At cruising altitude, airplanes emit a steady stream of nitrogen oxides into the atmosphere, where the chemicals can linger to produce ozone and fine particulates. Nitrogen oxides, or NOx, are a major source of air pollution and have been associated with asthma, respiratory disease, and cardiovascular disorders. Previous research has shown that the generation of these chemicals due to global aviation results in 16,000 premature deaths each year.
source https://techxplore.com/news/2021-01-concept-hybrid-electric-plane-aviation-air.html