Anthony Wexler, an atmospheric scientist at the University of California, Davis, studies how very small particles -- measured in nanometers, or billionths of a meter -- contribute to air pollution and affect human health and climate. Wexler believes the role of these atmospheric nanoparticles is one of the largest unknowns in understanding global climate change. Clouds form when water droplets form around nanoparticles, and the thickness and whiteness of clouds affects how much heat from the sun is reflected back into space. Wexler has developed new equipment for analyzing single nanoparticles in polluted air. He is participating in an Environmental Protection Agency project to monitor air quality in selected cities including Pittsburgh, Pa., Houston, and Fresno, Calif. He also studies how particles of different sizes are carried through the airways into the lungs, and how they can affect human health.