ISEE-0629 Background and Objectives: Indoor air quality can be affected by pollutants generated outdoor. In hospitals this situation has been insufficiently investigated. Methods: We analyzed two intensive care units located at a same hospital, measuring air pollutants and weather variables inside and outside these units. Automatic monitors were used to measure PM2.5 while passive samplers were adopted to measure both NO2 and O3. A questionnaire of symptoms was applied to the employees. Results: In unit 1, the mean value of indoor PM2.5 was 40.0 μg/m3 (SD = 43.9) while outdoor measure reached 28.4 μg/m3 (SD = 18.3) (P < 0.01). Also, it was two fold higher than indoor PM2.5 in unit 2. For NO2, outside values were higher than those measured inside and, in unit 1, this difference was statistically significant (P < 0.01). Ozone measurements in both outside and inside ICU 2 were higher (five fold) than those observed in ICU 1 (P < 0.01). From 18 investigated symptoms, tiredness or fatigue (66%), tension and irritability (65%), pain in back shoulders or neck (65%), headache (62%), runny nose (50%), sneezing (41%), sore and dry throat (30%), difficulty of concentrating (29%), nauseas and upset stomach (27%), dry and itchy skin (23%), depression (15%), and cough (15%) were the most reported. Frequencies of symptoms were similar in both units and among professional groups. Conclusions: Our results showed that isolation between indoor and outdoor environments through air conditioning systems may vary allowing high indoor concentrations of fine particles and infiltration of gases. Moreover, frequencies of respiratory symptoms and others of different diseases were higher than those reported in the specific literature.