Research by Martin Luther University Halle-Wittenberg in Germany has found that 78% of deaths attributed to COVID-19 in Italy, Spain, France and Germany have occurred areas that typically have the highest levels of NO₂. In only a correlation at this stage, the finding may spur further research to determine a possible causal link, with NO₂ already linked to lung damage in previous research related to the public health effects of air pollution. Other studies of COVID-19 have suggested that air pollution in general (in work on northern Italy) and previous exposure to small particulates (in work on the USA) also affect mortality from the virus. The Martin Luther University Halle-Wittenberg has not taken into account factors such as age distributions (or, it appears, even the distribution of known cases of the virus), and so should be treated with caution, but it would not be surprising if causation was proven, and such evidence would give further weight to calls to reduce air pollution as a matter of public health priority.