The Chemical Infant Protective System (CHIPS) is a special hood-like system into which a small battery-operated blower delivers filtered air. Because it is a semiclosed system, there is a risk of dangerous CO2 accumulation within the device, which particularly affects infants with acute or chronic respiratory disorders. Eleven infants hospitalized with various respiratory illnesses wore the device for 15 minutes. Inspired O2, inspired CO2, heart rate, respiratory rate, oxygen saturation, and inside temperature and humidity were measured before and during this test period. Inspired O2 and heart rate during the test period were significantly lower than baseline levels (O2, 19.1 vs. 20.1%; heart rate, 133 vs. 142 beats/min). Inspired CO2 and inside temperature during the test period were significantly higher than baseline levels (CO2, 0.23 vs. 0.06%; temperature, 25.0 vs. 23.1 degrees C). Oxygen saturation, respiratory rate, and humidity were not different from baseline levels. A short-term stay within the CHIPS in well-ventilated surroundings did not result in significant clinical and physiological impact for sick infants. Nevertheless, trends were identified that may be worrisome during longer periods and in sealed rooms.