The indoor air quality is very important for the well-being of occupants, especially in the case of young babies. This research focuses on the air quality of the surrounding air inside a crib with sleeping infants. To study the effects of different sleeping positions of the baby with in the crib a measurement setup was created in the laboratory. The breathing of an infant was simulated by means of a baby doll with air supply mixed with CO2 and measured at different sensor locations for different sleeping positions. The results show an enormous increase in the CO2 concentration (up to 4 times) depending on the sleeping position of the infant, and also show the effect of a more open crib. The effect of the position of the baby on the CO2 concentrations inside the crib are compared with the background level in the sleeping quarter.