LONDON — London’s mayor has issued air quality alerts across the capital for the first time because of high pollution levels.

The alerts are being displayed Thursday at 2,500 bus stop and river pier information signs and at the entrances of all 270 subway stations. Dozens of signs next to the busiest main roads into London instruct drivers to switch engines off when stationary to reduce emissions.

The air quality forecast was provided by King’s College London , which said an area of high pressure over Britain has led to poor pollutant dispersal. It said a “high” pollution level meant anyone experiencing sore eyes, cough or sore throat should reduce their outdoor activities.

It was the first time the alerts were issued since Mayor Sadiq Khan announced the initiative in August.