Jill Meara is a public health spokeswoman:

　　"Today, there is forecast to be high or very high levels of air pollution across a lot of the south of England, so we want to warn people if they already have lung disease, asthma or heart disease, that they may need to be extra cared for and maybe stay inside and limit their exercise during this smoggy weather."

　　This is a rare occurrence in the UK.

　　Professor Martin Williams, from the Environmental Research Group at King's College London, explains why there are such high levels of air pollution this week:

　　"We've been analyzing the measurements and looking at the various components of the particle mix that we measure and by doing that we can have a very good idea of where do the sources come from and it looks as if the large part, maybe even 70% of particle pollution measured in the southeast UK yesterday afternoon and today has been imported from the rest of the Europe, as some of the rest are contributed by the UK sources. But there's also an unusual combination of Saharan dust, which means winds from the Sahara desert can lift dust from the desert and transport the dust across Europe. And it looks as if there's been a transport of Saharan dust all the way through Europe, through Spain and France and up to the UK, so there are three main components of the pollution, mostly from continental Europe, some from the UK itself and some from the Saharan dust. "

　　Professor Williams says today's pollution mix is different from that which plagued the UK in its industrial past, these days the number one pollutant threatening the country's air quality is produced by diesel engines.

　　Medical researcher Dave Newby says that invisible particles in the air, from these engines, run deeper into the human body and stay longer.

　　"The particles are released by diesel engines, they are so small they get very deep down, even into the blood stream. And we think that these can cause very bad effects on blood vessels, they can cause bad effect on the hardening of the arteries, it could even cause or precipitate heart attacks."

　　Last month, Paris was caught in a similar polluted air situation and imposed restrictions on the numbers of cars running on the road.

　　Public transport is also made free of charge to the public to relieve the pressure on traffic.

　　It is unsure of whether Britain will follow suit.

　　But Professor Williams points out that while the government is introducing tighter emission standards, it does not necessarily cut down the quantity of emission in real practice.

　　"Now that's seems a little bit odd, but it's a fact of life. We've shown that diesel emissions, for example emissions from diesel cars haven't really improved in the last 20 years even though the standards have reduced quite a lot. That's because the test the cars have to pass, the regulatory test, isn't appropriate to what happens in the real world. So there's always that little question mark about emission standard for vehicles."

　　For CRI, I'm Duan Xuelian reporting from London.