Burns et al 1 report a significant excess of deaths due to amyotrophic lateral sclerosis (ALS) in a cohort of Dow employees potentially exposed to the herbicide 2,4-dichlorophenoxyacetic (2,4-D), but then argue against the plausibility of a causal association, concluding that the association “is not consistent with previous human or animal studies”.

This conclusion and the authors' characterisation of the relevant epidemiological studies seem to rely entirely upon the significance of the statistics, which downplays the importance of their finding. Firstly, the authors state that “cohort studies of people with exposure to 2,4-D (have not) reported increased rates of ALS,” citing two studies,2 3 both of which have limited power to detect the risk of ALS. One …

Dr C J Burns cburns{at}dow.com