Boris Johnson was accused of "dangerous complacency" after urging Londoners to "keep a sense of proportion" over the latest pollution scare.
Official figures showed the toxic content of London's air peaking at the maximum Level 10.
The London Ambulance Service reported a 14 per cent spike in calls from patients with breathing problems.
Prime Minister David Cameron cancelled his early-morning run, but the mayor again rejected calls for a Paris-style ban on cars.
"I understand that everybody is excorcised about it today but it is my view and the view of our experts that the situation is not palpably worse than it has been before."
"I think it would be draconian and unnecessary to hit a lot of people in the pocket, paralyse a lot of businesses by banning cars when the situation is not actually worse today."
The mayor told ITV News he had not noticed the pollution as he cycled to work.
"I think we need to keep a little bit of a sense of proportion.
"I cycled this morning and it seemed perfectly fine to me."
Dr Onkar Sahota, a west London GP and Labour spokesman on the London Assembly called on the Mayor to bring forward his Ultra Low Emission Zone planned for 2020.
"The Mayor's comments are dangerously complacent.
"This latest smog episode shoyuld act as a wake up call to our our Mayor.".