exposure to ozone ( o3 ) , a toxic component of photochemical smog , results in significant airway inflammation , respiratory discomfort , and pulmonary function impairment . 
 these effects can be reduced via pretreatment with anti - inflammatory agents . 
 progesterone , a gonadal steroid , is known to reduce general inflammation in the uterine endometrium . 
 however , it is not known whether fluctuations in blood levels of progesterone , which are experienced during the normal female menstrual cycle , could alter o3 inflammatory - induced pulmonary responses . in this study , we tested the hypothesis that young , adult females are more responsive to o3 inhalation with respect to pulmonary function impairment during their follicular ( f ) menstrual phase when progesterone levels are lowest than during their mid - luteal ( ml ) phase when progesterone levels are highest . 
 nine subjects with normal ovarian function were exposed in random order for 1 hr each to filtered air and to 0.30 ppm o3 in their f and ml menstrual phases . 
 ozone responsiveness was measured by percent change in pulmonary function from pre- to postexposure . 
 significant gas concentration effects ( filtered air versus o3 ) were observed for forced vital capacity ( fvc ) , forced expiratory volume in 1 sec ( fev1 ) , and forced expiratory flow between 25 and 75% of fvc ( fef25 - 75 ; p < .05 ) . 
 more importantly , the pulmonary function flow rates , fev1 and fef25 - 75 , showed a significant menstrual phase and gas concentration interaction effect , with larger decrements observed in the f menstrual phase when progesterone concentrations were significantly lower . 
 we conclude that young , adult females appear to be more responsive to acute o3 exposure during the f phase than during the ml phase of their menstrual cycles.(abstract truncated at 250 words)imagesfigure 1 .