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Parents in London were today advised to “take care” when taking their baby outside because of toxic air pollution levels.

The warning came from the pioneering Plume Labs company whose aim is “making the air more transparent”.

The firm put the level of air pollution in London as “high” this morning based on World Health Organisation limits, which are stricter for some pollutants than EU rules used by the Government and other experts.

Following hour-by-hour “high” readings, Plume Labs’ advice was for parents to “take care” when they “bring baby out” and when doing outdoor sports, and to “take it easy” when cycling or eating outside.

The firm’s chief executive Romain Lacombe said: “We are not saying that you should not go out with your baby. We are advising to avoid major roads and high traffic areas.”

Experts at King’s College London, using the EU limits, put air pollution at “high” in Brent mid-morning and “moderate” in Sutton, Westminster, the City, Lambeth, Croydon and Ealing.

Advice from Public Health England for moderate air pollution is: “Adults and children with lung problems, and adults with heart problems who experience symptoms, should consider reducing strenuous physical activity, particularly outdoors.”

Other people should “enjoy your usual outdoor activities”.

Only when dirty air levels become “high” should the general population consider reducing activity if experiencing discomfort.

Simon Birkett, of Clean Air in London, said: “It’s fair to say that any vulnerable people, young or old, should consider reducing activity today if they feel symptoms such as difficulty breathing or tightness in the chest. But please check the official Government advice.”

Professor Frank Kelly, air quality expert at King’s College, added: “If we really want to do more to improve public health we would follow the WHO guidelines.”

Shirley Rodrigues, deputy mayor for environment and energy, said children’s lungs were particularly vulnerable to “London’s illegal and filthy polluted air” and urged the Government to do more to tackle “this diabolical situation”.

The Environment Department is forecasting moderate to high pollution in urban areas in southern England, after a very high peak in London yesterday. Winds from the Atlantic were expected to ease pollution later today.

Meanwhile, Transport Secretary Chris Grayling signalled that ministers will not “water down” air quality rules after Brexit to allow a third runway to be built at Heathrow.

The Cabinet minister insisted the west London airport can be allowed to expand within limits on nitrogen dioxide which are currently being widely breached in the capital.

Environmentalists and campaigners dispute this and there are concerns that when the UK quits the EU, the Government could abandon the EU pollution limits. But Mr Grayling told the Commons environmental audit committee: “I cannot conceive of this Government or a successor government wanting to water down air quality standards.

“We all now recognise that there is a very real issue.”