Story highlights Activist says "incredible" level of pollution detected on south London's Brixton Road

Environmentalists say health is being affected and called for urgent action

(CNN) London breached its annual air pollution limits five days into the new year, Mayor Sadiq Khan said Friday.

An air monitoring site at Brixton Road in south London began recording levels of nitrogen dioxide above the threshold early Thursday and continued to do so beyond the 18-hour legal limit, according to data from King's College London.

Nitrogen dioxide is a gas emitted by diesel engines that causes lung disease and respiratory problems.

Pedestrians should have been warned, activist says

An environmental activist said an "incredible" level of pollution was detected on Brixton Road -- a busy shopping and transport hub -- and that bus services should have been suspended and pedestrians warned to leave the area.

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