This is pulled from bike helmets direct
http://www.bikehelmetsdirect.com/Giro_Ionos_Bike_Helmet.html
"What makes the Ionos so desirable? It boils down to a few key points: One is its weight. Giro makes liberal use of new lightweight composite materials to make the Ionos nearly 10% lighter than the vaunted Atmos. And then there's its ventilation. You get 21 huge vents -- in fact Giro states that they designed the structure of the Ionos around the vents, specifically to maximize head coolness. Giro calls it "Wind Tunnel Technology", and their testing indicates that you get 15% more airflow over your head in comparison to the Atmos."