Parts of England should be braced for the highest possible levels of air pollution, environment officials have said. Here are some questions answered about the current pollution levels.

Why are the pollution levels so high?

The current high levels of air pollution have been caused by a combination of factors; the continental air flow, air pollutants from the UK, a south easterly breeze bringing in pollutants from the continent and dust that has travelled thousands of miles from the Sahara Desert.

Dr Helen Dacre, a meteorologist at the University of Reading, said the combination of factors have led to a 'perfect storm' for air pollution. 'British car drivers and heavy industry create bad enough smog on their own, but the weather is also importing pollution from the industrialised urban parts of Europe, which is blowing across Britain,' she said. 'Saharan dust gets blown over to Britain several times a year - the current episode has been whipped up by a large wind storm in North Africa. This has all combined to create high concentrations of pollutants in the air.'

How does dust from the Saharan Desert end up in Britain?

The Met Office said Saharan dust is lifted by strong winds and can reach very high altitudes. From there it can be taken to anywhere in the world by gusts of wind. The airborne particles are deposited during rain showers. Paul Hutcheon at the Met Office said: 'We usually see this happen several times a year when big dust storms in the Sahara coincide with southerly winds to bring that dust here. More dust rain is possible during showers expected later this week.'

Which areas are worst affected?

The Department for Environment, Food and Rural Affairs (Defra) ranks air pollution from one to 10, with one being the lowest and 10 the highest, at present most of East Anglia can expect the highest level of air pollution recorded. Meanwhile swathes of southern and central England should be braced for moderate to high levels of pollution, forecasters said.

What are the implications to people's health?

Around the globe seven million people died in 2012 as a result of air pollution, according to estimations from the World Health Organisation (WHO). The body said that air pollution is 'the world's largest single environmental health risk'.

WHO said there is a link between air pollution and heart disease, respiratory problems and cancer.

When pollution levels are deemed to be 'very high', people in Britain are encouraged to reduce physical activity, particularly outdoor activity. Older people and those with lung or heart problems are urged to avoid strenuous physical activity. Meanwhile people with asthma may need to use their inhalers more frequently. When pollution levels are classed as 'high' people who experience sore eyes, coughs or sore throats are urged to consider reducing activity, particularly outdoors. People with health problems are urged to reduce physical activity.

What can be done?

Green campaigners at Friends of the Earth said that while we can't do anything about the dust from the Sahara, officials should be doing 'far more' to deal with road traffic emissions. Campaigner Jenny Bates said: 'We need cleaner vehicles, a serious strategy for tackling traffic levels, including the provision of better public transport and cycling facilities, and an end to plans to build new roads.'

When will it improve?

The current situation is expected to improve by Friday when most of the UK can expect low levels of air pollution because of cleaner south-westerly winds, Defra said.