In The Lancet, Hong Chen and colleagues1 reported that higher exposures to nitrogen dioxide (NO2) and airborne particulate matter were greatly associated with dementia. Unfortunately, Chen and colleagues1 were unable to directly measure these airborne pollutants, relying instead on the proxy measure of proximity to pollution sources, namely major roadways. The use of proximity to pollution sources as a proxy measure for airborne pollution was supported by a range of alternative measures, including satellite and ground-based monitoring. However, nearness to major roadways is associated with other potential biological risks; one of the more potent risks being previous exposure to lead.