Boris accused of using dust suppressants to hide pollution levels before Olympics Mr Birkett accused the Mayor of London's office of using the suppressants to clear the air before readings are taken. Mr Birkett said pollution levels are already above European legal limits but it is being hidden by the use of dust suppressants, meaning the authorities are not being forced to act. Pollution levels are measured in tiny particles of soot and dirt blown in from as far away as forest fires or the Sahara or from traffic emissions. The particulate matter, known as PM10s, can cause lung and heart problems when inhaled by humans. London was supposed to meet the target by 01 January 2005 but has had to ask for a series of extensions. Clean Air London claim levels are regularly breached in the heart of the capital. The latest deadline is this September and if London fails again then the UK faces a fine of £300 million. The Mayor's clean air strategy reads: "There are other days where a location only just exceeds the EU limit value. Mr Johnson has insisted the suppressants are part of a short term strategy to reduce pollution alongside long term measures including bringing in more low emission zones, retrofitting buses and planting trees. His office explained that it makes sense to have monitors at the most busy traffic spots so that it forces the UK to clean up its dirtiest air, in the same way this is the most important area to use dust suppressants. There is also growing concern about nitrogen dioxide fumes in London, which are twice legal health limits by London's busiest streets, according to Clean Air London. The Department for the Environment recorded the first "summer smog" over the weekend of 22nd March after high ozone levels were recorded. This occurs when sunlight reacts with pollutants and is the most dangerous kind of pollution for athletes. Beijing managed to bring down nitrogen dioxide PM10s and ozones during the 2008 Olympics, but it was through closing down factories and restricting traffic rather than the use of any short cuts like dust suppressants, and it will seriously embarrassing if London fails to clean up its air.