In homes equipped with gas-burning stoves or unvented space heaters, levels approaching 0.05 ppm have been measured, and peaks of 0.2 to 0.5 ppm occur. Studies of human populations suggests that young children exposed to nitrogen dioxide at home may have an increased incidence or severity of respiratory infections. These findings are supported by laboratory investigations, showing that mice are more susceptible to bacterial respiratory infection after exposure to nitrogen dioxide. Susceptibility to respiratory tract infection is influenced by the effectiveness of the lung's defense mechanisms. Aleveolar macrophages, cells that form the first line of defense against inhaled bacteria, are one component of a multifaceted system of lung defenses.
Macrophages protect against infection by several mechanisms, one of which is killing bacteria.
The researchers divided their study into two parts. First, they determined the lowest level of nitrogen dioxide that decreased the effectiveness of the lung's defenses against infection by mycoplasma. They counted the number of mycoplasma in the lungs 24 hours after infection to determine how effectively they had been killed. Second, the researchers determined which lung cells were involved in killing the mycoplasma, and whether mycoplasmal killing facilitated by these cells was less effective in mice exposed to nitrogen dioxide. A single four-hour exposure of mice to 5 ppm nitrogen dioxide, a concentration much higher than humans normally encounter, decreased the effectiveness of the lung's defenses against mycoplasma infection.