Annual legal air pollution limits have been reached in London within one month, figures have revealed.
Hourly limits for nitrogen dioxide (NO2) have been exceeded 18 times so far this year - the maximum allowed under European Union rules.
The Mayor of London, Sadiq Khan, said the air quality in the capital in January was still the best in 18 years.
Campaigners said the windy start to the year was likely to have reduced pollution spikes.
A recent NO2 study found Marylebone Road and Hyde Park Corner were the most polluted postcodes in Britain.
Under EU law the average hourly level of NO2, mostly caused by diesel vehicles, must not exceed 200 micrograms per cubic metre more than 18 times in a year.
The capital breached those limits by 6 January every year for a decade up to 2017.
Mr Khan has appointed King's College London to continuously monitor air pollution in London.
Scientists will use new technology to deliver air quality alerts to those most vulnerable to the impacts of poor air, such as schoolchildren.
The mayor hopes the monitoring will help people "learn more about the air they breathe and what they can do to improve it".
Campaigners welcomed action by Mr Khan but said central government must do more to improve air quality.
Oliver Hayes from Friends of the Earth said: "A decent scrappage scheme to compensate diesel drivers must go hand-in-hand with a network of genuinely effective clean air zones across the country."
He said in mid-January that the windy start to the year is "likely to have had as much to do with the reduced number of pollution spikes as anything else".
Speaking about the government's plan to tackle poor air quality, Environment Minister Thérèse Coffey said on Tuesday: "Poor air quality affects public health, the economy, and the environment, which is why we are determined to do more."