Why is it suggested to keep a window open about one inch while traveling?

• To prevent carbon monoxide poisoning
  Explanation:

  "Carbon monoxide poisoning can often be fatal, and even small amounts can cause you to become sleepy or unconscious and lose control of the vehicle. For this reason, it is suggested that you keep a window open about one inch while traveling."
• To improve fuel efficiency
• To reduce wind noise
• To maintain a comfortable temperature inside the vehicle