Naturally occurring levels of nitrogen oxides and nitric acid in the atmosphere may not be as high as was previously thought. John Ritter, Donald Stedman, and Thomas Kelly of the University of Michigan reported at a recent American Chemical Society meeting that they had measured the levels of NOx in rural Michigan and on a relatively isolated mountaintop in Colorado using a detector which measures fractions of a part per billion. They found very low levels of NOx, punctuated by high surges from man-made sources. All of the high concentrations were confined to the layer of air close to the ground and were directly traceable to plumes of man-made pollution. They concluded that a ‘background level’ of nitrogen oxides (NOx) was virtually meaningless.