Ambient air pollution and depressive symptoms in older adults.

Wang et al. (2014) investigated the relationship between ambient air pollution and depressive symptoms in a prospective study of elderly people over age 65 years. We are very surprised that the authors found no association between ambient air pollution and depressive symptoms, which is inconsistent with most previous studies (e.g., Banerjee et al. 2012; Calderón-Garcidueñas et al. 2014). A study of a population of elderly men indicated one potential biological mechanism may be methylation, which was decreased after acute exposure to fine particulate matter (Madrigano et al. 2012).

We identified three issues with the study by Wang et al. (2014). First, these authors used outdoor air pollution as the exposure of interest. Long-term exposure was estimated based on residential distance from the nearest major road. Considering only residential distance to the nearest major roadway, without taking into account other sources of exposure, may be insufficient to accurately estimate long-term exposure to traffic pollution. Short-term exposure to ambient air pollution was estimated based on pollutant levels measured at only one monitoring site, the Harvard–U.S. Environmental Protection Agency Supersite stationary ambient monitoring site. Although the monitoring site was located < 20 km from the home of any study participant, the number of monitoring sites was insufficient. Thus, participants’ exposures to ambient pollutant might be misclassified. Moreover, considering that the participants were elderly, they were less likely than other groups to spend time outdoors (Kerr et al. 2012). Hence, outdoor air pollution might not reflect their true exposures. Lacking validation of these estimated exposures, the negative conclusion is unconvincing.

Second, the Revised Center for Epidemiologic Studies Depression Scale (Eaton et al. 2004) is not the first choice for measuring depressive symptoms in the elderly; the Geriatric Depression Scale (Yesavage et al. 1982) is more suitable. The evidence from clinical diagnosis of depression shows a positive association between air pollution and depression. For example, Cho et al. (2014) found that ambient air pollution was positively associated with emergency department visits for depressive episodes among 4,985 Koreans using clinical diagnosis of depression episode as outcome. Of note, clinical diagnosis of depression is a more objective measure than depressive symptoms.

Third, Wang et al. did not address health status in their reported association. There is convincing evidence supporting the association between cardiovascular disease (CVD) and both ambient air pollution (e.g., Grahame and Schlesinger 2010) and depression (e.g., Sun et al. 2013). Hence, previous history of CVD could confound or modify the association between ambient air pollution and depressive symptoms. Stratified analysis according to health or CVD status is warranted. Without such analysis, the negative conclusion is not well supported.