Whether results are assertions or evidence is unclear

EDITOR—Fielder et al reported increased rates of congenital malformations in the area surrounding the Nant-y-Gwyddon landfill site, having examined five “exposed” wards and 22 comparison wards.1 They imply that their findings are relevant to landfill sites receiving domestic, commercial, and industrial waste in general rather than being specific to the Nant-y-Gwyddon site.

Studies such as this are subject to several scientific constraints.2 Paramount among these are that comparison populations should not be exposed to potential risks from landfill sites and that exposed areas should be relatively unaffected by alternative potential pollutant sources.

Examination of site licence records suggests that there are up to seven other operating or closed landfill sites whose zones of potential influence include the comparison wards, including sites receiving similar wastes to the Nant-y-Gwyddon site. Given the absence of comment from the authors, we conclude that these sites are not having any measurable effects. Hence the study seems to show that one landfill site may have certain health effects while seven others do …