we used daily time - series analysis to evaluate associations between ambient carbon monoxide , nitrogen dioxide , particulate matter [ less than and equal to ] 10 microm in aerodynamic diameter ( pm(10 ) ) , or ozone concentrations , and hospital admissions for cardiopulmonary illnesses in metropolitan los angeles during 1992 - 1995 . 
 we performed poisson regressions for the entire patient population and for subgroups defined by season , region , or personal characteristics , allowing for effects of temporal variation , weather , and autocorrelation . 
 co showed the most consistently significant ( p<0.05 ) relationships to cardiovascular admissions . a wintertime 25th-75th percentile increase in co ( 1.1 - 2.2 ppm ) predicted an increase of 4% in cardiovascular admissions . 
 no(2 ) , and , to a lesser extent , pm(10 ) tracked co and showed similar associations with cardiovascular disease , but o(3 ) was negatively or nonsignificantly associated . 
 no significant demographic differences were found , although increased cardiovascular effects were suggested in diabetics , in whites and blacks ( relative to hispanics and asians ) , and in persons older than 65 years of age . 
 pulmonary disease admissions associated more with no(2 ) and pm(10 ) than with co. pulmonary effects were generally smaller than cardiovascular effects and were more sensitive to the choice of model . 
 we conclude that in los angeles , atmospheric stagnation with high primary ( co / no(2)/pm(10 ) ) pollution , most common in autumn / winter , increases the risk of hospitalization for cardiopulmonary illness . 
 summer photochemical pollution ( high o(3 ) ) apparently presents less risk.imagesfigure 1figure 2