UK & World News
Air Pollution: 10 Things You Need To Know
People with lung and heart problems have been warned to avoid strenuous activity as air quality falls to its lowest possible level across parts of the country. We look at what is behind the increase in air pollution and ask just how serious the problem is.
:: What is causing air quality to drop?
The poor air quality levels sweeping across much of England and Wales are caused by a combination of dust blown in from the Sahara desert and harmful emissions from both the UK and Europe.
Light winds have allowed this cocktail of natural and man-made pollutants to linger in the skies above the country.
:: What is in the air?
As well as dust and sand particles from the Sahara desert, traffic pollutants such as carbon monoxide, nitrogen dioxide (NO2), volatile organic compounds (VOCs) and particulate matter - combined with the ozone created when sunlight reacts with NO2 and VOCs - cause air quality to worsen.
Although these pollutants can cause air pollution close to where they are emitted, they can also travel long distances, with emissions from mainland Europe adding to the current problems in southeast England.
The dust falls to the ground when it rains, leaving a fine residue on car windscreens and other outdoor surfaces.
:: What are the health implications?
Air pollution can cause runny eyes and noses, as well as coughs and sore throats, but the effects can be more serious among the very young, the elderly and those with existing lung or heart problems.
Dr Keith Prowse, former chairman of the British Lung Foundation and an honorary medical adviser, told Sky News: "People with asthma or COPD (chronic obstructive pulmonary disease) should make sure they have their reliever inhalers with them, and those who use preventer inhalers may have been told to double their dose.
"The best advice is not to go out when pollution levels are high and not to take part in strenuous exercise."
:: Will people need to start wearing face masks?
Worsening air pollution in places like China has made face masks a must-have accessory for many people.
However, Dr Carol Cooper, a London GP, said: "A good piece of advice is not to bother. They're not generally very effective and wearing one can actually make breathing more difficult.
"Wearing a face mask in somewhere like southeast Asia is more of a cultural thing than a medical one."
:: How does sand from the Sahara end up in the UK?
"There are currently strong dust storms across the Sahara and because the winds in the upper atmosphere are blowing in a southerly direction, this is sending the dust and sand particles over western Europe and into the atmosphere over England and Wales," Sky News weather presenter Nazaneen Ghaffar said.
"The reason we've seen the orange deposits is because of recent rainfall. Any rain clears the dust from the upper atmosphere, bringing it down to lower levels, and as the water evaporates it leaves behind the orange dust.
:: Is the dust likely to cause any disruption to flights?
Many air passengers will remember the chaos caused by the volcanic ash cloud generated by the eruption of Icelandic volcano Eyjafjallajokull in 2010.
Flights were grounded across the UK and Europe amid fears dust could cause engine damage.
However, a spokesman for Nats, the British air traffic control service, said it was "not aware" of any likely disruption to flights, while a British Airways official added: "We are not expecting our customers to be affected in any way."
:: How is air pollution measured?
The Department for Environment Food and Rural Affairs (Defra) measures air quality on a scale of one to 10, with each number given a different colour to visualise the extent of pollution on a map of the country.
London and the South East, including East Anglia, Essex and Kent, were currently coloured purple on Wednesday, indicating "very high" levels of air pollution.
By contrast, the North, Scotland and Northern Ireland appeared green, suggesting levels were much lower.
:: How does air pollution in London compare to other countries?
London appears way down a World Health Organisation (WHO) list of the most-polluted cities in the world, with similar pollution levels to other major cities such as Budapest and Dusseldorf.
The capital is around 13 times less polluted than the city with the worst air quality - Ahwan in southwestern Iran - and has air seven times cleaner than that of Delhi, although it lags behind places such as Munich, Tokyo and Canberra.
Pollution is measured by the average number of tiny particles in the air per cubic metre.
:: Has air quality not been improving in the UK?
There has been a marked improvement in urban air quality over the last two decades.
In 1993, air pollution was at moderate or higher levels for roughly one day in every six. Using the same methodology, that figure is now around one in every 36.
There has been a huge drop in the amount of carbon monoxide and sulphur dioxide in the air, but although pollution at the roadside has shown signs of long-term improvement, it has remained relatively stable since 1998.
In February, the European Commission launched legal proceedings against the UK over claims it is years late reaching agreed EU standards.
It says levels of toxic gas nitrogen dioxide remain "excessive" and are contributing to respiratory problems and premature deaths.
:: How long is this latest bout of air pollution likely to last?
Air quality is expected to return to higher levels by the weekend, helped by outbreaks of rain in the West that will effectively wash away dust and other contaminants from the atmosphere.
Ghaffar added: "Rain is only a brief relief from the dust in the atmosphere. A change in wind direction is needed to stop the flow of particles blowing through and from Thursday, that is what we can expect."