Because of the existence of overlapping buffer zones, around different types of site, a rule based procedure was used.
Thus, ‘unexposed’ postcodes were those which lay outside the 2 km buffers around all known landfill sites, whenever they had been operating (these thus remain fixed in all years).
Operational sites are those which were classified as open in that study year.
Closed sites were those which had been closed at some earlier date during the study period. Postcodes were classified as ‘before opening’ if, in that year, they lay within the buffer zone of a site which opened at some time later in the study period. Postcodes around all other sites, including those for which there were missing opening or closing dates, were masked out (excluded) from the analysis.