The composition of the atmosphere has changed remarkably over the last few millenia. This paper will look at the history of air pollution and the social and economic pressures that force societies to adopt new fuels. It will then trace the history of the effects and of public reaction to the new and more polluting fuels. Modern chemistry and meteorology allow us to take these descriptive materials and the statistical data gathered by early administrators to make a more objective assessment of the levels of pollution in early cities. In particular it is possible to draw up fairly detailed pictures of the composition of the atmospheres of London and York from 600 AD on. Such pictures should enable us to understand the effect of a changing atmospheric environment on the urban fabric over the last 400 years. Furthermore it is necessary to consider the parallel changes in the composition that have so changed the chemical climate against which we examine the air in the cities.