The aim is to study the level of deaths and hospital admissions to find if it was raised by a very heavy dose of particulates - tiny particles which penetrate deep into the lungs.
In Leeds, a government monitor detected unprecedented levels peaking at 800 micrograms per cubic metre around midnight on Saturday. Normally 200 micrograms is regarded as very severe pollution.
But in Birmingham, Bristol, Cardiff, Newcastle, Southampton and Bexley levels also reached 200 at the weekend. In some cities 300 was exceeded.
``Everything conspired to produce unusually high pollution levels - very still, stagnant cold air, no atmospheric mixing and all those fires,'' said Geoff Dollard of AEA Technology, which runs the Government's network of pollution monitors. ( Graphic omitted )