The serious harm caused by microscopic dust really made me wonder about diesel. You noted that the submicron-scale spheres in the lungs of a nonsmoking resident of Mexico City appeared to be diesel-exhaust particles. While Europeans worry about what will happen in 100 years from global warming, could they be killing themselves now with their huge jump in diesel use in recent years?

Ernest Lieberman
New York, N.Y.

Your article discusses the hazards of particles under 2.5 microns in size. For people living in highly polluted areas, are there any filters for windows, air conditioners, or air purifiers that will trap particles of that size?

Meredith Warshaw
Newton, Mass.

Probably not, since these particles are so tiny .—J. Raloff