People living and working in London awoke to a thick cloud of smog covering much of the capital on Thursday.
Britain has been placed on a health alert as a potentially dangerous cloud of air pollution blows over the country.
Experts said the smog could cause fatal asthma attacks and have warned the elderly and those with health problems to be cautious.
The Department for Environment, Food and Rural Affairs (Defra) said air pollution was blowing in from Europe and mixing with home-grown smog to create the problem.
Officials warned this could trigger health difficulties for those with respiratory illnesses and the elderly, although it does not pose a health threat for the general population.
Large parts of England will see air pollution climb to moderate levels, including London, while parts of the north will see it hit nine out of 10.
Our colleagues over at Mirror Online filmed this footage of the smog covering the capital on Thursday morning (March 19):
A Defra spokeswoman said: "Winds bringing in pollution from the continent, combined with locally generated pollution and still weather conditions has led to some high pollution measurements across the UK.
"Levels are expected to return to low by Friday."
Kay Boycott, chief executive of Asthma UK, said: "Two thirds of people with asthma find that air pollution makes their asthma worse, putting them at an increased risk of a potentially fatal asthma attack.
"When air pollution is high it's vital people with respiratory conditions including asthma check air pollution forecasts, carry their reliever inhaler with them at all times, and ensure that they are taking their preventer inhaler every day because this will help build resilience to asthma triggers like air pollution.
"People with asthma have told us that on days when air pollution levels are high they feel that they can't even leave the house for fear that it will trigger an attack."
Every year, it is estimated 29,000 premature deaths here are caused by poor air quality.
What is smog?
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.
This affects everyone as when you breathe in, you breathe in the harmful substances too.
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.
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.
The Met Office reports that air pollution levels are due to remain moderate or high on Thursday across "Wales, Northern Ireland and many parts of England under high pressure and light winds"
Joggers have been urged to not to jog in the smog as pollution hits danger levels in many areas of the UK.
