In this issue of Journal of Epidemiology & Community Health, Lee et al¹ (see page 591) demonstrate an association of both learning disabilities (LD) and attention-deficit disorder (ADD) with concurrent serum levels of persistent organic pollutants (POPs) among 12–15-year-old participants in the National Health and Nutrition Examination Survey (NHANES 1999–2000). For example, among children with detectable versus non-detectable levels of heptachlorodibenzo-p-dioxin (HPCDD), the adjusted prevalence odds ratios (95% CI) for LD and ADD were 2.08 (1.17 to 3.68) and 3.41 (1.08 to 10.8), respectively. This is perhaps the first published study to describe a potential POP-associated increased risk of LD or ADD. Previous studies have primarily assessed continuous cognitive or behavioural measures among generally healthy populations. LD and ADD are clinical end points that have obvious public health impacts in terms of childhood (and adult) morbidity as well as educational, medical and social costs of supporting individuals with these diagnoses.² If confirmed, Lee et al’s findings could make a substantial contribution to the identification of potentially remediable risk factors for these increasingly common disorders.^3,4

However, the strength of the associations observed in Lee et al’s study are surprising. Previous epidemiological findings generally support a modest association of early-life exposure to some POPs (eg, polychlorinated biphenyls (PCBs)) with poorer attention,^5,6 impulse control,^7,8 memory and learning skills,^6,9,10 and school achievement.¹¹ Furthermore, the Lee et al analysis had a small sample size (278 NHANES children had available exposure and outcome data) including only 44 cases of LD and 26 cases of ADD; used dichotomised exposure measures (levels were categorised as detectable vs non-detectable); measured exposure concurrently (whereas …