Nitrogen dioxide is an ubiquitous air pollutant resulting from the combustion of fossil fuels. When inhaled at high levels, it reacts with and damages lung cells, including those cells that fight infection. This damage can affect breathing and may increase the risk of respiratory infections. Dr. J.K. Davis and his colleagues at the University of Alabama, Birmingham examined whether exposure to lower levels of nitrogen dioxide (less than 5 ppm) compromises mouse lung defense. The investigators sought to identify the mechanism by which mycoplasma are killed in the lung, and to determine whether exposure to nitrogen dioxide decreases the effectiveness of this mechanism. First, they determined the lowest level of nitrogen dioxide that decreased the effectiveness of the lung\'s defenses against infection by mycoplasma. The investigators exposed mice to a range of nitrogen dioxide concentrations or to clean air for four hours, and then to the mycoplasma. 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.