Ventilation on goggles helps control the climate inside the goggle. Every brand has different features to allow air to flow through the goggle to prevent moisture from building up inside the goggle and to minimize fogging. The greater the ventilation, the less likely your goggles will fog up.
Low - These goggles have some ventilation but tend to fog up when you stop moving due to a decrease in airflow which is used to push out the warm, moist air.
Medium - These goggles have an adequate amount of ventilation that generally prevents goggles from fogging up during low-to-moderate activity. You may experience some fogging under high activity.
High - These goggles have maximum venting to allow cool dry air into your goggles which pushes warm moist air out in order to prevent your lenses from fogging.
Fan - These goggles are equipped with a fan that will continuously circulate air in order to prevent moisture from building up inside your goggles and keep your goggles from fogging up.