The image below is a magnified cross-section of exterior wall paint from a London house, built in 1705. This particular house was painted rather often, on average every 4.2 years, providing a 300 year long regular record of paint types, weathering, etc. A lovely find for a material culture specialist. It also provides a rather graphic visual of the impact of environmental policy. As nicely noted for us by Patrick Baty, it shows how the British Clean Air Act of 1956 ended a 250-year pattern of soot and industrial pollutant accumulation, visible as dark lines between the layers (marked by the 4th text-box down on the right-hand side):
All that filth, gone. Filth that stuck to everything it touched. Filth that used to get in people's lungs and on their skin. Filth that covered crops and feed and livestock in the fields and was consumed in every meal. Today's air-borne pollutants are less visible, but they just as surely accumulate on surfaces, whether of buildings or human bodies or food sources.