Toxic air warnings will be displayed at hundreds of Tube stations and bus stops during pollution peaks in London.
Mayor Sadiq Khan announced that filthy air levels, and advice on how to respond, will appear on a huge network of public signs across the capital.
The alerts will be issued when air pollution reaches “high” or “very high”, which has happened four or five times a year recently. They will be shown on 2,500 bus countdown signs, electronic update signs in the entrances of all 270 Underground stations, 140 roadside message signs on the busiest main roads, and river pier signs.
The system will go live from Monday August 15. Mr Khan said: “Londoners have a right to know about the quality of the air that they breathe.
“These new alerts will allow them to take precautions and help them plan ahead to avoid the worst instances of air pollution.” Information on the signs will depend on the level of pollution and may include telling motorists to switch their engine off when stationary to cut emissions and advising people to walk, cycle or use public transport.
It could also include warnings to reduce strenuous activity if someone experiences symptoms or advising asthma sufferers and other vulnerable groups that they may need to use their reliever inhaler more often or take other precautionary measures.
Four out of five of nearly 15,000 people who responded to a public consultation on Mr Khan’s air quality master plan said they wanted to be told when pollution is high or very high so they can take action to protect their health and reduce its impact.
The warnings will be displayed on the day before forecast high and very high air pollution, as well as the day itself.
Leon Daniels, Transport for London’s managing director of surface transport, said: “We are working with the Mayor to deliver an ambitious and wide-ranging programme to improve air quality across the capital.
“An important part of this work is to raise awareness and provide advice to people on how they can personally contribute to this work.”
Londoners with particular health needs will continue to have access to the airTEXT service for regular air quality forecasts via text, voicemail, email, mobile app and online.
Mr Khan has also consulted on a new “T-charge” of £10 a day for the worst-polluting vehicles —broadly those registered before 2005 — to drive into central London from next year.
In addition, he has proposed introducing the central London Ultra Low Emission Zone one year earlier in 2019. Cars, vans and motorbikes not meeting set emission standards would have to pay £12.50 a day, and lorries, coaches and buses £100.
The ULEZ may also be extended London-wide for lorries, buses and coaches from 2020.