Taken from "Babycenter", a mainstream website for mothers:
Your preschooler needs fresh air as much as he needs sleep and good food. Unfortunately, modern life has moved largely indoors. That's why it's important to build time into every day for your child to be in your backyard, a park, or a playground. Encourage your child to use outside time to run around and work his muscles. Being outdoors also provides sensory stimulation (the sun, the birds, the dirt) that can't be replicated indoors.