Babies need fresh air and light, and they need to get out. A healthy baby can go outside every day, even in winter, as long as the temperature is not too cold (down to about –12°C) and it's not too windy. She'll be stimulated and distracted, and get plenty of oxygen. Fresh air cleans children's lungs, ridding them of impurities such as car fumes and dust, which playing outside will help clense. In a home with poor air circulation, you might think the most obvious problem is that we are all breathing in and recycling each other's breath.