To the Editor: The article on pathologic changes in young cigarette smokers by Niewoehner et al. (N Engl J Med 291: 755–758, 1974) makes a very important analogy. It compares the changes seen in the respiratory bronchioles of cigarette smokers with the pathologic alterations in hamsters exposed to nitrogen oxides. Few doctors realize that mainstream cigarette smoke contains 250 ppm of nitrogen dioxide or dinitrogen tetroxide, as reported in the Surgeon General's “Smoking and Health” in 1964. Depending upon the depth and timing of an inhalation, the safe level for this gas, 5 ppm, is quite likely to be exceeded. No extract is available for articles shorter than 400 words.