it is estimated 29,000 premature deaths in the UK are caused by poor air quality.
