Mayor of London statement: 'High' air pollution alert triggered

25 July 2018

As London continues to experience very high temperatures, the Mayor of London has triggered a ‘high’ air pollution alert for tomorrow (Thursday). Sadiq Khan said:


"This week has seen London experience extremely high temperatures. The heat, combined with London’s toxic air, a lack of cloud cover and emissions travelling from the continent, means I am triggering a ‘high’ air pollution alert today, for tomorrow, under our comprehensive alert system. This is the second time in six months that we have had to use the ‘high’ alert system and shows just why air pollution is a public health crisis."

Notes to editors

  • Public Health England advice for coping during high temperatures can be read here:
  • This is the ninth time the air quality alert system has been used since Sadiq Khan became Mayor. Eight times for ‘Very High’ and once for ‘High’.
  • Before Sadiq Khan became Mayor, social media and text alerts have been used on a small number of occasions to make Londoners aware of air pollution episodes.  Sadiq Khan believes it is important that Londoners are fully informed about toxic air quality and since being elected has delivered a comprehensive alert system using bus countdown signs, roadside signs and electronic updates at underground stations, plus social media and text alerts.
  • Air-quality alerts will be displayed at:
  • 2,500 bus countdown signs and river pier signs across London.
  • 140 road-side dot matrix message signs on the busiest main roads into London, with instructions to switch engines off when stationary to reduce emissions.
  • Electronic update signs in the entrances of all 270 London Underground stations.

What is the air quality index?


The air quality index is how we communicate about air pollution levels in a simple way. It is numbered 1 to 10 and divided into four bands, low (1) to very high (10). This system was recommended by the government’s Committee on Medical Effects of Air Pollutants (COMEAP).


What is causing this episode?


This episode was caused by high temperatures, no forecast cloud cover, and a southerly air feed bringing emissions from the continent, coupled with London's filthy air. This will add to the preceding day’s pollution and combined with London's own emissions to likely produce 'High' levels of ozone.


Note: even though the borough average is moderate or low, conditions next to busy roads may still be high or very high. Most people are exposed to pollution next to busy roads.


What is the health advice for ‘high’ pollution?


The Government’s Committee on the Medical Effects of Air Pollutants (COMEAP) advises that adults and children with lung problems, and adults with heart problems, should reduce strenuous physical exertion, particularly outdoors, and particularly if they experience symptoms. People with asthma may find they need to use their reliever inhaler more often. Older people should also reduce physical exertion.  Anyone experiencing discomfort such as sore eyes, cough or sore throat should consider reducing activity, particularly outdoors.


When was the last episode like this?


Episodes of pollution with high concentrations occur a few times per year in London, but very high pollution episodes are rare.


The last recorded high pollution in London was on Monday 7th May.



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