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City chiefs today issued an unprecedented warning to joggers to avoid pollution blackspots in the Square Mile.

They warned Londoners to avoid Upper Thames Street into Lower Thames Street, Bank Junction, Bishopsgate into Gracechurch Street and Farringdon Street into New Bridge Street on high pollution days.

Jon Averns, the City of London Corporation’s public protection director, told The Standard: “During very bad pollution episodes, even a normally fit and healthy person can experience issues.

“Strenuous exercise means deeper breathing and more particles inhaled into the lungs, so we are calling on Londoners to plan their travel routes and avoid jogging through pollution hotspots.”

The warning came as London was set to enjoy its warmest day this year, with temperatures set to soar to 26C — and a day after campaigners said that more than 1,300 Londoners have so far this year died prematurely due to toxic air in the capital.

The Corporation, the local authority for the Square Mile, said that the capital was “vulnerable to summer smog air pollution” which happens during hot weather in built-up areas.

The Department for Environment, Food and Rural Affairs publishes daily forecasts for pollution, rating it from a low of one to a “very high” 10 — at which point health chiefs advise that people should “reduce physical exertion, particularly outdoors, especially if you experience symptoms such as cough or sore throat”.

If the pollution prediction is high, between seven and nine, “anyone experiencing discomfort such as sore eyes, cough or sore throat should consider reducing activity, particularly outdoors”.

Separate advice, urging more caution, is issued for “at-risk individuals” such as adults and children with heart and lung problems.

The forecast today is “low”.

The Corporation has produced a free mobile phone app to help people find the least polluted way to work or for other journeys. Users can specify on the City Air App whether they are a pedestrian, cyclist, jogger, driver, business or an “at-risk individual”.

They can sign up for alerts of a high pollution episode to be sent by text message or email.

The app, designed by experts from Kings College London, offers a choice of three routes to avoid pollution.

Ruth Calderwood, the Corporation’s environmental policy officer, said: “Modern day pollution is invisible, but the ground-breaking augmented reality 3D view of air pollution across London really helps [the app] users see their city in a new way.

“We want to alert Londoners to the problem areas so they can take action and manage their own behaviour — even if it’s just to go jogging in the evening rather than the morning.”

Public Health England London said that the benefits of taking regular exercise, such as walking and cycling, were thought to be more important for health than the possible negative effects of air pollution. But a spokeswoman added: “Individuals can reduce their exposure to air pollution by choosing to travel on less-trafficked routes.”

MPs will debate London’s pollution next week amid calls for more drastic action to improve air quality.