This study will evaluate effects of sildenafil on exercise performance while breathing high levels of emission exhaust ultrafine and fine particulate matter. Exercise performance will be measured by work accumulation (total kJ) during a 6-min maximal effort cycle ergometer ride (CER) done immediately after 30 min cycling at 75% of 6 min mean watts determined from familiarization trial. Peak oxygen consumption, cardiac output, pulmonary artery pressure, diffusion capacity(DLco), and SaO2 will be determined for each trial. Blood and urine will be analyzed for sildenafil using LC/MS.
