(The Journal of Clinical Investigation) The study put 20 asthmatics (ranging from 15 to 40 years old) in an exposure chamber, increasing nitrogen dioxide levels to test if they showed adverse effects. More than half of the subjects exhibited noticeable effects, and the study concluded that “very low levels of NO2 can adversely affect some asthmatics."
Orehek, J., Massari, J., Gayrard, P., Grimaud, C., Charpin, J. (1976) Effect of Short-Term, Low-Level Nitrogen Dioxide Exposure on Bronchial Sensitivity of Asthmatic Patients. The Journal of Clinical Investigation Vol 57. Pgs 301-307 https://www.ncbi.nlm.nih.gov/pmc/articles/PMC436654/pdf/jcinvest00145-0063.pdf