Nitrogen-Dioxide Remains a Valid Air Quality Indicator

In epidemiological studies, both spatial and temporal variations in nitrogen dioxide (NO 2 ) are a robust predictor of health risks. Compared to particulate matter, the experimental evidence for harmful effects at typical ambient concentrations is less extensive and not as clear for NO 2 . In the wake of the “Diesel emission scandal—Dieselgate”, the scientific basis of current limit values for ambient NO 2 concentrations was attacked by industry lobbyists. It was argued that associations between NO 2 levels and medical endpoints were not causal, as NO 2 in older studies served as a proxy for aggressive particulate matter from incineration processes. With the introduction of particle filters in diesel cars, NO 2 would have lost its meaning as a health indicator. Austria has a high percentage of diesel-powered cars (56%). If, indeed, associations between NO 2 concentrations and health risks in previous studies were only due to older engines without a particle filter, we should expect a reduction in effect estimates over time as an increasing number of diesel cars on the roads were outfitted with particle filters. In previous time series studies from Vienna over shorter time intervals, we have demonstrated distributed lag effects over days up to two weeks and previous day effects of NO 2 on total mortality. In a simplified model, we now assess the effect estimates for moving 5-year periods from the beginning of NO 2 monitoring in Vienna (1987) until the year 2018 of same and previous day NO 2 on total daily mortality. Contrary to industry claims of a spurious, no longer valid indicator function of NO 2 , effect estimates remained fairly stable, indicating an increase in total mortality of previous day NO 2 by 0.52% (95% CI: 0.35–0.7%) per 10 µg/m 3 change in NO 2 concentration.