With the rapid increase of motor vehicles in the recent years, chronic effects of low level NO₂ and CO on health has attracted much interest. In the present study the morphological changes in the lungs of mice continuously exposed to low concentrations of NO₂ and CO were examined.
Electron microscopically, inhalation of NO₂ gas for 30-45 days was found to induce shortening and reduction of cilia of the ciliated epithelial cells. Edematous changes of the alveolar epithelial cells and edema of the alveolar interstitium were observed.
Light microscopically, the mice exposed to 0.5-0.8 ppm of NO₂ for one month revealed so-called hyperplastic foci due to proliferation of epithelial cells of the peripheral bronchus. It is well known that this kind of epithelial proliferation is also observed as an early change of pulmonary adenoma in mice. Therefore, it is very noteworthy that a finding completely identical to the hyperplastic foci in pulmonary adenoma was also observed in the lung of mice exposed to NO₂ in the present study.

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From 1950 to 1970 we observed 33 bronchiectasis cases in children up to 15 years of age in Gunma University and 18 in Tomo Hospital. Recently the incidence of this disease, particulary that of severe cases has decreased (fig. 1). Its decrease may be attributed to the introduction of antimicrobial therapy of respiratory infections in early childhood, because bronchiectatic patients have decreased in number in order of the year of their birth and the increasing prevalence of antimicrobial drugs (fig. 2 and 3).
In Japan penicillin has been in widespread use since 1948, streptomycin since 1951, tetracycline and chloramphenicol since 1953, and erythromycin since 1957 (R. Fujii 1962).
The incidence of bronchiectasis, in order of the year of their birth, markedly decreased a few years after 1953, when broad spectrum antibiotics attained widespread usage in Japan.
Patients were devided into two groups according to the extent of their lesions; those with bronchiectasis involving over 8 lung segments (severe cases), and those less than 8 segments (moderate and mild cases). Seventeen out of 18 severe cases were born before 1953, the year of widespread of broad spectrum antibiotics.There were 5 patients with the so-called “honeycomb lung”, namely, saccular ectasis of all the bronchi of one lung. All of them were born before 1947, the preceeding year when penicillin attained broad usage in Japan.
Our observation, that the incidence of this disease markedly decreased among children born after the widespread use of antimicrobial drugs, readily agrees with the assumption that the majority of bronchiectasis patients might have aquired this desease in their early childhood due to severe respiratory infection and demonstrates the preventability of this disease. The so-called “honeycomb lungs” have been considered congenital by many authors, except Malory and Rappaport who considered this as the result of extensive damage of the developing lung in early infancy. We are of opinion that this type of bronchiectasis may be also attributed to severe bacterial infection in early infancy as this “honeycomb lung” was only observed among children born before the prevalence of penicillin, and none was observed thereafter.

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