Nitrogen dioxide and mortality: review and meta-analysis of long-term studies

Abstract

Exposure to ambient nitrogen dioxide (NO₂) has been linked to increased mortality in several epidemiological studies but the question remains of whether NO₂ is directly responsible for the health effects or is only an indicator of other pollutants, including particulate matter. The aim of the present review was to provide pooled estimates of the long-term effects of NO₂ on mortality, which are potentially useful for health impact assessment.

We selected 23 papers, published from 2004 to 2013, evaluating the relationship between NO₂ and mortality, also including an assessment of the effect of particulate matter exposure. A random-effects meta-analysis was carried out on 19 studies.

The pooled effect on mortality was 1.04 (95% CI 1.02–1.06) with an increase of 10 μg·m⁻³ in the annual NO₂ concentration and 1.05 (95% CI 1.01–1.09) for particulate matter <2.5 μm in diameter (PM_2.5) (10 μg·m⁻³). The effect on cardiovascular mortality was 1.13 (95% CI 1.09–1.18) for NO₂ and 1.20 (95% CI 1.09–1.31) for PM_2.5. The NO₂ effect on respiratory mortality was 1.03 (95% CI 1.02–1.03) and 1.05 (95% CI 1.01–1.09) for PM_2.5. Four bipollutant analyses with particulate matter and NO₂ in the same models showed minimal changes in the effect estimates of NO₂.

There is evidence of a long-term effect of NO₂ on mortality as great as that of PM_2.5. An independent effect of NO₂ emerged from multipollutant models.