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A toxic air “red alert” was issued across London today, with millions of people urged to consider taking it easy to protect their health.

Mayor Sadiq Khan raised the alarm about a filthy cloud of pollution for the first time this year at thousands of bus stops and all Tube stations.

City chiefs advised commuters, cyclists, joggers, drivers and pedestrians to avoid busy, pollution-clogged roads.

Mr Khan said: “The extent of our air quality crisis means that I have triggered a city-wide alert, ensuring Londoners have all the information possible.

"It’s crucial that London- ers who are vulnerable, such as asthma sufferers, are able to take appropriate measures to protect themselves.”

Pollution was predicted to reach “high” — the same as red — in at least seven boroughs and the City as a cloud of toxic air from Germany’s industrial heartlands was set to sweep in, smothering a swathe of central and west London.

Scientists at King’s College London said: “There is a good chance that particulate pollution will continue to rise into the ‘high’ threshold across London.”

The particulate pollution was expected to mix with locally produced nitrogen dioxide, much of which is spewed out by diesel vehicles.

The “high” alerts were issued early this morning for Westminster, Kensington and Chelsea, Hammersmith and Fulham, Tower Hamlets, Wandsworth, the City, Kingston and Hounslow — with the rest of the capital rated “moderate”.

AirText messages from City Hall before 8am to the worst-affected boroughs stated: “High air pollution forecast for Thursday. Consider spending less time outdoors, take medication. If unwell contact GP.”

By mid-morning, air quality warnings were being sent out to 2,500 bus countdown signs and river pier signs, electronic update signs in the entrances of all 270 Underground stations and 140 road-side message signs on the busiest main roads into London, with instructions to switch engines off when stationary to reduce emissions.

A red alert is issued when the scale indicating air toxicity stands between seven and nine out of 10. It hit nine yesterday near MI6’s Vauxhall HQ.

City Hall issues a London-wide alert if more than four boroughs, exclud- ing the City, are forecast to hit “high” air pollution.

When filthy air rises to that level the official health advice is: “Anyone experiencing discomfort such as sore eyes, cough or sore throat should consider reducing activity, particularly outdoors.”

Adults and children with lung problems and adults with heart problems should reduce strenuous physical exertion, especially outside. People with asthma may find they need to use their inhaler more often. Older people should also reduce physical exertion.

A City of London Corporation spokesman said: “This spike in air pollution is a huge health problem. We encourage pedestrians, cyclists, joggers and drivers to use our free City Air app for lower pollution travel routes.”

Research has shown that air pollution inside cars can be particularly bad. A “black” warning — 10 on the toxicity scale — would trigger an alert urging all of the public to “reduce physical exertion, particularly outdoors, especially if you experience symptoms such as cough or sore throat.”

The King’s College London experts were forecasting that an area of high pressure over the continent would continue to bring cold and settled weather, with “near calm conditions” at the start of the day.

“Air mass back trajectories indicate that air arriving in Greater London will have passed over industrialised and urban areas of Germany and the near continent,” they added.

“The imported particulates will combine with local emissions which have been poorly dispersed over the previous 24 hours.”