Air quality in major U.S. cities has improved considerably over the  past 40 years.  New studies show that photochemical ozone and aerosol  pollution has significant health effects at ambient concentrations  currently found in most major urban areas.  Recent air quality studies  in Houston, Los Angeles and Mexico City have strategically combined  the collection of meteorological and atmospheric chemistry data to  better understand the processes resulting in pollution exceedance  events.  Findings from these studies have emphasized the critical  relationship between meteorology and air quality.  The new more  stringent EPA air pollution standards that have been enacted in the  past two years in particular highlight the significant role that long  range transport of pollution plays on local air quality.  As cities  continue to grow, air quality challenges may also become more  difficult to address.  Consequently, new technologies and energy  sources may be needed to address this considerable public health issue.