article: So what exactly is the problem, and how serious is it? By UK standards, air pollution levels are high. The Department of the Environment measures outdoor air pollution on a one-to-10 scale. Parts of north-west Norfolk hit level 10 on Tuesday. On Wednesday, levels were recorded at seven - meaning high - in towns and cities in East Anglia and rural parts of south-east England. Pollution levels of five were reached in London, the Midlands and Yorkshire and Humberside. The tipping point seems to have been dust from storms in the Sahara. Normally it settles in the countries of southern Europe - Spain, Malta and Greece, for example. However, winds from the south and east have brought the dust to the UK, along with industrial pollution from Europe. And because those weather conditions are stable and not changing, those particles are not being dispersed. But that is only part of the problem. Most air pollution in the UK comes from road transport and residential emissions. Leaving aside the Sahara dust, the pollution is made up of various substances, including nitrogen dioxide, sulphur dioxide and ammonia. These form particles in the atmosphere. The two most common measurements are particulate matter measuring 2.5 micrometres or less, known as PM2.5; and larger particles measuring 10 micrometres, known as PM10. The World Health Organization (WHO) uses PM10 levels to measure pollution, expressed in micrograms per cubic metre. It recommends mean exposure over 24 hours to PM2.5 of no more than 10 micrograms/m3, and to PM10 of no more than 25. PM2.5 particles are thought to be particularly damaging because they are so small, they can penetrate into the deepest parts of the lungs. WHO air quality guidelines Why is the UK doing so badly? Last month, the EU announced it was taking legal action against the UK because it was persistently over the safe limit for air pollution - in particular, levels of nitrogen dioxide. But the UK is not alone - most EU countries struggle to meet the targets, says Martin Adams of the European Environment Agency. In fact, the EU is currently taking action against 17 out of 28 member states with serious air quality problems. EU standards make allowances for natural sources of air pollution, including Saharan dust for Spain and ozone for coastal countries such as the UK. Frank Kelly, professor of Environmental Health at King's College London, puts the blame for the UK's pollution woes squarely on the shift to diesel vehicles over the past 10 years. Six out of 10 cars sold today in the UK are diesels, and these produce a lot of nitrogen dioxide, he says. King's College London air quality monitoring project (London only) In all, the WHO says air pollution is responsible for seven million premature deaths a year around the world. That includes indoor as well as outdoor pollution - typically, smoke from cooking stoves in developing countries. About 80% of deaths related to outdoor pollution are linked to heart disease and strokes, while 14% are due to lung or respiratory diseases, and 6% to
summary: Health warnings have been issued for parts of England, as air pollution reaches high levels.